TOPIC 12.
THE ELEMENTS - the Periodic Table.
For millennia, humans have been discovering and extracting elements from nature
and using them either in their elemental state or in combination with others as
compounds. This process has accelerated over the past few centuries and in today’s
highly technical environment we are dependent on a continued supply of many
elements, the names of some of which were rarely mentioned or even recognised by
chemists just a few decades ago. Without an assured supply, much of the
technology that is now widely taken for granted would no longer be viable and
potential future developments will be hampered or rendered impossible. As an
example, note how dependent current technology has become on the supply of the
so-called rare earth elements which underpin many of the advances made in
computing, communications and the many applications in which powerful rare
earth magnets are the basis. Other less exotic elements are even more important for
today’s living standards - consider how dependent agriculture is on a continued
supply of phosphorous in the form of phosphate fertilisers, a supply which may be
fated to be fully depleted in the future. In these notes, a selection of elements
which are among those that are essential components of technology today are
discussed in the context of the Periodic Table. This system of classification of the
elements is not only a convenient summary of the chemistry associated with various
families of elements in which the Table’s origin lies, but in its modern form the
Periodic Table provides the scaffold underlying the electronic structures of the
atoms and upon which their various properties and reactions depend.
The following are a few illustrative examples to ponder concerning the elements
which are discussed in this Topic.
Nitrogen molecules in the atmosphere contain one of the most stable bonds yet
about half the nitrogen atoms in our bodies were extracted artificially from the air.
Salt is destroying vast areas of agricultural land in Australia and has undesirable
consequences if consumed in excess in our diet yet is a valuable source of essential
products.
The air surrounding us contains a large proportion of a highly corrosive gas that
originated as the most polluting ever and which reacts with almost all other
elements yet is indispensable to all animal life - oxygen.
The rechargeable batteries which power mobile phones and computers are based
on the extremely small size of the lithium ion.
Many nuclear diagnostic procedures rely on using artificially produced atoms that
originate from nuclear reactors such as that at Lucas Heights in Sydney.
An irreplaceable gas with vital applications in magnetic resonance imaging
machines is used and lost - filling party balloons!
An object made from some metals retains a memory of its initial shape and if
distorted, it will return to the original shape when heated.
XII - 1
XII - 2
Origin of the elements.
As discussed in Topic 1, there are 90 naturally occurring elements. In addition,
there are about 28 other elements which have been produced synthetically but some
of these exist only as very short-lived radioactive species which have been
produced in extremely small quantities using high energy particle accelerators.
How did the 90 naturally occurring elements originate? The most widely accepted
version of the origin of all matter in the universe is the BIG BANG THEORY
which proposes that in an instant, all of space, energy and matter which had been
confined to a volume the size of a grain of sand at an infinitely high temperature
underwent an explosive expansion. Within 10 minutes of the big bang, nuclei of
mostly hydrogen and helium were formed from more basic particles and over the
next million years these nuclei cooled enough to capture electrons and form atoms.
After about a billion years, the gravitational attraction between atoms which were
still mostly hydrogen and helium, lead to clumps of matter which by gravitational
attraction gradually increased in size. With increasing size, the temperature of
these clumps also increased and in some regions of space the larger clumps became
hot enough to initiate fusion reactions between nuclei, forming stars which are in
effect giant nuclear fusion reactors. Our own sun, like all stars, converts hydrogen
to helium with the concurrent release of extremely large amounts of energy known
as the BINDING ENERGY associated with the strong nuclear force which was
discussed briefly in Topic 2. Within a few billion years vast numbers of new stars
were formed and these in turn, through gravitational attraction, clustered to create
galaxies, each of which contains enormous numbers of stars. By that point in time,
the universe would have looked much as it does today. As the hydrogen fuelling
the fusion reactions in a star is consumed, other fusion reactions can occur in which
heavier elements form. Fusion reactions leading to new atoms of elements as heavy
as iron all release energy and can continue to fuel a star. Ultimately, when a star
has consumed most of its available fuel, it may simply cool and dim or in some
cases it may initially implode and then undergo an enormous explosion which
flings much of its constituent material and energy out into space in what is called a
SUPERNOVA EVENT. It is only during supernovae that elements heavier than
iron are formed. It is estimated that the present elemental composition of the
universe is 92.7% hydrogen atoms, 7.2% helium atoms and just 0.1% atoms of all
the other elements. The shock waves of energy and material sent into space from
supernovae may interact with existing clouds of gas, ice and dust to eventually form
new stars and planets such as our solar system. Our sun was not one of the original
stars in the universe but is probably a second or third generation star, formed in part
from the energy and residues released by previous supernovae. Although already 5
billion years old, it still contains 71% hydrogen and 27% helium, so the sun will
burn for several billion more years.
Thus on the basis of this theory, the elements which constitute all matter on earth
apart from hydrogen were originally formed from stars that existed before our sun
and which had consumed all their available hydrogen, converted it to helium and
progressed to other nuclear fusion reactions that created heavier elements and
finally underwent a supernova explosion in which more of the heavier elements
were produced.
XII - 3
Discovery and isolation of the elements.
Most of the non-gaseous elements on earth are chemically combined with other
elements as compounds. Few non-gaseous elements are found in the free state. For
thousands of years gold, silver, copper, sulfur and carbon had been known because
they do occur in the free form, although they were not necessarily recognised as
elements - indeed the concept of an element as we know it today was not firmly
established until the 18th century through the visionary work of Lavoisier. While
some metals such as gold and silver are so unreactive that they can be found as free
elements, most elements occur as compounds in MINERALS. Extraction of
metals such as copper and tin from their ores by the process of SMELTING was
probably accidentally discovered when minerals were used as fireplaces. The
extraction process relied upon the use of bellows made from animal hides to
increase the heat obtained from a fire to the point where decomposition could occur
of rocks containing for example copper (malachite, a copper carbonate compound)
and tin (cassiterite, an oxide of tin). Charcoal in the fire reduced the copper and tin
compounds to the free elements. Later it was discovered that mixing about 9 parts
of copper and 1 part of tin together and melting them produced an alloy called
BRONZE which is much harder than either of its constituent elements, a feature
exploited in the bronze age from about 5000 years ago. Later, iron was isolated
from its ores by similar means - probably more than 3000 years ago. Prior to 1600,
the elements gold, silver, carbon, sulfur, copper, tin, lead, iron and mercury had
been discovered by persons unknown. However arsenic, isolated by Albertus
Magnus in 1250, is the first recorded instance of an attributed method of isolation
of an element.
The prehistoric technical advance of using crude furnaces to smelt ores set the
pattern for future discoveries of elements which resulted from newly devised
processes. The rate of isolation of the elements was a process that tended to occur
in steps where development of a new method or technology gave impetus to a flurry
of discoveries, often followed by a quieter period prior to another new method or
technology being devised. This is shown in the chart on page XII-5 which plots the
number of known elements against dates of discovery based on the data given in the
Tables on Pages XII-4 and 5. In the Table on Page XII-6, the dates of discovery of
those elements known in prehistoric times are simply listed as <1600 while in the
chart the prehistoric discoveries have been placed undated prior to 1600 along the
axis.
Prior to 1600, the driver of isolation of elements such as copper, tin, lead and
mercury was augmentation of the use of fire with bellows to create a furnace.
Isolation of elements in the 18th century became increasingly rapid, boosted by
several new technologies. Methods for analysing minerals reached a high state of
development, particularly as a result of the advent of the blowpipe. This tool allows
the ready decomposition of minerals on a carbon block, a prerequisite for analysis
of their components. Elements discovered through the improved analytical
techniques during the 18th century were cobalt, bismuth, platinum, zinc, nickel,
manganese, molybdenum, tellurium, tungsten and chromium.
However, the greatest advance in new techniques during the 18th century was the
development of apparatus designed to handle gases. This not only allowed the
isolation of the elements hydrogen, nitrogen, oxygen and chlorine as well as
gaseous compounds such as carbon dioxide, but lead to our current understanding
of the nature of chemical processes and to fundamental laws such as the law of
conservation of matter.
XII - 4
The development of the battery and its application to isolating elements via the
method of electrolysis was exploited, particularly by Humphry Davy, in the early
years of the 19th century. He used large batteries made from copper and zinc to
electrolyse molten salts of elements from Groups 1 and 2, isolating potassium,
sodium, calcium and barium in rapid succession in 1807 - 1808.
Throughout the 19th century methods for analysing minerals continued to develop.
These methods included the use of the blowpipe to obtain high temperatures in
conjunction with acid digestion of minerals, selective precipitation of salts,
gravimetric analysis and fractional crystallization allowing separation of pure salts
from mixtures. Along with improved analytical techniques, the development of two
new pieces of apparatus, the Bunsen burner and the spectroscope by Bunsen and
Kirchoff respectively lead to discovery of elements initially by their atomic
emission spectra. The Bunsen burner allowed high enough temperatures to produce
atomic emission spectra from salts and the spectroscope allowed the unique pattern
of spectral lines of each element to be observed and recorded. In 1860 they
discovered the element caesium initially through its spectral lines which they found
in samples extracted from minerals containing other Group 1 elements. In 1878
holmium and in 1879 samarium were first discovered by the presence of their
spectral lines in extracts from ores. In 1868 Lockyer observed previously unknown
spectral lines in sunlight and he attributed them to an unknown element which he
called helium. Subsequently in the 1890's it was found that helium is present in our
atmosphere along with the other noble gases neon, argon, krypton and xenon. Their
discovery was in part prompted by Ramsay’s observation that there was room for
gaseous elements at the end of each Period of the Table. Removing all the known
gases from air left a small, unreactive component which was called argon.
However, it required yet another technological advance before neon, krypton and
xenon were isolated. This time the new technology was the ability to liquefy air.
Fractional distillation of liquid air after gases such as oxygen, nitrogen, argon and
carbon dioxide had been removed, left a small fraction which showed an unknown
spectrum - this element was krypton. Careful fractional distillation of this
remaining material revealed the presence of yet another gaseous element with an
unknown spectrum - neon. Xenon was finally isolated by repeated fractionation of
liquid krypton, again it was determined that they had obtained a new gaseous
element by observing its spectrum. Thus a completely new family of hitherto
unsuspected elements, the noble gases had been discovered as a result of combining
spectroscopy and fractional distillation of liquid air.
The development of sensitive apparatus for measuring radioactivity by the Curies
was central to their isolation of the elements polonium (1898) and radium (detected
in 1898 but not isolated until 1903).
The chart shows a long gap until another rise in the rate of isolation of elements
around 1940 when nuclear reactors were first invented and elements such as
plutonium and americium were isolated for the first time. More recently, the
development of high energy particle accelerators has been applied to production of
synthetically produced elements, generally in microscopic quantities.
XII - 5
Isolation of the elements - chronological listing.
Prehistory
Au, Ag, C, S, Cu, Sn, Pb, Fe, Sb
1250
1669
1735
1746
1751
1753
1755
1766
1772
1774
1778
1781
1782
1789
1790
1791
1794
1797
1801
1802
1803
1807
1808
1811
1817
1823
1826
1828
1830
1839
1843
1844
1860
1861
1863
As (but used earlier)
P
Co, Pt
Zn (but used earlier)
Ni
Bi (but used earlier)
Mg
H
O, N
Mn, Cl
Mo
W
Te
U, Zr, Be
Sr
Ti
Y
Cr
Nb
Ta
Os, Pd, Ce, Rh, Ir
Na, K
Ba, B, Ca
I
Li, Se, Cd
Al, Si
Br
Th
V
La
Tb, Nd, Er
Ru
Cs
Tl, Rb
In
1866
1875
1878
1879
1880
1885
1886
1894
1895
1896
1898
1899
1900
1907
1917
1923
1925
1937
1939
1940
1944
1945
1947
1950
1952
1953
1955
1958
1961
1965
1970
1974
1976
1982
1984
F
Ga
Yb, Ho
Sm, Sc, Tm
Gd
Pr
Dy, Ge
Ar
He
Eu
Po, Ra, Kr, Ne, Xe,
Ac
Rn
Lu
Pa
Hf
Re
Tc
Fr
Np, Pu, At
Cm
Am
Pm
Bk, Cf
Es
Fm
Md
No
Lr
Rf
Db
Sg
Bh
Mt
Hs
XII - 6
XII - 7
DATES OF DISCOVERY OF THE ELEMENTS
2
1
H
He
1766
3
6
7
8
9
1895
10
5
B
C
N
O
F
Ne
1817
4
Be
1798
1808
<1600
1772
1772
1866
1898
11
Na
12
Mg
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar
1807
19
1755
1823
1669
<1600
1774
1894
32
Ge
33
As
34
Se
35
Br
36
Kr
Li
25
26
27
28
29
30
1823
31
Mn
Fe
Co
Ni
Cu
Zn
Ga
1797
1774
<1600
1735
1751
<1600
1746
1875
1886
1250
1817
1826
1898
41
Nb
42
Mo
43
Tc
44
Ru
45
Rh
46
Pd
47
Ag
48
Cd
49
In
50
Sn
51
Sb
52
Te
53
I
54
Xe
1789
1801
1778
1937
1803
<1600
<1600
<1600
1782
1811
1898
74
W
75
Re
78
Pt
79
Au
82
Pb
83
Bi
84
Po
85
At
86
Rn
1802
1781
1925
1803
1735
<1600
<1600
81
Tl
1923
1803
77
Ir
1817
80
1863
73
Ta
1844
76
1803
72
Hf
1861
<1600
1753
1898
1940
1900
104
Rf
105
Db
106
Sg
107
Bh
108
Hs
109
Mt
1965
1970
1974
1976
1984
1982
65
Tb
66
Dy
67
Ho
68
Er
69
Tm
70
Yb
1843
97
Bk
1950
1886
98
Cf
1950
1878
99
Es
1952
1843
100
Fm
1953
1879
101
Md
1955
1878
102
1807
20
Ca
21
Sc
22
Ti
23
V
24
Cr
1808
1879
1791
1830
37
Rb
38
Sr
39
Y
40
Zr
1861
1790
1794
55
Cs
56
Ba
57-71
1860
1808
87
Fr
88
Ra
1939
1898
K
LANTHANIDES
ACTINIDES
89-103
Os
57
La
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
1839
89
Ac
1899
1803
90
Th
1828
1885
91
Pa
1917
1843
92
U
1789
1947
93
Np
1940
1879
94
Pu
1940
63
64
Eu
Gd
1896
1880
95
Am
1945
96
Cm
1944
Hg
No
1958
71
Lu
1907
103
Lr
1961
XII - 8
The Periodic Table
As each element has its own characteristic properties, this implies that one would
need to be familiar with more than 100 different sets of chemical properties in order
to understand the chemistry of all the elements. However the elements actually
consist of families or groups, each of which contains a number of elements that all
share many similar properties. Thus by knowing the general properties of each
group of elements, the task is made much easier. Further, recognition of the
existence of these groups has led to an understanding of why various properties are
associated with each of them. The arrangement of the elements as chemical groups
constitutes the PERIODIC TABLE, one of the fundamental cornerstones of
chemistry which not only embodies the outward properties of elements, but also
incorporates the inner atomic structure of their atoms.
Development of the Periodic Table.
The earliest suggested grouping of elements was simply on the basis of very
obvious properties such as being shiny or malleable (classed as METALS) or not
(classed as NON-METALS). Metals were further grouped as COINAGE
METALS (silver, gold, copper) or as REACTIVE METALS. From this simple
beginning, the modern Periodic Table evolved.
One of the first classifications into families was by Dobereiner (1829) who noted
that there were often groups of three elements which shared similar properties, e.g.
Ca, Sr, Ba
Li, Na, K
S, Se, Te
Cl, Br, I
Fe, Co, Mn
reactive metals
very reactive soft metals
foul smelling hydrides
highly corrosive non-metals
hard metals, coloured salts
In each case, the atomic weight of the middle member was approximately the
arithmetic mean of the other two. As chemical knowledge increased along with the
number of elements isolated, other bases for classification became possible. One
basis tried was to arrange the elements in order of increasing atomic weight.
Newlands (1864) observed that the chemical properties seemed to be repeated every
8 elements when this order was used, leading to his law of octaves: "the span
between repetitions in chemically similar species is an octave". While Newlands's
classification appeared valid for the lower atomic weight elements, at higher atomic
weights there were many obvious absurdities with elements of very disparate
properties being classed as a family. This problem arose because many elements
had not yet been isolated in 1864, and no spaces had been left for them in his
classification. Dobereiner’s suggestion of chemical families being related to atomic
weight apparently inspired Mendeleev in 1868 to assemble a small fragment of a
Periodic Table, based on more accurate atomic weights and chemical properties.
element
atomic
weight
element
atomic
weight
element
atomic
weight
Cl
35.5
K
39
Ca
40
Br
80
Rb
85
Sr
88
I
127
Cs
133
Ba
137
XII - 9
In 1869 Mendeleev proposed a classification of the then known 65 elements which
placed priority on allocating elements to each family on the basis of similar
properties with special emphasis on valence rather than atomic weight alone. He
left blanks in families where discrepancies would otherwise appear and
repositioned some elements, disregarding their accepted atomic weights and/or
valencies in recognition that these could be in error. In 1871 he produced an
updated version of the Table which included then unknown elements to which he
gave names such as eka-aluminium (following aluminium: now gallium) and ekasilicon (following silicon: now germanium). Using this Table he predicted the
properties of the missing elements based on the corresponding properties of the
preceding and following element in the Group. His predictions were remarkably
accurate when the missing elements were ultimately isolated.
The Periodic Law proposed by Mendeleev states: “When arranged by atomic mass,
the elements exhibit a periodic recurrence of similar properties”.
The modern Periodic Table.
With the discovery of the sub-atomic particles and the subsequent knowledge of the
structure of atoms, the fundamental basis for the periodic classification was realised
to be arrangement in order of increasing atomic number (the number of protons in
the nucleus), rather than atomic weight (Moseley, 1913). The difference from
atomic weight order is due to the various isotopes that contribute to the atomic
weight (the weighted average of all naturally occurring isotopes of that element) so
that an element which exists as an abundant heavy isotope but a lower atomic
number would be out of order and appear in the wrong Group. There are only three
instances where atomic weight order departs from atomic number order arising
from the proportions of the different isotopes of each element. Atomic number is
the number of protons in the atom’s nucleus and as atoms are electrically neutral, is
numerically the same as the number of electrons in the atom. As the arrangement
of electrons around the nucleus depends on how many electrons are present, then it
is the electronic structure of the atom which determines the properties of each
element. Thus what began as a classification solely on the basis of properties of
elements was found to be a classification based on the atomic structure of the
elements. A copy of the modern Periodic Table is given on the last page of this
book.
The Periodic Table in Review.
(i) The periods.
Each horizontal row or PERIOD of the Table starts with a Group 1 element and
ends with a Group 18 element. [Note: There are several different Group numbering
systems in use. One uses Roman numerals and extends from Group I to VII plus 0
or VIII for the noble gases while the current IUPAC system uses normal numbers
and extends from 1 to 18. The IUPAC system is used in this Topic.] In between,
there are various numbers of elements as follows:
1st period H, He
2 elements
2nd period Li v Ne
8 elements
3rd period Na v Ar
8 elements
4th period K v Kr
18 elements
5th period Rb v Xe
18 elements
6th period Cs v Rn
32 elements
7th period Fr v
see “The Search Continues” on Page XII-20.
Elements in each period do not constitute a family and there is no value in
specifically committing to memory the members of each Period of the Table.
XII - 10
Indeed, across each Period there is a steady change in properties from those
elements classed as metals (on the left hand side) to those classed as non-metals (on
the right hand side). Properties of metals compared with non-metals have been
mentioned in previous Topics, but the following extended summary of physical and
chemical properties of metals compared with non-metals is appropriate at this point.
METALS
Good conductors of heat and electricity
Malleable, ductile
Shiny appearance when freshly cut
React with non-metals to form cations in salts
Dissolve in acids to form cations
Ionic halides
Have oxides which are ionic and dissolve
in acids to form salts (basic oxides)
NON-METALS
Poor conductors
Brittle, often powders or gases
Dull appearance
Form anions in salts
Not soluble in acids
Covalent halides
Have covalent oxides which are
insoluble in acids but may
dissolve in bases to form salts
(acidic oxides).
The basis for the above properties and why they are associated with a metal or a
non-metal are now well understood in terms of the structure of each element’s
atoms.
Variation of atomic properties across Periods
The sizes of the atoms (as measured by the atomic radius) decreases from left to
right across any Period of the table. [See diagram on page XII-10] This may seem
strange at first, given that additional electrons are being added to the structure of the
atom as the Group number increases. The explanation lies in the fact that as the
atomic number increases for each extra electron added to the structure, there is also
an extra proton present in the nucleus. For a given Period, the added electrons are
all in the same orbit and so all outer electrons experience the additional attraction of
the increased number of protons, leading to reduced atomic radius. At the end of
each Period, the last element is a noble gas after which there is no more room in the
electron orbit that is being filled to accommodate further electrons. The next
element (a Group 1 element) is the first in the following Period and its added
electron must occupy a higher orbit, further out from the nucleus. Each time a new
orbit is occupied, the electron allocated to it is partially SCREENED from the
attraction of the nucleus and so its atomic radius increases to become the largest for
that next Period. Moving across this next Period, the same process is repeated with
atomic radius decreasing from left to right, becoming smallest for the next noble
gas, then again increasing for the subsequent Group 1 atom as electrons occupy the
next highest orbit, and so on throughout the Table.
Consequently if one examines the attraction felt by outer electrons of the atoms
from left to right across any Period, it increases until reaching a maximum for the
noble gas element at the end of that Period. This can be expressed as the charge
actually felt by the outer electrons called the EFFECTIVE NUCLEAR CHARGE
Sometimes this is called the CORE CHARGE. Noble gases are unreactive
because their outer electrons experience a very large effective nuclear charge and
too much energy is required for them to be removed in a chemical reaction.
Thesingle outer electron of atoms of Group 1 elements experiences a much smaller
XII - 11
XII - 12
effective nuclear charge due to the screening of that electron by all the other
electrons and so only a relatively small amount of energy is required to convert
Group 1 atoms to 1+ ions. Having lost an electron to form the 1+ cation and thus
become isoelectronic with a noble gas, a very large amount of energy would then be
needed to remove a second electron - the high effective nuclear charge of the noble
gas structure is reinforced by the excess 1+ charge on the ion - and so a 2+ ion does
not form for Group 1 elements.
For Group 2 elements, it is not until after two electrons have been removed that the
noble gas structure is attained with its large effective nuclear charge. While it does
require more energy to remove the second electron from a Group 2 atom than was
required to remove its first electron, stable 2+ ions still result for Group 2 elements
(apart from Be2+ due to the small radius of the Be atom).
Similarly stable 3+ ions are the norm for most elements in the Group starting with
boron, but not boron itself due to its extremely small atomic radius. From the
Group starting with carbon onwards across a Period, the large excess positive
charge that would result on the nucleus prevents stable cations forming.
The atoms of the Group just prior to the noble gas at the end of each Period (the
halogens) have an effective nuclear charge which is only slightly less than that of
the adjacent noble gas and they have room for one more electron in that outer
energy level. Consequently these atoms are readily able to form 1! anions by
gaining an electron and actually release some energy in the process. Hence all the
halogen atoms are readily converted to the corresponding halide ions carrying a 1!
charge. However, in order to gain a second electron and form a 2! ion, that second
electron would have to occupy the next outer atomic orbit and thus be screened
from the nuclear charge by all the other electrons, so reducing the effective nuclear
charge it would experience. To achieve this would require the input of too much
energy to form a stable anion and so no 2! ions of halogens exist.
Similarly, outer electrons of the atoms of the Group starting with oxygen which are
all just 2 electrons short of the structure of the noble gas at the end of the Period are
subject to a high effective nuclear charge and can form anions with a 2! charge, but
formation of a 3! ion is energetically too difficult.
Thus it can be seen that the transition from metal to non-metal from left to right
across a period can in part be attributed to the accompanying increase in effective
nuclear charge of the atoms.
(ii) The Groups.
Each of the vertical columns of the Table headed Groups 1 through to 18 constitutes
a chemical family of elements. Each family has many chemical properties in
common. (You will recognise eight of these as the groups that you committed to
memory at the beginning of this course.) The chemical similarities within each
Group are attributable to there being the same number of electrons in the outer level
of the atoms of elements in that Group. For example, atoms of all Group 1
elements have 1 electron in their outer level; all Group 2 atoms have 2 electrons in
the outer level and so on. This is the reason for there being common valencies
within any given Group, a property to which Mendeleev gave priority in devising
his version of the Table.
While the elements of each Group have many properties in common, there are also
differences in properties within the Group. These differences typically are
exhibited as a trend from the first to the last element within each Group. The most
notable is that metallic properties (which are associated with the ease of removal of
outer electrons) increase down the Group. This is because the outer electrons are
partially screened from the nuclear charge by the inner electrons and thus attracted
XII - 13
less strongly to it and so are located at an increasing distance from the nucleus. The
larger the atomic radius, the weaker the outer electrons are held.
Probably the most striking instance of this transition is to be seen in the elements of
Group 15. At the top of this Group, the element nitrogen is totally non-metallic - it
is a gas, exists as covalently bonded N2 molecules and forms an anion with a 3!
charge in salts. The element bismuth at the bottom of Group 15 is a solid with
metallic bonding between atoms and forms 3+ cations in salts.
Another good example of how increasing atomic radius leads to greater reactivity
as a consequence of the outer electrons being further from the attraction of the
nucleus is to be found in the reactions of Group 1 elements with water to form
hydrogen gas and hydroxide ions. The element at the top of this Group, lithium,
only reacts mildly with water. The element at the bottom, caesium, reacts violently.
The elements in between become increasingly reactive to water down the Group.
The d-block and f-block elements.
In addition, there is a large block of elements, called the d-block, located towards
the middle of the table containing Groups 3 to 12. These elements are often called
the transition elements. You will also recognise some of these elements as those in
the last section of Table 2 from Topic 1.
Finally, there is another block of elements called the f-block located near the
bottom of the Periodic Table.
Properties of the Groups
The following notes provide an outline of the more important chemical properties
of each of the main Groups and also very briefly discusses the d-block and f-block
elements. Also a range of the elements are discussed in more detail with emphasis
on those properties which have significant useful applications.
GROUP 1. Li, Na, K, Rb, Cs, (Fr)
Group Overview.
Group 1 elements are known as the alkali metals because they react with water to
form hydroxides which are all soluble in water. They are all soft, very reactive
metals which can be cut with a knife and tarnish rapidly so must be kept under oil
to protect them from reacting with air.
They all react with water to form hydrogen gas and hydroxide ions, the reaction
becoming increasingly violent down the Group. This is a redox reaction in which
the metal is oxidized and hydrogen atoms in water are reduced.
2Na + 2H2O
v
2Na+ + 2OH! + H2(g)
They form ionic compounds called "salts" with non-metals. For example, sodium
and chlorine combine in another redox reaction to form sodium chloride, a
compound known as common salt or table salt. The elements in such compounds
are no longer made up of electrically neutral atoms of free element, but instead are
present as species called ions - atoms which have gained or lost electrons so as to
have an electrical charge. When a salt forms, the Group 1 metal atoms have each
had one electron removed, leaving them as +1 charged cations. As all Group 1
elements have a single outer electron, they all form only the M + ion in their
compounds because this arrangement leaves the ions with the noble gas electron
structure. In all compounds of Group 1 elements, each atom shows a combining
power or valence of 1 only. To form cations with a 2+ or higher charge would
XII - 14
require too much energy. The physical and chemical properties of Group 1
elements can be clearly related to this aspect of their atomic structure. The lone
outer electron of Group 1 elements in the solid state can move from atom to atom
easily when an electrical voltage is applied, causing these elements to be good
conductors. As there is only the one outer electron available to bind each atom of
the metal to its neighbours in the solid state, the metallic bond between the atoms is
easily broken and causes the softness of Group 1 elements. In later groups where
there are more outer electrons available to participate in metallic bonding, the
elements become harder.
The increased reactivity down the Group is a
consequence of the outer electron being further from the nucleus and therefore more
easily removed.
The atoms of a non-metal reacting with Group 1 elements gain the electrons that
were removed from the metal atoms when their cations formed and so the nonmetal atoms also become ions but with a negative charge (anions). The two
oppositely charged ions are then held together by electrostatic attraction to form an
ionic compound. These are characteristic reactions of all Group 1 elements.
For example, in the reaction of sodium with the non-metal chlorine, the process
could be shown as
2Na + Cl2 v 2Na+Cl–
However, the charges on the Na+ and Cl– in such compounds are not usually shown
in their formulas. It is a typical property of metals that they form ionic halides
whereas non-metals form covalent halides.
When a salt is melted or when it is dissolved in water, it shows electrical
conduction. This is because the ions are no longer held together in a solid crystal
but instead, under the influence of an electrical voltage, move to the electrode of
opposite charge and there undergo a reaction in which cations gain electrons and
anions lose electrons to form free elements again. This process is called
electrolysis and is the reverse of the reaction by which the original salt was formed.
Electrolysis of molten sodium chloride will produce the elements sodium and
chlorine at the electrodes according to the following equations.
2Na+(l) + 2e– v 2Na(l)
and 2Cl–(l) v Cl2(g) + 2e–
Reactions of this type, redox reactions, were examined in more detail in Topic 11.
Sodium and potassium ions are important in the conduction of nerve impulses.
Lithium ions provide a treatment for schizophrenia. Common compounds of Group
1 include sodium hydroxide (caustic soda), used in oven cleaners and as a starting
material in many industrial processes and sodium hydrogencarbonate (NaHCO3)
which is used in cooking.
FURTHER NOTES FOR LATER READING - lithium, sodium, caesium.
LITHIUM
Lithium is the third lightest atom and has the smallest radius of any metal. The Li+
ion is the smallest stable metal cation, a property which makes it ideal for use in the
lithium ion battery. [See the table of radii on page XII-11].
Lithium is extracted from salt beds in the deserts of some South American
countries. Unlike other Group 1 elements, it has an insoluble carbonate which
provides an easy method of extraction and purification. Until about 1990, lithium
had only minor uses, the main one being as a treatment for bipolar disorder which is
readily controlled with lithium carbonate. Prior to the introduction of this
XII - 15
treatment, psychiatric hospitals housed large numbers of sufferers but following the
introduction of lithium treatment, they were emptied. The mode by which it works
is not known but is assumed to replace sodium ions in some of the functions of the
brain in a way that calms the depression and suicidal thoughts that are part of
bipolar disorder.
At that time, apart from treating bipolar disorder, the other main application for
lithium compounds was in greases where lithium soaps are added to normal grease
to make it flow better.
Lithium aluminium alloys are now being used in aircraft construction.
Lithium batteries.
The market for lithium increased vastly when batteries based on lithium were
devised, especially the lithium ion battery. Lithium batteries now power most
portable electronic devices where the light weight and in the case of lithium ion
batteries, the ability to recharge them are unchallenged at present. They are also
being used to power electric vehicles and to store electricity from renewable
sources to smooth out the variation in supply. These batteries have the advantages
of large charge density and being rechargeable many times before they degrade.
There are two types of lithium battery:
(i) Non-rechargeable. These are typically the small button-sized batteries used in
small electronic devices. This type contains elemental lithium and the oxidation of
Li atoms to Li+ ions is the source of the electron flow from the negative electrode.
The positive electrode is typically MnO2. Because lithium is so reactive, this type
of battery is not suitable for large applications.
(ii) Rechargeable (lithium ion) battery..
The lithium ion battery is so named because the lithium is not present as lithium
metal but only as Li+ ions and there is no redox reaction involving them.
The Li+ ions are so small that they can move through solid graphite which
constitutes the negative electrode and also through the compounds of various metals
such as CoO2 used for the positive electrode.
In a lithium ion battery the graphite anode and the cathode containing a ‘mixed
metal’ oxide, typically LiCoO2 are divided by a separator which allows Li+ cations,
but not electrons to flow through it in the same way that H+ ions move through the
familiar lead battery.
When the battery is being charged, the charging voltage causes electrons to be
removed from the metal electrode which in the case of cobalt undergoes an
oxidation number increase to +IV. To preserve electrical neutrality in this
electrode, Li+ ions move out from the mixed metal electrode into the electrolyte and
through the barrier into the other half cell containing the graphite electrode. At that
electrode, the charging voltage forces electrons into the graphite layers and the
resulting excess negative charge is neutralised by the flow of Li+ which squeeze
into the spaces between the carbon layers. When fully charged, the battery has the
lithium ions packed between the layers of carbon atoms in the graphite anode.
When discharging, the reverse process occurs with the electrons flowing from the
graphite, through the device to be powered and into the mixed metal electrode
where the cobalt accepts the electrons and undergoes reduction from +IV to a lower
oxidation state. To preserve electrical neutrality in the battery, the lithium ions
move from the graphite electrode into the sponge like structure of the mixed metal
oxide.
XII - 16
This process is represented in the following diagram,
A typical lithium ion battery produces 3.7 V compared to 1.5 V of a standard dry
cell. There is intense research in this area to discover an electrode system that can
work with the larger, and much cheaper, sodium ion. Because the Na+ ion is too
large to fit between the layers of graphite, chemical reactions are being used to
expand the gap sufficiently. Also under investigation is the use of silicon, another
Group 14 element, to replace graphite.
SODIUM
The symbol Na for sodium is from the Latin, natrium. Elemental sodium is
produced by electrolysis of molten sodium chloride. Large deposits of sodium
chloride occur in dried-out lake beds.
Elemental sodium is used in sodium vapour lamps in which the metal is heated by
an electric current under vacuum inside a glass envelope
Sodium atoms are
released into the gas phase and electrons are excited to higher energy orbits before
falling back to the ground state and releasing the yellow light which is often used
for street lighting. Sodium vapour lights are much more efficient that incandescent
lamps, converting 70% of the electrical energy to light.
Sodium compounds are so ubiquitous that they are the source of the yellow flashes
from their atomic spectrum that can be seen in gas flames and fires,
Sodium chloride.
The common name “salt” used for sodium chloride is derived from the Latin and is
incorporated into numerous English words such as salacious, salsa and salary - the
latter because Roman soldiers were paid in salt. Historically salt has played a
major role in commerce and social changes over millennia. The vital need for salt
to preserve food and to keep people and animals healthy meant that controlling the
supply of salt was most advantageous both commercially and militarily. Salt was
of equal value to gold the middle ages and was the basis for the existence and
prosperity of cities such as Venice which had a monopoly of salt supply in that
region.
XII - 17
Capitalising on monopolies of salt through salt taxes were one of the underlying
causes of the French revolution and the American war of independence. The
beginning of civil disobedience in India was started by Ghandi’s march across the
country to fight the British monopoly of salt there. Lack of salt was one factor for
Napoleon’s defeat in Russia and the defeat of the confederate army in the American
civil war.
By the 20th century, refrigeration and modern packaging reduced the need for salt to
preserve food. Because it is readily accessible, salt is the ideal starting material for
the manufacture of industrial chemicals such as sodium hydroxide and chlorine.
In countries where snow falls, large amounts of sodium chloride are spread on roads
to melt snow and ice.
Sodium chloride is still used in some cases as a food preservative and additive but
before the advent of refrigeration, food such a meat was “salted” for storage and
hence the expression “to salt something away” meaning to conserve it.
Sodium ions are an essential component of the nervous system because conduction
of currents through nerves is achieved by movement of Na+ ions through
membranes rather than movement of electrons as in metallic conduction. Sodium
chloride also plays an important role in regulating blood pressure. This is because
the sodium and chloride ions attract water molecules - in particular, each Na+ ion
can attract more than 20 H 2O molecules around it. The effect of excessive salt
concentrations in the blood is to increase water uptake causing the volume and thus
blood pressure to increase. However, lack of salt for example when suffering from
diarrhea can be fatal. Lack of circulating salt in the blood can cause cramps. The
normal mass of sodium chloride in the human body is about 250 g.
Salt licks are used to attract cattle to various locations in paddocks.
Sodium hydroxide.
Sodium hydroxide in water solution provides the strong base, OH!. Such strongly
basic solutions have many industrial applications, such as:
Paper manufacture - wood consists of cellulose fibres held together with lignin. In
the Kraft method of paper production, wood chips are boiled in a mixture of sodium
hydroxide and sodium sulfite which removes the lignin. Subsequent squeezing out
of the water solution and drying results in paper sheets,
Soap - soaps are salts of fatty acids which are the components of fat. Heating with
sodium hydroxide causes the fats to break down to the sodium salts of these
component acids. Soaps dissolve dirt and grease because their anions consist of a
negatively charged end and a long hydrocarbon end. The negative end of the anion
can interact with water through ion-dipole attractions while the hydrocarbon end
can interact with non-polar entities such as grease through dispersion forces (see
Supplementary Topic 4). As a result, particles of grease and dirt or oils can be
dispersed into water as very tine droplets.
Sodium hydrogencarbonate.
Also commonly known as sodium bicarbonate, NaHCO3, this compound is used in
cooking to make cakes rise. In water solution it has a pH of about 8 and is used
where a mildly alkaline environment is required and it is used in antacid tablets.
Soft drinks used to contain sodium bicarbonate which released bubbles of carbon
dioxide by reacting with acid in the drink and hence the name soda pop although
now the carbon dioxide is gas forced in under pressure.
Heating causes carbon dioxide to be released from sodium hydrogencarbonate
which is why it is used in cooking where it is usually mixed with an acidic
ingredient. It is also utilised in solid state fire extinguishers.
XII - 18
Sodium carbonate.
Sodium carbonate, Na2CO3, is soluble (as are all sodium salts) and produces a basic
solution of pH about 10 . It is used in washing powders where it functions as a
water softener. Sodium carbonate is also an important component in the
manufacture of glass. Some toothpastes include sodium carbonate and sodium
hydrogencarbonate to counteract the acids in the mouth resulting from eating
carbohydrates.
CAESIUM
Caesium, symbol Cs, has a very low melting point, 28o C. A vial of solid caesium
will melt if held in the hand. It is the softest of all the solid elements. Being at the
bottom of Group 1 of the Periodic Table, its outer electron is the most readily
removed of any element. Thus caesium is extremely reactive and must be protected
from the atmosphere but its compounds are very stable.
Caesium was the first element to be initially discovered by observing its spectrum
in a flame. This discovery was made by Robert Bunsen using his newly invented
gas burner in conjunction with Gustav Kirchoff, the inventor of the spectroscope.
Salts of caesium such as caesium formate are used in drilling fluids in the mining
industry where the relatively high density of the Cs+ ion causes less dense liquids
such as oil to be displaced.
The radioactive isotope 137Cs which is derived from nuclear reactors is used for
various medical applications.
Caesium clocks.
Extremely accurate methods of determining time are of increasing importance.
High precision is needed for GPS devices, electricity grid stability and
telecommunications amongst many others. Use is made of the frequency at which a
particular electron transition occurs in the 133Cs atom which can be measured with
extreme accuracy. Using this standard, an accuracy of 1 second in 20 million years
is currently available and using a different basis, it is anticipated that an accuracy of
1 second in 13 billion years will be achieved, a greater time interval than the age of
the universe. Because the rotation of the earth is not constant and is slowing, in
order to keep the time as set by the atomic clock synchronised with “normal” time,
a leap second is introduced occasionally.
GROUP 2. Be, Mg, Ca, Sr, Ba, (Ra)
Group Overview.
These are also metals, but are harder than Group 1 elements and their reactions are
slower because more energy is required to remove the valence electrons.
Beryllium is hard enough to scratch glass but at the bottom of the Group, barium is
only slightly harder than lead. Increasing hardness compared with Group 1
elements can be attributed to the doubling of the number of outer electrons and a
resulting increase in the strength of the metallic bonds. Apart from beryllium, they
generally form salts with non-metals, always showing a valence of 2 in their
compounds due to the presence of 2 outer electrons in all their atoms. Removal of
the two outer electrons leaves all Group 2 elements with the noble gas structure.
Beryllium forms covalent compounds with non-metals, a property more like that of
a non-metal than a metal. The reason for this is that the Be atom is very small and
the energy required to remove even one of its outer electrons is relatively high.
Removal of both outer electrons from subsequent atoms in the Group becomes
easier due to the increased size of the atoms and therefore less attraction between
XII - 19
outer electrons and the nucleus and this explains the increased reactivity observed
down the Group.
Some typical reactions of Group 2 elements (i) burn Mg in air
2Mg
(ii) Ca in water
Ca
+
+
O2
2H2O
v 2MgO
v
Ca(OH)2
+
H2
calcium hydroxide
Alloys containing beryllium are used in aerospace applications. Beryllium is used
as windows in X ray sources as it is transparent to X rays. It is also used in the
nuclear industry because it reflects neutrons. However beryllium dust can cause
fatal lung diseases. Mg is the central atom in chlorophyll. The compound calcium
carbonate (CaCO3) is present in shells and calcium phosphate (Ca3(PO4)2) is of
particular importance as a component of teeth and bones. Large amounts of
calcium carbonate (limestone) are used commercially in the manufacture of glass
and cement. Radium has been used in radiotherapy treatment of cancer.
FURTHER NOTES FOR LATER READING - calcium.
CALCIUM
Calcium is found as various salts, mostly as calcium carbonate of which limestone
is constituted. Calcium compounds in the earth’s crust are slowly eroded and
washed into the oceans where calcium ions are combined with carbonate ions
formed from carbon dioxide which has dissolved in the water. Over long periods
the insoluble calcium carbonate settles to the ocean floor, joined by shells from
microscopic organisms. The usual geological processes result in compression and
ultimately uplifting to reveal limestone such as the huge cliffs that border the shores
of much of the Southern Ocean in Western Australia. Thus limestone represents a
vast storage of carbon dioxide, extracted from the atmosphere in previous eras.
Shells of sea creatures and exoskeletons of some marine organisms are made from
calcium and carbonate ions present in sea water. Carbon dioxide dissolved in water
forms a small amount of carbonic acid. The consequence of increasing CO2 in the
atmosphere is an increase in the acidity of the oceans. Because calcium carbonate,
the main constituent of marine invertebrates, dissolves in acids, the acidity of their
environment is critical and much marine life is threatened by the increased
atmospheric CO2 concentration.
Cement.
The most widespread use of a calcium compound is for manufacturing cement
which is the vital ingredient of concrete. Limestone is heated strongly to drive the
carbon dioxide off leaving calcium oxide or lime. Adding water to lime produces
slaked lime, a strong waterproof bonding material. However, the process of driving
the CO2 off the limestone contributes significantly to the increased concentration of
this greenhouse gas in the atmosphere, not only by the heating needed but also the
CO2 released from the limestone. The production of 1000 kg of cement releases
800 kg of carbon dioxide to the atmosphere and amounts to 5% of the world’s total
annual CO2 release.
Calcium sulfate.
Calcium sulfate (gypsum) is another calcium compound found in large deposits and
it is used in the building trade for plastering and also for setting broken bones in
which application it is known as Plaster of Paris.
XII - 20
Teeth and bones.
Calcium compounds are a large proportion of the body’s weight as bones and teeth
are made of calcium phosphate. Bones form when special cells produce a scaffold
made from protein which gives the bone its tensile strength and then calcium and
phosphate ions form the solid bone which provides its compressive strength.
Calcium ion intake is required throughout life to maintain healthy bone.
Other physiological functions of calcium ions.
Apart from solid structures in the bodies of most living creatures, calcium ions are
also a vital component in numerous physiological processes. For example, calcium
ions are used in the transmission of nerve impulses, facilitate the operation of
muscles and serve as cofactors that helps various enzymes to operate in functions
such as stabilising blood pressure and blood clotting.
In plants, calcium ions are vital in the mechanism that causes the stomata to close
when required as well as many other functions.
GROUP 13. B, Al, Ga, In, Tl
Group Overview.
Boron is a non-metal, having a black powdery appearance and not having any of the
usual properties of metals. Boron is a non-conductor of electricity and has a very
high melting point (2040 o C), indicating covalent network bonding of the boron
atoms in the solid. It has an acidic oxide, typical of non-metals and it does not form
ions when it reacts to produce compounds, but instead bonds by sharing electrons
with the bonded atoms to form covalent compounds. This is the method by which
non-metals bond to each other. The other elements of Group 13 mostly form ionic
compounds. The Group 13 elements usually have a valence of 3 in their
compounds, due to the presence of 3 outer electrons in their atoms.
Aluminium, the third most abundant element in the earth’s crust, has considerable
commercial application due to its very high strength to weight ratio and also its
being a particularly good conductor of electricity. Aluminium is produced in large
quantities by electrolysis of aluminium oxide ores such as bauxite. Although
aluminium is very reactive, it forms an oxide layer (Al2O3) on its surface which
protects the metal from further corrosion, making it useful as a building material.
However, at high temperatures, aluminium burns vigorously.
This Group shows clearly what is a general trend whereby the elements display
increasing metallic properties down the Group. This trend is quite subtle in Group
1 where lithium does form a few covalent compounds and is more pronounced in
Group 2 where beryllium forms compounds that are predominantly covalently
bonded but retains many other properties of a metal including metallic bonding in
the solid element. In Group 13, boron is essentially a non-metal in appearance and
in physical and chemical properties. Aluminium forms both ionic and covalent
compounds as do the other elements of this Group, but all exist as stable cations in
solution. This trend from non-metallic to metallic properties down a Group
continues to be apparent in Groups 14 - 16, attributable as discussed previously to
the larger size of the lower atoms in each Group resulting in weaker attraction
between nucleus and electrons and thus less energy being required to form cations.
FURTHER NOTES FOR LATER READING - boron, aluminium, gallium,
indium.
XII - 21
BORON
Boron forms an oxide, B2O3, which dissolves in water to form a weak acid, boric
acid. Both compounds have widespread applications.
Compounds of boron have some mild anti-microbial properties. Boric acid is used
to keep contact lenses free from bacteria and as an eye wash. Its sodium salt is used
as an insecticide which is harmless to humans but effective against insects and it is
typically the active ingredient of ant and cockroach traps. It is also used in trace
amounts as fertilizer for plants.
The very small size of the B atom allows boron atoms to fit into small spaces in
metals’ crystal lattices where they prevent sliding of planes of metal atoms and thus
impart greater strength to the crystal structure.
Boronsilicate glass.
Adding up to 15% of boron oxide to normal glass made from silicon dioxide (silica)
imparts the property of having minimal expansion on heating. This allows its use in
applications such as laboratory glassware and cooking utensils where heating would
cause cracking of normal glass. Consequently one can heat test tubes, beakers,
distillation flasks and other equipment made from boron-containing glass without
risking breakage. Varying the boron content of glass allows any coefficient of
expansion to be produced for a given task.
Lenses made from normal glass suffer from achromatic distortion whereby the lens
fails to focus the various wavelengths (colours) of light equally, leading to blurred
images. Using boronsilicate glass eliminates this difficulty and achromatic lenses
can operate down to the limits imposed by the wavelength of light.
Abrasives.
The compound boron carbide is one of the hardest materials known, third only to
diamond and another boron compound, boron nitride. It is used as an industrial
abrasive (carborundum) and also to make body armour. Because boron is also a
very light element, the weight of boron carbide armour is minimised, an important
consideration when wearing it.
Reactor control rods.
Nuclear reactors operate because of a flow of neutrons from decaying 235U atoms
impinging on other uranium atoms and continuing the fission process which
produces more neutrons in a chain reaction. If the flow of neutrons is not
regulated, the reactor would overheat and meltdown of the core would occur. The
10
B atom’s nucleus has the ability to capture neutrons. Control rods packed with
boron oxide or boron nitride are placed beside the fissile material in the core. By
raising or lowering the rods, the reaction can be turned on or off as required.
Boron neutron capture therapy.
This technique, still being developed, relies on the ability of the 10B nucleus to
absorb a neutron and convert to the unstable 11B atom which then decomposes to
release an alfa particle and a lithium atom - the same process that is used in nuclear
reactors. In this procedure, a suitable boron compound is injected and the tumour
site is exposed to a focussed source of neutrons of the appropriate energy. The
release of alfa particles that ensues damages the tissues that have absorbed the
boron compound. To be effective, the boron needs to be well distributed
throughout the tumour.
XII - 22
ALUMINIUM.
Aluminium is the most abundant metal in the earth’s crust and occurs in easily
accessible deposits well distributed world-wide, so its supply is guaranteed. A lot
of electricity is required to liberate the free metal by electrolysis of its common ore,
bauxite, which is aluminium oxide or Al2O3. Consequently aluminium smelters are
usually located close to cheap sources of electricity such as hydroelectric
generators. Aluminium is very reactive but is stabilised by the formation of an
oxide layer on its surface. The low density of aluminium combined with
mechanical strength and resistance to corrosion are why it is used in so many
industrial applications which include motor vehicle,
aeroplane and boat
construction. Food and beverages are ubiquitously contained in aluminium cans.
Other application use the very shiny surface attainable on aluminium metal. Thus it
is used in insulation materials and thermal blankets and also, if sprayed onto glass
as a thin layer, it is used to make mirrors,
Sapphire.
Aluminium oxide is one of the hardest substances known and is used as an abrasive
including sandpaper. Sapphires are crystals of Al2O3 which was subjected to very
high temperatures and pressures deep in the earth’s crust before ultimately being
uplifted by geological processes. Sapphires are used in various applications which
take advantage of their property of hardness. These include as bearings that resist
wear such as in mechanical clocks and watches and as components of various
scientific instruments.
The colour of sapphires is derived from small amounts of impurities consisting of
atoms of elements of similar size to that of aluminium. Small amounts of iron
atoms results in the blue colour which is common. Other colours are attributed to
addition of atoms of titanium, copper, magnesium and chromium, all of which are a
neat fit to replace aluminium atoms in the crystal. Addition of chromium atoms
produces the red colour of ruby which is another gemstone of aluminium oxide.
Artificial sapphires are grown at very high temperatures around 2500o C
Recycling.
Aluminium is recycled very efficiently with typically about 50% being reused.
This is commercially viable because the amount of energy needed to reuse
aluminium metal is only 5% of that needed to produce it from its ore. There is no
limit on how often aluminium can be recycled. In theory, ultimately there would
be no need for newly smelted aluminium but given its expanding market, that
would be a long time into the future.
GALLIUM AND INDIUM.
These two consecutive elements in Group 13 are both soft metals with low melting
points. Indium is named not for the country but after the characteristic indigo
spectral line by which it was first discovered. The existence of gallium along with
some of its properties was predicted by Mendeleev when proposing his version of
the Periodic Table and he called the yet to be discovered element eka-aluminium
for “following aluminium”.
Gallium melts at 30o C and indium melts at 157o C . Both “wet” glass which means
that they adhere to glass which makes them ideal for soldering electrodes to glass
surfaces.
They are both soft, shiny metals. Indium has a crystal structure that causes it to
emit a crying sound when bent, like tin, due to planes of their atoms moving
relative to each other.
XII - 23
They do not occur in minerals in quantities worth mining but instead are extracted
as trace elements from ores of zinc (indium) or aluminium (gallium).
Compounds.
Alloys of gallium, indium and often tin are used for example as solders for special
applications. The compound indium tin oxide is a conductor of electricity and is
used to coat touch screens and liquid crystal displays. An alloy of gallium and
indium with tin (galinstan) is a liquid at room temperature and is used in medical
thermometers. With mercury being banned, its use in normal thermometers will
provide a substitute.
Light emitting diodes.
Compounds of gallium and indium with various Group 15 elements are used in the
production of light emitting diodes (LEDs) and semi-conducting lasers which are
the type found in modern electronic devices. To manufacture LEDs, a substrate is
used onto which is deposited very thin layers of atoms of gallium, indium and
various other elements. For lighting LEDs, the substrate is a small sapphire but
different materials including silicon are also used for other applications. LEDs are
much more efficient than conventional globes as almost all the electricity used is
converted to light whereas an incandescent light globe converts 95% of the energy
supplied to heat. As their cost is reduced, LEDs are rapidly replacing other types of
lighting because of their greater efficiency and very long lifetimes. About 35% of
worldwide electricity production is used for lighting so widespread replacement
with LEDs will make a significant reduction in carbon dioxide emissions. Another
rapidly growing application is the use of LEDs in TV picture tubes.
The principle of LEDs can be used in reverse to convert light energy to electricity
as solar panels. Compounds based on these two elements have produced
efficiencies of 40% in conversion of sunlight compared with 10% typically for the
normal silicon panels because unlike silicon, these compounds can utilise all visible
frequencies. At present the cost precludes their mass production but they are used
in some special environments. Using mirrors to focus sunlight onto a small area of
such materials overcomes their larger cost.
Solid state lasers.
The small blue-light lasers used in CD and DVD players are mostly made using
gallium(III) nitride, GaN, deposited onto saphire substrate. The process to do this
successfully was elusive for many years but finally was put on a commercial basis
in the late 1990s, a feat which earned its developers the 2014 Nobel Prize in
Physics. Blue light lasers lead to the development of high density DVD discs which
allow much more data to be stored because the blue light is of shorter wavelength
than that used previously.
Recycling.
Due to the lack of concentrated sources of ores containing gallium and indium,
significant amounts are obtained by recycling. Indium supplies are likely to be a
problem at some time in the future.
GROUP 14. C (non-metal) Si, Ge (intermediate) Sn, Pb (metals)
Group Overview.
Carbon occurs as graphite and diamond as well as an amorphous (non-crystalline)
form such as charcoal. Carbon also occurs as hollow, soccer-ball shaped
XII - 24
arrangements of up to 76 or more C atoms known as fullerenes. These are
examples of allotropes - different physical forms of the one element arising from
different arrangements in the way their atoms are bonded.
Diamond contains a large number of carbon atoms joined by network covalent
bonding. Each C atom is bonded to 4 other carbon atoms by covalent bonds which
are pointing to the corners of a tetrahedron. Graphite contains carbon atoms which
are bonded to 3 other carbon atoms, all in the same plane, and also has weak bonds
to carbon atoms in the plane above and below. It is these weak bonds which cause
graphite to easily peel off into flakes when it is used in "lead" pencils or in graphite
lubricants. Graphite is exceptional among non-metals in being a conductor of
electricity, also due to the mobility of the electrons that constitute the weak bonds
between the planes of carbon atoms.
Carbon has an acidic oxide, CO2, forms only covalent bonds in compounds
including its halides and apart from graphite, does not conduct electricity - typical
properties of a non-metal. Carbon is the fundamental element in living cells, and is
recycled through the carbon cycle. Many millions of carbon compounds exist as
natural products or through laboratory or commercial synthesis. Because carbon
has the ability to form molecules containing long chains of covalently bonded
carbon atoms along with atoms of many other elements, almost unlimited numbers
of carbon compounds are yet to be prepared. The chemistry of carbon compounds is
called ORGANIC CHEMISTRY.
Important inorganic compounds of carbon include carbon dioxide, CO2, a product
of RESPIRATION and the main GREENHOUSE GAS from burning fossil fuels.
Carbon monoxide, CO, which results from the incomplete combustion of carbon
compounds is a powerfully toxic substance. Fortunately, carbon monoxide rapidly
converts to carbon dioxide in the atmosphere and does not accumulate to large
concentrations.
Silicon is the fundamental element of mineral chemistry, and it makes up the
majority of the earth's crust - usually in combination with oxygen as compounds
called SILICATES. For example, the white sand on beaches is substantially
silicon dioxide (silica), SiO2. About 87 % of the earth's crust is made up of SiO2
and related compounds. Silicon does not form such extensive chains through
bonding as does carbon, and has nowhere near as many compounds. Apart from its
role in mineral chemistry, elemental silicon is used to make solid state electronic
components including photovoltaic cells for converting sunlight directly to
electricity. Elemental silicon displays network covalent bonding in the solid state.
Do not confuse the element silicon with the class of compounds called
XII - 25
SILICONES which are polymers containing silicon bonded to hydrocarbon groups
in particular. Silicon and germanium have some properties of both metals and nonmetals so are often called SEMI-METALS or METALLOIDS.
While carbon and silicon do not form cations, germanium, tin and lead are more
like metals in that they do form cations in compounds but they can also covalently
bond. Group 14 again clearly illustrates this trend observed in most Periodic Table
Groups towards an increase in metallic properties of the elements down each Group
and both tin and lead in the solid state exhibit metallic bonding.
Tin and lead are both relatively low melting metals used together to form solder.
Tin is used to plate the ubiquitous steel cans because, unlike iron, tin does not
corrode readily.
Lead has anti-fungal properties which make it valuable as a component in paint,
usually in the form of lead oxides. Concerns about health risks through ingestion of
paint have caused lead to be deleted from most paints at present. Lead is also used
in large amounts in the lead acid accumulator (i.e. the common rechargeable battery
used in cars). There, alternate plates of lead metal and lead(IV) oxide, PbO 2, are
immersed in sulfuric acid, all contained in a plastic case. Electricity is provided by
the redox reactions which take place at the electrodes. Redox reactions were
discussed in detail in Topic 11.
Group 14 elements usually have valence = 4, but lead and tin in particular exist also
in compounds with valence = 2, often as salts containing the ions Pb2+ or Sn2+.
FURTHER NOTES FOR LATER READING - carbon, silicon, tin, lead.
CARBON
Graphite.
The most common form of carbon is graphite and it can be converted to another
form, diamond, by application of heat and high pressure. The traditional use for
graphite has been as a dry lubricant. Oils eventually dry and form thick gums and
thus cease lubricating the surfaces that require it. Because the layers in graphite can
peel apart and slide over each other, they provide lubrication without these
problems. Writing with a “lead” pencil is in fact depositing thin layers of graphite
on the written surface. However, in recent years a number of revolutionary
materials based on carbon atoms has been developed and graphite has become an
important starting material for their production. Some of these materials are
discussed below.
Carbon fibre.
CARBON FIBRE was the first and it consists of C atoms bonded to each other in
long strings. On a weight basis, carbon fibres are much stronger than metals as the
bond energy of the covalent bonds between carbon atoms greatly exceeds that of
typical metallic bonds. [Recall how the bonding in metals relies on mobile outer
electrons - see Topic 3.] The carbon fibres can be woven into mats and set with
resin, the fibres providing the tensile strength. Before that stage, the carbon mat
can be shaped to simple or complex designs as required, for example in components
of aircraft, especially where stress resistance is paramount. Apart from strength
and the shaping advantages of carbon mat construction, its significantly lighter
weight makes it ideal for applications such as aircraft and motor car bodies. The
carbon fibres are made from preexisting chains of carbon atoms in polymers such as
rayon from which all the non-carbon atoms have been removed.
XII - 26
Graphene.
A very recent development among carbon materials is the discovery of graphene.
Graphene was initially made by putting sticky tape onto graphite and pulling it off.
Adhering to the tape is a single layer of C atoms bonded to each other in the same
hexagonal pattern as in the layered graphite structure which has been likened to
chicken wire in appearance. The backing material can be dissolved to leave the free
graphene. On a weight basis, it is one of the strongest materials known and has the
ability to stretch as much as 20% of it length without damage. Potential uses for
graphene are being discovered regularly. It is an excellent conductor of heat and
electricity and has promise as a component of batteries and also photovoltaic cells.
The C atoms in the chicken wire-like structure of graphene provide an impermeable
surface through which even helium atoms cannot penetrate. No other material has
this property. Its flexibility allows it to be made into curved surfaces which holds
promise for making curved solar panels. Another use mooted is for the touch
screens of mobile phones due to its transparency and electrical conductivity as well
as its ruggedness, all properties required for that application. Graphene can also be
used to make filters which admit only a specific sized entity by putting the desired
size hole in it so it could be used as membranes in desalination plants for example.
Nanotubes.
Tubes of carbon atoms called NANOTUBES is another newly discovered carbon
material with much potential. These can be made by passing a large electric current
through a graphite electrode and sheets of the chickenwire structured atoms fly off
and spontaneously roll up to form the nanotubes, so named because they typically
have a diameter of 1 nanometre. The C/C bonds in nanotubes are stronger than
those in diamond and carbon nanotubes are hundreds of times stronger than any
metal. Applications to take advantage of the properties of carbon nanotubes are still
being explored but already they are being used in bulk, added to polymers to
increase strength. Individual nanotubes potentially could be used in biomedical
applications, as transistors and in conjunction with copper to make high capacity
electricity conductors.
Diamond.
The very regular tetrahedrally arranged structure of the C atoms in diamond
coupled with the strength of the C/C bonds gives diamond the property of being the
hardest known material. Consequently it is in widespread use for cutting and
grinding.
Diamond also has another unique property in being the substance having the
greatest heat conductivity. As a non-metal, this might seem surprising because
most non-metals are poor conductors of heat. The explanation lies in the highly
regular arrangement of its atoms in the crystal. As described in Topic 3, heat
conduction arises by two mechanisms. One is the increased movement of electrons
which bump into atoms and other electrons and transfer energy along the material
from the heat source. Metals have mobile outer electrons associated with their
atoms and so this mode of conduction is exhibited by metals. The second is
transfer of vibrational energy by atoms moving in a synchronous manner under the
influence of increasing temperature. This mechanism requires very regular arrays
of atoms with no impurities or discontinuities in the solid. There are no free
electrons to move in diamond unlike in metals, but it does meet the other
requirements for transmitting heat by vibrations of the C atoms. The C atoms in
diamonds are arranged to a high degree of symmetry and few impurities are present
in the diamond crystal. Diamond is four times better at conducting heat than
copper. Consequently diamond is used in the windows of high powered infrared
XII - 27
lasers for cutting steel for example, allowing ten times more energy to pass through
than glass. Diamond is also used as heat sinks for solid state components on
electronic circuit boards where heat dissipation is a significant problem.
Artificial diamonds of industrial grade can be made from graphite by applying heat
and extreme pressure. By this method, a lump of graphite can be converted into a
mass of microscopic diamonds in less than an hour. High quality jewellery grade
diamonds can be made more slowly.
SILICON.
The element silicon from the Latin silax, symbol Si, is not to be confused with
silicones which are polymers containing silicon atoms.
Silicon and oxygen are the two most abundant elements in the earth’s crust,
frequently combined in minerals together. Silicon is a grey coloured solid and is a
semiconductor of electricity. Silicon and oxygen are the two most abundant
elements in the earth’s crust, frequently combined in minerals together. Combined
with oxygen, silicon forms a dioxide which unlike the molecular covalently bonded
CO2, is a network covalent solid named silica of formula SiO2 which is ubiquitous
as sand and many minerals. Like carbon, it forms four covalent bonds in its
compounds such as SiH4 (silane) and SiCl4 (silicon tetrachloride).
Glass.
Common glass is made from silica with some small amounts of additional
substances to improve its properties. Glass can be drawn into fibres and used for
fibre optic cables required for communications transmission. Chopped into small
lengths and set in resin, fibreglass is a common construction material.
Alloys.
Silicon is added to many different combinations of metals to form alloys with
specific properties. The most common alloy is a mixture of iron with silicon,
accounting for about 80% of total silicon production.
Bronze is an alloy of copper (ca 90%) with tin (ca 10%) which is much harder than
copper and was the basis for bronze age implements. Its properties can be
enhanced by adding small amounts of other elements including silicon. Silicon
bronze is ideal where corrosion resistance under severe conditions is required.
Semiconductors.
Silicon is one of a number of elements known as semiconductors which conduct
electricity less well than metals but better than non-metals. One of their properties
is the ability to pass electric current more easily in one direction than the other
which can be used to convert alternating current (AC) to direct current (DC).
When purified to an extreme degree, silicon forms the basis of transistors and
integrated circuits which underlie modern electronics.
Solar photovoltaic (PV) panels.
Silicon can also convert visible light to direct current electricity and is the most
common material for rooftop PV panels. The conversion efficiency is currently
about 20% when crystalline silicon is used but technology is rapidly improving this
performance.
Silicones.
Silicones are polymers of Si atoms bonded to O atoms as a chain with organic side
groups bonded to the two remaining tetrahedral positions on the Si atom as shown
below.
XII - 28
Silicone chains can be cross linked to make three dimensional polymers which
serve for example as rubbers, sealants, greases and cookware. In dissolved form,
silicones are used as lubricants and water dispersant sprays, ingredients of hair
conditioners and fabric waterproofing agents as well as many other applications.
Properties which make silicones so valuable include their inertness, non-toxicity,
electrical insulating capacity and resistance to water and UV light.
TIN.
Tin was one of the earliest elements discovered, around 3000BC, presumably as a
result of tin-bearing rocks being used in fires. When mixed with copper which was
also discovered in ancient times, an alloy of the two metals called bronze results.
Bronze (12% tin) has superior properties to either of its component elements and
gave rise to the weapons and tools of the Bronze Age.
Pewter is another alloy of tin (85 - 90% tin) and has properties such as being
resistant to corrosion and non-toxic, thereby making it suitable for the manufacture
of eating and drinking utensils.
Tin is a soft, low melting point metal which resists corrosion due to the formation
of a protective oxide layer. It used to be combined with lead to make solder for
electronics circuits and plumbing, but the toxicity of lead has resulted in tin now
being used alone. The lack of lead in solder allows tin to develop problems known
as tin whiskers and tin pest to develop.
Tin has a highly crystalline structure and bending a bar of tin causes layers of atoms
within the crystal to slide over each other leading to an audible sound, sometimes
called the “cry of tin”.
Properties of tin have resulted in it being used in organ pipes(50% with lead) to
produce their characteristic mellow sound and in bells (20% tin, 80% copper) from
which long resonances are produced when struck.
However tin solder is not without its faults because the instability of the crystal can
result in small “whiskers” of tin erupting and leading to possible failures of printed
circuit boards. Tin pest is an eruption of pustules on the surface of tin arising from
the same cause. These problems can be reduced by adding small amounts of other
elements such as bismuth and antimony to tin.
Tin plating of cans provides a corrosion resistant surface. However this application
is being superceded by polymer coatings or using aluminium cans.
Tin has an important role through a revolution in the manufacture of sheet glass.
Sheet glass manufacture used to be a very hot and dangerous labour intensive
occupation until a process developed by the Pilkington company in the mid-20th
century. The basis for the Pilkington process is to float the molten glass sheet on
top of a tank containing molten tin. The tin is hot enough to stop the glass from
solidifying as it floats out to form a flat sheet which then moves along a continuous
production line.
Compounds of tin with organic molecules find uses such as biocides and as
stabilisers in plastics such as PVC without which the polymer breaks down rapidly.
XII - 29
LEAD
Lead was one of the earliest elements discovered and isolated. Lead ores have been
mined in England from pre-historic times and at one period lead was exported to
Rome. The symbol for lead, Pb, is derived from its Latin name, plumbum, which
has also given rise to names of activities traditionally utilising lead such as
plumbing and devices such a the plumb bob. It is a soft, malleable metal which is
shiny when freshly cut but gradually tarnishes in air. Lead metal is very resistant to
corrosion even by strong acids such as sulfuric acid a property used in the lead acid
battery.
Many of the applications in which lead or lead compounds were used have been
largely discontinued because of health concerns. Lead compounds when ingested
or inhaled cause damage to the nervous system and in particular, are associated
with brain damage. Some psychologists even claim evidence shows that leaded
fuel caused increased incidence of violent crime due to its effect on parts of the
brain.
Applications now discontinued in most parts of the world include the following:
Lead solder - lead melts at 328o C and it wets copper and tin so it is an ideal solder
for electrical and plumbing applications, usually mixed with tin. Solder now
contains no lead.
Lead as a fuel additive - for many decades lead compounds were added to petrol to
improve its combustion properties and also to act as a lubricant for the engine’s
valves.
Lead compounds in paints - these provide excellent antifungal properties but when
old paint was subsequently being burnt off, the inhaled vapours are dangerous.
Flaking paint also poses an ingestion hazard. The reaction against leaded paints
has been so strong that even artists’ paints cannot be legally sold in a some
countries. White lead carbonate is acknowledged as the best primer for oil painting
as it does not crack and imparts unrivalled visual properties to the paint overlaid on
it but it too is banned from sale in some regions.
Lead batteries.
Most (90%) of the lead mined today is used in the production of the ubiquitous lead
battery that is used in every motor vehicle and as standby power sources in the
event of power supply failure. These batteries are very reliable and simple and the
materials from which they are constructed can be recycled an unlimited number of
times. These include the sulfuric acid electrolyte, the lead plates that are part of the
electrodes, the lead dioxide and lead sulfate from the electrodes as well as the
polypropylene battery casing. Over 90% of all lead acid batteries sold are
eventually recycled and plants that once smelted lead ores are now largely involved
in recycling.
The main drawbacks of lead batteries are their weight and limited lifetime.
Lead sheeting.
Lead sheeting is the ideal material for sealing tiled roofs and in similar building
applications because of its flexibility which allows moulding to the required shape
and its low melting point which makes it easy to seal. Lead is extremely resistant to
corrosion and so lead sheeting is used to protect electrical cables, especially from
sea water.
Ballast.
The large number of protons and neutrons in the nucleus of lead atoms endow this
element with exceptionally large density which, combined with its resistance to
XII - 30
corrosion, makes it ideal to use as ballast.
applications.
It especially is useful in marine
Radiation shielding.
Again because of its nuclear structure, lead can block radiation from sources such
as X-ray machines and nuclear radiation. When undergoing body scans, lead
covering is used to protect organs from dangerous radiation and limit exposure to
the region under examination.
GROUP 15. N, P, (non-metals)
As, Sb, (intermediate) Bi (metal)
Group Overview.
Again, a transition from non-metal to metal is observed down this Group. Arsenic
and antimony are usually regarded as metalloids. Nitrogen is one of the few
elements to occur as a gas at room temperature and pressure. The other elements of
Group 15 are solids. The elements nitrogen and phosphorus exhibit molecular
covalent bonding, arsenic and antimony have network covalent bonding but
bismuth has metallic bonding in the solid state as expected of a metal. Elements of
this group commonly have valencies of 5 or 3 in their compounds. Compounds of
nitrogen and phosphorus are of vital importance in biological systems - nitrogen as
a component of all proteins and both nitrogen and phosphorus as components of
DNA. Phosphate groups are an essential component of the system by which cells
store and use energy. Elemental nitrogen is rather unreactive because it occurs as
the highly stable N2 molecule which constitutes 78 % by volume of the atmosphere.
Simple compounds of nitrogen found in the natural environment as part of the
nitrogen cycle include ammonia (NH3), ammonium salts (containing the NH4+ ion),
nitrate salts (containing the NO3– ion) and nitrite salts (NO2– ion). The process
whereby atmospheric nitrogen is converted to such salts requires considerable
energy input and is called NITROGEN FIXATION. It is accomplished by
lightning strikes and by various microorganisms. Today, about 50 % of all nitrogen
fixation is man-made through synthesis of ammonia from nitrogen and hydrogen
gases.
Phosphorus occurs in four different allotropic forms, all of which react vigorously
with oxygen in air, and thus must be stored under water. Phosphorus occurs most
commonly in the natural environment as phosphates which contain the PO43– ion.
Washing powders often contain phosphates.
Arsenic, antimony and bismuth occur less frequently and are of less importance in
biological systems.
Being mostly non-metals, many compounds of Group 15 are covalently bonded,
although compounds containing ions such as As3+, Sb3+, Bi3+ and N3– commonly
occur. Bismuth forms the usual salts containing the Bi3+ ion as expected of a metal.
FURTHER NOTES FOR LATER READING - nitrogen, phosphorus.
NITROGEN
The element nitrogen occurs as a diatomic gas, N2, at atmospheric conditions but
can be liquefied by using high pressures and lowering its temperature. Liquid
nitrogen boils at –176 o C at atmospheric pressure. Nitrogen constitutes almost 80%
of the atmosphere but is quite unreactive due to the very strong triple bond joining
the two N atoms of the N2 molecule. The bond in the nitrogen molecule is the
strongest of any molecule. The nitride ion, N3–, does occur in some ionic
compounds and the black tarnish that appears rapidly on the surface of freshly-cut
lithium metal is the compound lithium nitride. Being easy to produce, liquid
XII - 31
nitrogen finds many uses such as freezing biological specimens for future use and
freezing water pipes to allow disconnection for maintenance. Every doctor’s
surgery has a container of liquid nitrogen used to freeze and remove skin cancers.
Many applications where strong magnetic fields are required make use of the low
temperature attainable from liquid nitrogen. This allows greatly enhanced magnetic
fields to be produced because electrical resistance in metals which carry the
electrical current through the electromagnets decreases at low temperatures.
Gaseous nitrogen is used as an inert atmosphere in food packaging whereby
excluding the reactive element oxygen prolongs the storage life of perishable items.
Breaking the triple bond
Nitrogen atoms are one of the essential components of all the vital molecules
involved in living systems. For example the basic molecules of living cells - the
enzymes, DNA and RNA - all contain N atoms in their structures in combination
with atoms of other elements, predominantly H, C and O.
Both DNA and proteins contain N atoms within their structures where one of their
roles involves both intermolecular and intramolecular hydrogen bonding. The allimportant shapes of proteins are largely maintained by intramolecular hydrogen
bonds between the amino acids which constitute the protein chain. The opening
and rejoining of the links between the double strands of DNA are dependent on
hydrogen bonds breaking and reforming between N atoms and some O atoms on the
two strands.
However, extracting the N atoms from the molecular form of nitrogen, N2, requires
the input of a lot of energy, 944 kJ per mole of N 2 molecules split. Although
nitrogen molecules are abundant in the atmosphere, this large energy requirement to
convert them to a more accessible form is a limiting factor for all living systems.
The triple bonds in the elemental N2 molecule must be replaced preferably by single
bonds in nitrogen compounds. This feat is achieved in nature partly by lightning
strikes which produce vast amounts of energy and can split N2 and O2 molecules
and the atoms released may combine to form oxides of nitrogen. The main source
of converting atmospheric nitrogen to more accessible compounds is done by some
plants called legumes which host bacteria that can break the triple bond and
incorporate the N atoms into compounds that the plant can use. Subsequently
animals eat the plants and gain the desired nitrogen compounds. The process
whereby nitrogen molecules are converted to compounds in this way is called
nitrogen fixation. The food chain in which animals eat the plants to derive their
source of nitrogen and other animals eat those animals then continues the nitrogen
cycle. In due course, the death of the nitrogen fixing plants and all the other forms
of life return the fixed nitrogen to the soil or the ocean. Until a method of
artificially fixing nitrogen was devised in the early 20th century, this vital element
was mainly available by recycling using plant mulch and manure from animals for
example. One problem encountered by the First Fleet when establishing the
settlement in Sydney was lack of manure due to the shortage of animals available.
Due to the very strong triple bond in N2, nitrogen availability for living creatures
including humans was once very limiting for food production. As world population
increased and farming expanded in the 19th century, the traditional means of
recycling nitrogen in fields as mulch and manure became inadequate. Significant
deposits of nitrates existed in Chile in particular and also use was made of guano
(bird faeces) which have accumulated for centuries in some South American
locations. This material is rich in nitrogen compounds and was ideal to use as a
fertilizer. So valuable was this source that wars were fought over it in the latter part
of the 19th century in South America where large deposits occurred.
XII - 32
The ionic compound potassium nitrate occurs in deposits where it has crystallised
from water as the water evaporated, usually in very dry environments such as the
lakes in the high regions of inland South America.
However the natural and recycled sources of fixed nitrogen were insufficient to feed
an increasing world population and it is only as a result of the production of
synthetic ammonia that it is possible to feed 7 billion people today. The driving
force to develop the process used known as the Haber Bosch synthesis of ammonia
was the cornering of the guano supplies from Chile by Britain. The German
chemist Fritz Haber succeeded in combining hydrogen and nitrogen gases over an
iron catalyst at high temperatures and pressures in the lab in 1905 and the process
was developed commercially in conjunction with Carl Bosch. The Haber Bosch
synthesis requires vast amounts of energy due to the high temperatures and
pressures needed and about 2 % of the total energy used world wide is consumed by
this process. About half of all the nitrogen now fixed on the planet is produced by
this process and about 40% of the nitrogen atoms in each of us was fixed by the
Haber synthesis. Without the Haber Bosch synthesis of ammonia, mass starvation
would occur - it is estimated that the world’s population would be about 3.5 billion
today instead of 7 billion. Following the commercialisation of the process, nitrate
production in Chile fell from 2.5 million tons annually to 800,000 tons within a
decade. Today annual production of ammonia by the process is more than 100
million tons.
Haber was awarded the Nobel Prize for chemistry in 1918 for his discovery.
This was a controversial decision by the Nobel Prize committee as Haber had also
developed the war gases used in the First World War and others that would be used
in concentration camps during the second world war. Despite his services to
Germany, Haber who was Jewish had to flee and died of a heart attack on his way
to Israel in 1934.
Biofixation of nitrogen.
The mass use of ammonia fertilizers has had severe detrimental environmental
consequences. Fertilizers are sprayed on fields in far greater quantities than
actually used by crops and much runs off to contaminate waterways and to change
acidity of soils. It is estimated that only about 20% of the fertilizer deployed is
actually used with the bulk being lost by various means. To avoid these undesirable
consequences, much research is in progress trying to harness the methods that some
plants already use to derive their nitrogen requirements. These methods utilise an
enzyme called nitrogenase which is able to split the N/N triple bond to make
compounds that can be taken up by the plant which typically hosts the bacteria in
root nodules. These nodules protect the bacteria from oxygen in which nitrogenase
is ineffective. Legumes have this mechanism but not the cereal crops such as wheat
and the research aims to equip cereal plants with the ability to host nitrogen fixing
bacteria.
Explosives.
The majority of the ammonia synthesised is not used for food production but as the
basis for making explosives in which nitrogen compounds find considerable
application, replacing earlier products such as dynamite. One such is the compound
potassium nitrate which, mixed with charcoal and sulfur, produces gunpowder
which is believed to have originated in China in the 9th century AD. Potassium
nitrate is not an explosive on its own but another compound of nitrogen, ammonium
nitrate, NH4NO3, has widespread use in mining as a cheap and easily detonated
explosive. If heated or mixed with fuel oil, it explodes violently to form nitrous
oxide and oxygen. The hazzards associated with this compound were highlighted
XII - 33
in the great explosion that occurred in Texas Port in 1947 when a shipload of
ammonium nitrate fertilizer caught fire and detonated, causing massive destruction
and the death of at least 581 people, the injury of 5000 more and the destruction of
the sea port. Ammonium nitrate is made from ammonia and nitric acid which is
itself made via the Haber Bosch process and is an important industrial oxidizing
acid.
PHOSPHORUS.
Phosphorus compounds like those of nitrogen play a vital part in all living systems.
Bones and teeth contain phosphate compounds, but far more important is the role
of phosphate groups in DNA molecules and in the energy supply systems for cells
based on the compound adenosine triphosphate, abbreviated as ATP.
Phosphate groups in DNA.
The structure of the backbone of each strand of DNA consists of a series of sugar
groups each of which is bonded to one of four bases, the so-called “letters” of the
genetic code designated as A, C, G and T. Each of these sugar/base groups is
joined to the next one in the DNA strand by a phosphate group. This sequence
extends along the entire strand. In the cell, two complementary strands of DNA
form a double helix in which the two strands are joined to each other through
hydrogen bonding between N atoms on the bases. This is illustrated by the
following block diagram.
The strands open by
breaking the hydrogen
bonds to allow the
genetic code to be
transcribed to a molecule,
RNA, which is similar to
DNA but which uses a
different sugar. RNA is
single stranded and also
uses phosphate groups to
join the component
sugar/base groups.
Phosphate groups in the energy supply system, ATP.
All cellular processes need energy to drive them. A molecule containing a large
amount of accessible energy in its bonds is adenosine triphosphate.
XII - 34
The diagram above shows this molecule consists of a sugar molecule bonded to a
base (adenine) at one end and to a chain of three phosphate groups covalently
bonded to each other at the other end.
When energy is required , the ATP loses a phosphate group to become adenosine
diphosphate, ADP and releases 30.5 kJ of energy per mole of ATP reacting. A
further reaction can also take place in which a second phosphate group cleaves off
to form adenosine monophosphate and releases 61 kJ of energy per mole of ADP
reacting. The full sequence can be shown as
ATP 6 ADP + PO43! + 30.5 kJ
6 AMP + PO43! + 61 kJ
The sequence can be reversed to rebuild the ATP molecules from AMP and
phosphate groups using energy supplied from other processes.
Sources of phosphorus compounds.
Normally phosphorus compounds are recycled when living systems die and break
down but by eating plants and animals which consume them, the phosphorus
content of soils diminishes and severely limits plant growth. The lost phosphorus
content can be replaced as for nitrogen previously discussed, by using mulch or
manure but for high intensity farming which is essential now, additional phosphate
must be added to the soil. Sources of phosphate were again bird droppings and
when these were exhausted, phosphate rich rocks were mined and converted to
phosphate fertilizers. Initially suitable sources were to be found on some Pacific
islands such as Nauru but now the main source of economically viable phosphate is
from Morocco which controls 75% of the worlds known stock. Unlike nitrogen
which can be extracted from air, albeit with a huge energy requirement, there is no
alternative source of phosphorous compounds. There is concern about how
agriculture will deal with the ultimate depletion of the currently available stocks.
One approach now under way in England is to recycle the phosphates which are
excreted into sewage. A side benefit of this is to reduce phosphates entering
waterways where they stimulate algal growth which then leads to excessive
bacterial growth for the amount of oxygen in the water, and killing other marine
life. While recycling cannot totally replace the need for additional phosphate from
mining, it can help to reduce that need.
In summary, Group 15 contains two of the most vital elements to life as we know
it. Both constitute a limitation on how many people can be supported on earth. The
Haber process has overcome the problem of lack of suitable nitrogen compounds
but the only long term solution for the limited phosphorous availability seems to be
different farming practices and recycling.
GROUP 16. O, S, Se (non-metals) Te (intermediate) (Po - metal)
Group Overview.
All except the rare and radioactive element, polonium, are non-metals or metalloids.
They exist as 2– charged anions in compounds with metals and form covalently
bonded compounds with other non-metals. Oxygen is the only member to occur as
a gas, the others are solids at room conditions. Oxygen also occurs in small
amounts in the atmosphere as the allotrope called OZONE. Ozone is dangerous to
health when inhaled but plays a vital role in the upper atmosphere where it absorbs
much of the harmful ultraviolet light that would otherwise impinge on the earth’s
surface. Tellurium has network covalent bonding in the solid state while elemental
XII - 35
oxygen, sulfur and selenium are molecular covalently bonded. The common
valencies are 6 and 2, but oxygen has the valence of 2 only.
Oxygen is one of the most reactive non-metals, forming oxides with most elements.
The term OXIDATION originally referred to the reaction of substances with
oxygen. Apart from occurring as the diatomic element O2 (20.9 % by volume in
air), oxygen also occurs extensively as part of numerous compounds in the earth's
crust and in water. All aerobic organisms require elemental oxygen to survive as
part of the process of respiration. Compounds of oxygen may be ionic, containing
the oxide ion (O2–), or covalent (e.g. H2O, CO2). Oxygen also occurs as the
peroxide ion, O22–, in ionic compounds or covalently bonded to non-metals such as
in the antiseptic, hydrogen peroxide, H2O2, which has the structural formula
H!O!O!H.
Sulfur occurs free in large deposits as the element and in compounds containing, for
example, sulfide (S2– ) and sulfate (SO42–) ions. Covalently bonded sulfur atoms
are an important component of proteins, helping to maintain the required shape of
enzymes. The compound hydrogen sulfide (rotten egg gas), H2S, is a highly toxic
gas which is generated by anaerobic bacteria. It is produced naturally by such
bacteria in marshes and in sewage holding tanks where oxygen is excluded and can
sometimes be detected in the exhaust fumes of modern motor cars.
Selenium and tellurium are very similar to sulfur in their properties and their
hydrides, H2Se and H2Te, have an even more repulsive odour. Selenium is used
commonly as a rectifier for converting AC to DC. Foods containing selenium
compounds are in vogue at present because of the belief that they remove cancerforming free radicals from the body.
All Group 16 elements form covalent compounds with other non-metals (e.g.
H!O!H) and anions in ionic compounds with metals (e.g. Mg2+O2–).
FURTHER NOTES FOR LATER READING - oxygen, sulfur.
OXYGEN
Oxygen is the third most abundant element in the universe and in combination with
the most abundant element, hydrogen, it forms water which is the most abundant
compound. On earth, apart from atmospheric O2 gas, much more oxygen is present
in compounds including water and minerals, especially with silicon in silicon
dioxide. Two thirds of the mass of the human body is made up of oxygen atoms in
compounds, especially water.
When earth was formed about 4.5 billion years ago, its atmosphere contained no
elemental oxygen. Microbial life evolved in the absence of oxygen (anaerobic) but
after about 1 billion years, microbial species which contained organelles called
chloroplasts within their cells developed.
Chloroplasts use light energy
(photosynthesis) to split water molecules and release O2 gas in the process which is
carried out by plants and some types of algae. At that stage in earth’s development
there were huge quantities of iron present as dissolved salts and as the oxygen was
released, iron oxides formed and precipitated to form the vast beds which today are
the ore bodies being exploited in places such as Western Australia. Ultimately the
freely available iron was depleted so instead the oxygen gas was released into the
atmosphere where its present composition is 21%. This change had a drastic effect
on the earth’s future. Much of the preexisting microbial life was in effect poisoned
by the new pollutant and died out. Anaerobic life continued in places shielded from
the atmosphere such as swamps.
Oxygen is one of the most powerful oxidizing agents after fluorine. It reacts with
almost all metals to form oxides, many of which come away from the surface of the
metal which disintegrates. While oxygen gas is essential for aerobic life including
XII - 36
animals, within the body extremely reactive oxygen atoms rather than O2 molecules
damage cells. The body has defence mechanisms devoted to mopping up such
oxygen free radicals as they are called. Damage to cells by oxygen free radicals
may contribute to aging and be the cause of other deleterious effects. Anaerobic
organisms generally cannot survive in the presence of oxygen gas.
Oxygen in the atmosphere is inherently so reactive that constant replenishment
from plants and algae is needed to maintain its concentration. Due to the burning
of fossil fuels, the percentage of oxygen gas in the atmosphere is reducing.
Water.
In space, water can exist as a gas but mostly as ice in small grains or lumps and on
asteroids. Current theory holds that when the sun ignited about 4.5 billion years
ago, water along with much other old material swirling around the sun was
vaporised and condensed further out, possibly where Jupiter is now. Comets which
formed far out may have been the vehicle by which water came to be on earth is
such large amounts. There must also have been some special circumstances on
earth that allowed it to retain its water whereas other planets such as Mars which 3
billion years ago had oceans but now has none. Recently a comet lander has
analysed comet-borne water and found that it has a H:D ratio different from water
on earth so comet delivery is not confirmed as a source yet.
SULFUR
Sulfur is found as the free element near volcanoes and hot springs. It also occurs as
strata buried during previous geologic eras. The stable form of sulfur is the familiar
yellow coloured powder which contains 8-membered rings of S atoms covalently
bonded to each other. If heated, these rings of S atoms break apart and form long
chains of a treacle-like appearance. If this form of sulfur is cooled, it solidifies into
a less stable allotrope called plastic sulfur.
Sulfur burns with a blue flame forming an acidic oxide, SO2, sulfur dioxide, which
is soluble in water and forms sulfurous acid, H2SO3. In the mid-20th century,
burning of large amounts of sulfur-containing coal and petroleum lead to sulfurcontaining acids contaminating forests and waterways, falling as acid rain and
killing plant and animal life. In the 1970's the countries of North America instituted
the first and very successful emissions trading scheme in which polluters paid for
the amount of SO2 they emitted. Like the schemes currently used in some parts of
the world to abate CO2, any unused permits could be sold to other polluters who
could not meet the mandated targets. Reduced levels of acid rain soon followed as
it became uneconomic to burn “sour” coal and oil so that coal mines producing high
sulfur content coal closed and oil refineries installed equipment to remove sulfur
before it was incorporated in the end products. Now so much elemental sulfur is
extracted from oil that there is a world glut of it unused despite its being the starting
material for the manufacture of sulfuric acid, a vital and widely used industrial
material.
Elemental sulfur and slow release compounds of sulfur are now added to fertilizer
as sulfur is an essential nutrient for plant growth. Ironically this addition of sulfur
to fertilizer has become necessary as a result of the reduction of sulfur compounds
being emitted industrially.
Compounds.
Cysteine.
Sulfur containing compounds are essential components of living organisms, mainly
as atoms incorporated into amino acids that are the building blocks of proteins. The
XII - 37
enzymes which catalyse all metabolic reactions are proteins and they function
because of the fixed 3-dimensional shape which the long chain of amino acids
maintains (see Topic 13). To keep their required shapes, amino acids at different
locations in the chain intramolecularly hydrogen bond via N–H bonds but also S–S
bonds form between S the atoms of the amino acid which contains them, cysteine.
The combination of the hydrogen bonding and the S/S linkages provide the rigidity
needed for the enzyme to operate as a catalyst. The human body contains about
0.25% by mass sulfur in compounds.
The S–S bonds can be broken and reformed relatively easily. Hair contains strands
of protein with S/S linkages between them which provides hair’s strength. These
cross linking bonds are broken and reformed when hairdressers straighten or
permanent wave hair. The unpleasant smell from burning hair is due to sulfur
compounds being formed.
The formation of S/S linkages is the basis for producing rubber from latex which is
a liquid but which contains polymers with S atoms by mixing elemental sulfur with
latex and heating it to convert it into the stretchable material, a process called
vulcanising. The first vulcanised rubbers were developed by Dunlop in the 19th
century enabling the production pneumatic tyres.
Hydrogen sulfide.
Hydrogen sulfide is a highly toxic gas at room conditions. One might expect it to
be a liquid given that H2O from the period above boils at 100 o C but there is no
hydrogen bonding associated with S atoms due to their larger size compared with O
atoms, (See Supplementary Topic 4). It is well known as rotten egg gas because
some is formed from the sulfur-containing amino acids in eggs.
Anaerobic
conditions in swamps for example, make use of the redox reaction in which the S
atoms in compounds undergo the oxidation state change which in aerobic
conditions would be fulfilled by O atoms. Unpleasant odours can be detected
when microbial life is using this method of redox. Deep oceans sometimes contain
vents which release mineral-rich steam from beneath the surface. Surprisingly
these vents teem with living creatures despite their being too deep to receive any
sunlight which is normally the driver for life. Instead the heat from the vent and the
oxidation of hydrogen sulfide are the basis for the deep water life forms. It is
thought that the earliest forms of life may have started around such vents.
Sulfuric acid.
Sulfuric acid is the most widely used commercial acid due to its low cost. Large
amounts are used in many industrial processes including the production of
detergents. Detergents contain molecules which consist of a hydrocarbon group at
one end which is attracted to dirt and grease by dispersion forces (see
Supplementary Topic 4) and an ionically charged group at the other end which is
attracted to polar solvent such as water. This structure allows molecules of
detergent to surround dirt particles and carry them into water. The most commonly
used ionic group used is derived from sulfuric acid.
Sulfuric acid is used to extract phosphate from calcium phosphate which is mined
and the other end product is calcium sulfate, also known as gypsum, which is used
to make plasterboard. Sulfuric acid is also used to extract metals from their ores by
leaching and then reducing the leachate to form the free metal - copper and nickel
in particular are extracted this way.
Sulfuric acid is made by converting sulfur dioxide over a catalyst to sulfur trioxide
which is dissolved in water.
XII - 38
Sulfur dioxide.
Sulfur dioxide derived from burning sulfur in air or roasting sulfides of iron is the
starting material for the production of sulfuric acid.
Sulfur dioxide was used to sterilize wooden wine barrels between vintages - sulfur
was burnt in the barrel as the SO2 gas kills bacteria. Today sulfites are still used as
preservatives in wine as one can read on he labels.
Dimethyl sulfide.
Most sulfur compounds have foul smells - one of them, dimethyl sulfide, is added
to natural gas which is odourless so that gas leaks are detected.
Dimethyl sulfide is released from the decomposition of marine organisms including
seaweeds. It is then oxidized in the air to other compounds including sulfuric acid
as an aerosol which can act as nuclei for the formation of drops of water which
may become clouds. As vast amounts of dimethyl sulfide are released over the
oceans, it is thought that this process may be significant in controlling the earth’s
climate.
GROUP 17. F, Cl, Br, I, (At) also known as the halogens.
Group Overview.
Fluorine and chlorine are diatomic gases at room conditions, bromine is a liquid and
iodine is a low melting solid. All exhibit molecular covalent bonding. The
halogens are very reactive non-metals which can combine with metals to form ionic
compounds containing the halide ion (F–, Cl–, Br– or I–). Species which can enter
into such reactions are called OXIDIZING AGENTS, and all the halogens are
therefore good OXIDANTS. Because of its oxidizing power, chlorine gas is
introduced into the drinking water supply to oxidize organic contaminants such as
bacteria and viruses which could be harmful, as well as to remove algae. Chlorine
solutions can also be obtained for household use from the compound sodium
hypochlorite (NaOCl) which is sold as bleach and also as an agent for maintaining
clean swimming pool water. Iodine dissolved in alcohol (tincture of iodine) is used
as a disinfectant for cuts of the skin, and is used to sterilise dairy equipment. Lack
of iodide in food is one of the most common deficiency disorders in Australia
where once it was mandatory to add iodide to table salt but this practice has ceased.
Some compounds of Group 17 elements are covalent (e.g. HCl) while others are
ionic (e.g. Na+Cl–). They occur as anions in the latter case. The only ionic valence
shown is 1 but, apart from fluorine, they have numerous other valencies when
covalently bonded e.g. as the polyatomic anions in the salts NaClO, NaClO2,
NaClO3, NaClO4.
Important compounds of halogens include hydrogen chloride (HCl) which is a gas,
but when dissolved in water it breaks down (ionizes) to form a solution of H+ and
Cl– called hydrochloric acid. The halide ion Cl– is an essential component of the
nervous system and the fluoride ion, F– plays an important role in developing strong
enamel on teeth. As natural levels of F– in drinking water are often too low, the salt
sodium fluoride, NaF, is commonly added to drinking water to prevent tooth decay
as a public health measure.
FURTHER NOTES FOR LATER READING - fluorine, chlorine, bromine.
FLUORINE
Fluorine is the most reactive element because of its small size and proximity to the
noble gas neon so consequently it has the largest effective nuclear charge.
XII - 39
Conversely, having reacted with other atoms, the resulting compounds of fluorine
are usually extremely stable given the large bond strength of covalent bonds to F
atoms.
Fluorine is so reactive that it is difficult to store and handle and extremely
hazardous if contact is made with living organisms. It is an even stronger oxidant
the chlorine and oxygen and it reacts with all the other elements except a few of the
noble gases.
Fluorine is mined as the mineral fluorite which is essentially calcium fluoride. The
mineral can be crushed and used directly as flux in steel making. Fluxes lower the
temperature at which a metal melts. Elemental fluorine is extracted from fluorite by
heating it with sulfuric acid.
Fluorine is unusual in that it only exists in a single isotopic form, a property made
use of in the separation of isotopes of uranium - see uranium hexafluoride below.
Hydrogen fluoride.
Hydrogen fluoride dissolves in water to form hydrofluoric acid which is analogous
to hydrochloric acid but unlike the other hydrogen halides, hydrogen fluoride is a
weak acid.
Even so, if contact results in it penetrating beneath the skin,
hydrofluoric acid reacts with any calcium ions which control vital functions of the
body and converts them to solid calcium fluoride thereby disrupting those
functions.
Hydrogen fluoride is produced from fluorite by mixing it with sulfuric acid
Hydrofluoric acid must be stored in plastic containers and cannot be stored in glass
which it dissolves. Use is made of its ability to attack glass when glass etching is
required for example, the security coding on car windows. Hydrofluoric acid is
essential in many industrial processes, for example in the smelting of ores of
aluminium by electrolysis and production of refrigerants.
Sulfur hexafluoride.
While fluorine is extremely reactive, it follows that its compounds are extremely
stable. One of many extremely stable covalent fluorine compounds is sulfur
hexafluoride, SF6, which has an application as a spark retardant in high voltage
electricity transformer stations. The extreme stability of SF6 which is a gas
prevents any spark from ionizing the air and thus being propagated and damaging
the electrical equipment. However sulfur hexafluoride is an extremely potent
greenhouse gas with a lifetime of 3000 years so its use must be carefully monitored.
Fluorocarbon refrigerants.
Compounds of carbon with fluorine have been used as refrigerants for decades but
it has been discovered that when released they destroy ozone molecules in the
upper atmosphere, opening the way for UV light to penetrate to lower levels with
serious health and environmental implications. Refrigerants are compounds that are
gases at room conditions and which can be liquefied by compressing and cooling
externally. The liquid refrigerant is then passed through a small hole in a sealed
unit and it converts back to the gaseous state with the absorption of heat. This is
the basis for the operation of all refrigerators. A desirable refrigerant needs to
undergo these phase conversions readily and to be non-toxic in case of leakage.
One of the early refrigerants was ammonia but it fails the toxicity test.
Hydrocarbons in which the H atoms have been replaced by F and Cl atoms are ideal
refrigerants but since the discovery of the damage they were causing to the ozone
layer, the original versions have been banned worldwide. The Montreal protocol
devised in 1987 brought an international agreement to phase out their production.
XII - 40
The replacements are chlorofluorohydrocarbon compounds which are similar but
retain one or two H atoms on each of the carbon atoms. These compounds usually
break down too rapidly to reach the upper atmosphere but if they do, they are found
to be extremely potent greenhouse gases with very long lifetimes. It is estimated
that 15% of the total greenhouse gases released by humans is made up of such
compounds.
Polymers of fluorocarbons.
One of the most versatile polymers is teflon, a polymer of tetrafluoroethylene which
is an ethylene molecule in which all four H atoms have been replaced by F atoms.
In the polymer both C atoms of each ethylene molecule join to other
tetrafluoroethylene molecules. The double bond is broken in this process and the
electrons used to form the two new bonds. Thus the structure of the polymer is an
unbroken chain of C atoms, each of which is bonded to two F atoms and its formula
could be represented as !(CF2)n! where n is a very large number. The chemical
name for the polymer is polytetrafluoroethylene or PTFE. Teflon is a very versatile
polymer. It is extremely resistant to chemical attack and can be machined to any
shape. Thin teflon tape is used to seal joints in plumbing. It has self-lubricating
properties which find application as a material for bearings and in a powdered form,
as an enhancement with other lubricants. One of its best known applications is the
surface of non-stick saucepans.
Fluoride in teeth.
Unlike the element, having achieved the noble gas configuration of neon, fluoride
ions are very stable. The compound calcium fluoride, CaF2, is insoluble. The
enamel on the surface of teeth can be given a protective coating of glass-like
calcium fluoride by bringing F!ions into contact with teeth which consist of calcium
compounds. The fluoride content of most natural water supplies is insufficient to
achieve this so most public water utilities add additional fluoride ion as sodium
fluoride. Dentists also typically finish an examination by coating the teeth with a
fluoride-rich gel.
Uranium hexafluoride.
Uranium hexafluoride, (UF6), is a stable gas above 57o C. It is vital in the
separation processes used to separate the uranium isotope U235 which undergoes
fission, from the non-fissile U238 which occur as a mixture in uranium ores. By
converting all the uranium to UF6 and either using gas diffusion or gas
centrifugation , the slightly lighter isotope can be separated based on the mass
difference. In gas diffusion, the U235F6 will move faster through a membrane than
U 238F6 and by setting up many gas diffusion chambers in series, gradually the
diffused gas becomes richer in the desired compound. Gas centrifugation uses
centrifuges to spin the gas samples at high speed and the heavier 238UF6 is
concentrated lower in the container and mixture richer in the lighter compound can
be drawn off from the top. Again, a large number of the centrifuges must be used
in sequence to achieve the desired level of enrichment. Both processes can only
work because the F atom exists as just one isotope so the mass difference between
the 235UF6 and 238UF6 is solely attributable to the difference in the masses of the two
U isotopes.
CHLORINE
Chlorine was known but not recognised as an element prior to it being named by
Humphrey Davy in 1810 - the name is derived from the Greek meaning “green” on
XII - 41
account of the pale yellow green colour of gaseous chlorine. It is produced by
electrolysis of concentrated sodium chloride solution in a cell in which the anode
and cathode are separated by a membrane which allows electrical neutrality of the
electrolyte to be maintained but prevents the gaseous products formed at the
electrodes from mixing. At the positive electrode, chlorine gas forms and at the
negative electrode hydrogen gas and hydroxide ions form, both from the water.
Metallic sodium cannot form by electrolysis of an aqueous solution, only by
electrolysis of molten sodium chloride.
Chlorine like all the halogens is a diatomic element and it is a powerful oxidising
agent. Because the element is so reactive, the compounds it forms are consequently
generally stable. This is especially useful with regard to organic chloro compounds
in which Cl atoms replace H atoms. Chlorine atoms are incorporated into hundreds
of compounds used for example in the building industry and in the manufacture of
pharmaceuticals. Apart from such compounds where Cl atoms are part of the final
product, chlorine is used in many processes to form intermediate compounds
without being part of the final product. Two common examples are the polymers
nylon and polyurethane, neither of which contain Cl atoms but synthesis of both
requires chlorine. The following discusses a few of the many compounds which
incorporate chlorine atoms.
Sodium chloride.
Sodium chloride is easily obtained from underground deposits or by evaporation of
salt water. It is the basic feedstock for both elemental sodium and chlorine and the
manufacture of their compounds.
Because chlorine is very reactive, the Cl! ion formed when the Cl atom has gained
an electron is very stable and along with sodium ions is an essential component for
maintaining the correct water balance in the blood stream.
[See sodium chloride under Group 1 elements.]
Sodium hypochlorite.
This compound, NaClO, is sold as common household bleach and is also used in
water treatment, especially swimming pools. It is made by reacting chlorine gas
with sodium hydroxide solution, the chlorine being released in solution as it is used.
The Cl2 molecules kill microbial life because of their strong oxidizing properties.
Chlorine gas is a very toxic substance if inhaled and it has been used as a war gas it damages the linings of the lungs and causes a fatal build up of fluid.
Consequently for most disinfection purposes it is much safer to use a solution of
sodium hypochlorite rather than elemental chlorine gas.
Polyvinylchloride.
An organic compound of chlorine, vinyl chloride is very stable and forms the
polymer polyvinyl chloride, commonly known at PVC.
XII - 42
Because of its extreme stability, PVC is used in many building products such as
drainage pipes, guttering and window frames. Production of PVC is the largest
single use for chlorine.
Hydrogen chloride.
The hydride of chlorine, hydrogen chloride, HCl, reacts vigorously with water to
form the strong acid hydrochloric acid in solution. Hydrochloric acid is one of the
most widely used in industrial applications.
Stomach acid.
Hydrochloric acid is secreted in the stomach as part of the digestion of food. It
plays a role in enabling the enzymes to break down amino acids and when food is
being digested, the pH of the stomach juices falls to the range 1 - 2. After a meal
has been digested, the pH increases to its normal value of 4 - 5.
DDT (dichlorodiphenyltrichloroethane).
DDT is a potent insecticide widely used in agriculture until the 1970's by which
time its propensity to accumulate in fat cells in living creatures lead to its disuse in
most situations. However in some locations, DDT is still used as a means of
controlling mosquitoes that carry malaria parasites.
Apart from DDT, a number of other organochloro compounds have been used as
very effective insecticides but again, due to their long lifetimes and unwanted
indiscriminate effects, most have been banned from sale.
BROMINE.
Bromine, symbol Br, is a red-brown liquid at room conditions, boiling at 59o C.
Like the other halogens, it is a corrosive oxidizing agent and its reactivity lies
between that of chlorine and iodine. The halogen Group illustrates how the strength
of intermolecular attractions through dispersion forces increases with the number of
electrons (see Supplementary Topic 4) as reflected in the boiling points. The main
forces acting between the molecules in these non-polar diatomic elements are
dispersion forces. Thus at room conditions fluorine and chlorine are both gases,
bromine is a liquid and iodine is a solid.
XII - 43
Like chlorine and iodine, bromine is used as a disinfecting agent.
Many of the uses made of bromine in the past have been discontinued recently on
health and environmental grounds - for example some fire extinguishers once
contained organic bromo compounds but their ozone depleting properties have
caused this use to cease.
Organic bromo compounds.
The main application for bromine is as a fire retardant. Organic molecules
containing Br atoms replacing some H atoms, when heated, release Br atoms which
have the property of extinguishing a fire. Organic bromo compounds are
incorporated into plastics which normally will burn readily if heated, for example in
TV sets and laptop computers if a fault develops and overheating occurs. Also
bromo organic compounds can be soaked into materials to reduce flammability. In
these roles, bromine compounds have undoubtedly saved many lives and much
damage.
Drilling fluids.
When drilling oil wells and similar, the pressure outside the well must be
maintained to match that of fluids or gas inside the well. This is achieved using
concentrated solutions of bromide salts which are very dense because of the large
size and mass of Br! ions.
Mercury scavenger.
Bromide salts such as calcium bromide are sprayed onto coal just prior to its being
burnt in coal fired power stations. These salts remove the mercury which is always
present in coal and normally is released through the flu gases unless captured.
Photography.
Classical black and white photography using film is based on the ability of light to
reduce the silver ions in silver salts to silver metal. This is why silver salts and
their solutions are kept stored in dark containers to reduce light exposure. The most
favoured silver compound for photography is silver bromide, a cream, insoluble
salt. When light is focussed by the camera lens onto film which contains silver
bromide in a solid emulsion attached to a transparent base, the Br! ions are
converted to Br atoms and release an electron which the Ag+ ion accepts and
converts to an Ag atom. The more light striking a given spot on the film, the more
Ag atoms are produced. Developing processes amplify the effect and fixing
removes any unreacted silver bromide from the film. The image is reversed so the
areas that would be white appear as black on the negative produced while areas that
received no light from the lens appear transparent. To obtain a positive print, the
process is used again, shining light through the negative to expose silver bromide
layered onto paper and developing the print in the same way as was done for the
negative. The Br atoms released in these processes are removed in a layer
containing a bromine acceptor on the top of the emulsion . The exposed silver
appears as black rather than shiny because the particle size of the individual grains
of silver is very small.
GROUP 18. He, Ne, Ar, Kr, Xe, Rn - the noble gases or inert gases.
Group Overview.
All occur as monatomic gases at room conditions - the only monatomic elements.
They have virtually no reactions due to the very stable atomic structure which has
XII - 44
the maximum effective nuclear charge for that period and which is therefore
associated with very large energy requirements for the gain or loss of electrons
required for reactions to occur.
Helium is found trapped in oil wells along with gas or crude oil. It occurs there
because helium is a product of the radioactive decay of large, unstable nuclei such
as uranium. Helium is an extremely useful substance because it has the lowest
boiling point of any element. It constitutes a significant part of the mass of the sun
where the nuclear fusion reaction of hydrogen to form helium provides most of the
energy released.
The gases neon, argon and krypton are all present in the atmosphere in small
amounts - argon to the extent of 0.9 %. Neon and argon are used in situations
where an inert atmosphere is required, such as in neon lighting tubes and in argon
arc welding. The element radon is a powerful carcinogen which is ubiquitous in all
minerals where it too arises from radioactive decay processes. Radon presents a
particular hazard for miners involved in uranium mining, but is also released from
the burning of coal and even from the clay of house bricks.
FURTHER NOTES FOR LATER READING - helium.
HELIUM.
Helium is named after Helios, the sun god because it was first discovered by
spectroscopic analysis of sunlight in which previously unknown spectral lines were
observed. Later it was recognised as a gas released from some radioactive minerals
where it originates from alfa particles expelled when unstable nuclei decompose.
Alfa particles contain two protons and two neutrons but no electrons so is in effect a
helium nucleus. The required electrons are picked up rapidly in air to form He
atoms, atomic number = 2 and mass number = 4. This nuclear arrangement of 2 p +
2 n has a particular stability which accounts for why so much helium was formed in
the short interval following the hypothesised big bang.
Helium is rare on earth because it is so light that it escapes gravity and is only
found trapped in the impermeable formations along with natural gas and oil from
which it is commercially separated.
Helium is used to dilute pure oxygen breathed by deep sea divers as it is non-toxic.
When inhaled, it causes one’s voice to take on a squeaky Donald Duck sound due
to the much greater speed of sound in the lighter atmosphere as the timbre of the
voice is very dependent on the speed of the sound waves. Its most obvious
although trivial presence is as the gas used to fill party balloons and blimps but it
has a common industrial use as a gas to protect metals from reacting with oxygen or
nitrogen while being welded.
Because of its inertness, helium is used to provide an inert atmosphere when
growing and handling crystals of semiconductors such as silicon.
Cryogenic applications.
Helium has the lowest boiling point of any substance because of its small atomic
size and extremely weak intermolecular interactions. It liquefies at !269o C, a
property essential in superconducting magnets such as those used for magnetic
resonance imaging (MRI). At such low temperatures, electricity travels through
copper wires with no resistance allowing the generation of powerful magnetic fields
that then can interact with hydrogen atoms from which images of bodily structures
and organs can be obtained unobtrusively and without tissue damage. Likewise,
many instruments used for scientific research rely on the low temperatures
achievable with liquid helium.
XII - 45
Although helium is one of the two most common elements in the universe, it is rare
on earth and there is concern that supplies of helium are not being husbanded
carefully enough.
The "d-block" elements.
Block Overview.
This is a block of elements located between Groups 2 and 13 following Ca, Sr and
Ba. In the current IUPAC system of Group numbering, they are numbered as
Groups 3 to 12. They first appear after Ca in the fourth period of the Table, and
there are ten d-block elements in each period where they occur. Note that there are
no d-block elements in Periods 1, 2 or 3. They are all metals, most being hard with
high melting points, although mercury is a liquid at room temperature.
Well known elements from this block include chromium, iron, manganese, nickel,
cobalt, copper, zinc, cadmium, tungsten, platinum, silver, gold and mercury. Some
of the special characteristics of these elements include the following:
(i) They all form cations in salts (e.g. MnCl2) but also can form covalent bonds to
non-metals such as oxygen (e.g. Mn in the polyatomic ion MnO4–).
(ii) They often have more than one ionic valence state (eg Fe occurs as the ions Fe2+
and Fe3+ in its compounds) as well as a number of valence states in covalent
compounds.
(iii) Their compounds are frequently coloured (eg copper(II) sulfate crystals - blue)
whereas compounds of other metals are usually white.
FURTHER NOTES FOR LATER READING - vanadium, chromium,
manganese, nickel, technetium, tungsten, gold, mercury.
SCANDIUM (Z = 21) and YTTRIUM (Z = 39)
See “rare earths” in f-block
VANADIUM (Z = 23).
Vanadium is a silver-grey metal located in the first row of the d-block, between
titanium and chromium. Consequently it has many similarities to these two
elements. Vanadium added in very small amounts to steel forms alloys with vastly
improved strength and heat resistance, the latter being an essential for high speed
drills.
Oxidation states and colours.
Vanadium has four common oxidation states, +II, +III, +IV and +V. Vanadium
ions in solution in each of these oxidation states have their own characteristic
colours as below:
+II
lilac
+III green
+IV blue
+V yellow
The range of colours of vanadium compounds lead to its being named after the
goddess of beauty, Vanadis. Coloured compounds are characteristic of d-block
elements and result from the large number of electrons and transitions between
orbitals that are available in their outer levels. This is in contrast to s and p-block
elements which rarely form coloured compounds.
XII - 46
Vanadium flow battery.
Transitions between the various oxidation states of the V atom is the basis for the
vanadium flow battery. In the usual rechargeable or non-rechargeable batteries one
or both of the electrode materials undergoes a redox reaction. These redox
reactions involving the electrodes proceed in one direction when discharging and in
the reverse direction when charging. In the vanadium flow battery, the electrodes
are inert and merely act as part of the external electron transport. The redox
reactions take place entirely through vanadium ions in the electrolyte in each half
cell changing their oxidation states. Thus it is oxidation and reduction of the
electrolyte rather than the electrodes that provides the electron flow.
The following diagram illustrates a vanadium battery.
When discharging, in the negative half cell vanadium ions undergo the change from
the +IV (VO2+) to the +V (VO2+) states with the release of electrons through the
external circuit where they move to the positive electrode. There another vanadium
solution undergoes the change from the +III state to the +II state. The electrodes
made of an inert material play no part in the operation of the cell which is
maintained by pumping electrolytes of each type through the relevant half cell. For
this reason, it is known as a flow battery. To reverse the reaction and recharge the
cell, the spent electrolyte solutions are pumped back through their half cells while
the voltage needed to reverse the reaction is applied to the electrodes. The charge
balance within the cell is achieved by H + ions migrating through a suitable
membrane that keeps the vanadium ions from intermixing.
This type of cell has many advantages - it can be reused an unlimited amount of
times and can hold/supply large amounts of electricity, limited only by the size of
the electrolyte tanks. The main drawbacks are related to the bulk which limits their
use to fixed applications such as back up for the grid rather than for transport.
Sulfuric acid manufacture.
One of the main uses for vanadium is for the oxide, vanadium pentoxide, which is
used as a catalyst for the conversion of SO2 to SO3 in the contact process by which
sulfuric acid is produced. In this process, sulfur is burnt or sulfide compounds are
heated to release sulfur dioxide gas which needs to be further oxidized to sulfur
trioxide before reacting with water to form sulfuric acid. Without the catalyst the
process is too slow to be commercially viable.
XII - 47
CHROMIUM (Z = 24).
Chromium is a shiny metallic element found in ores often in company with iron.
The name comes from the Greek, “chroma”, meaning “colour”. This relates to the
many chromium compounds which are brightly coloured and are used as pigments
in paints and elsewhere. More recently these uses have diminished due to health
concerns about ingested chromium compounds
When chromium is mixed with iron, stainless steel results. First developed in
1913, stainless steel largely resists rusting unlike normal steel because the
chromium atoms form an extremely hard oxide on the surface and this prevents the
attack on the Fe atoms by water and oxygen. Up to 30% of chromium is mixed
with iron to make stainless steel and the higher the chromium content, the more
resistant it is to corrosion. Unlike iron, chromium is not magnetic and high quality
stainless steel can be detected by its inability to be attracted to a magnet. Alloys of
chromium and iron, sometimes with other metals, are used for high temperature
environments such as the valves in car engines.
The property of extreme hardness coupled with the ability to resist corrosion is the
reason that chromium plating is much used for both protection and appearance.
When a thin layer of chromium atoms is plated onto normal steel and polished, a
bright shiny surface results giving the familiar chrome plating used to decorate and
protect many types of metal objects and notably, on motor cars from previous eras.
Chromium also is used in the tanning of leather. Chromium ions seep into the hide
and cross link the collagen polymers of the skin, making it more pliable and water
resistant and able to absorb coloured dyes.
Rubies and emeralds are crystals of aluminium oxide and they owe their colours to
Cr atoms occupying places within the crystals. This is possible because Al and Cr
atoms are of similar size so a chromium ion can easily replace an aluminium ion in
a crystal lattice. Ruby crystals were used in the first lasers where the properties of
the imbedded Cr ions allow the production of the coherent light which lasers
produce.
However chromium in one of its many oxidation states is very toxic and can cause a
range of medical problems including potentially cancer. Chromium in oxidation
state VI also known as hexavalent chromium as for example in dichromate ions,
Cr2O72–, was shown to be associated with widespread illnesses among the
population of the town of Hinkley in California by Erin Brockovitch. In a landmark
class action in 1996, a fine of $333M was awarded against a gas company which
had been dumping polluted water into the groundwater which was later drunk by
local well users. Her work which was also featured in the film bearing her name as
the title, has lead to stringent limits on the amount of hexavalent chromium allowed
to be released into the environment worldwide. One consequence has been a much
reduced use of chrome plating which, along with tanning, used to be responsible for
some of that release.
MANGANESE (Z = 25).
Manganese, symbol Mn, shows all the characteristic properties of d-block elements
especially the wide range of colours of its compounds and the range of valence
states (oxidation numbers). The element has a silvery grey appearance and occurs
as compounds in minerals, the dioxide MnO2 being the most common. Manganese
is a vital atom in the photosynthetic processes used by all plants to generate
oxygen. Small amounts are also essential for human health, playing a vital part in
development, metabolism and antioxidant processes.
XII - 48
Applications.
Most of the manganese mined is used in the production of steel and as a component
of alloys with iron and aluminium.
Manganese dioxide is used as one component in the standard zinc dry cell. The
brown staining liquid from leaking dry cells is this oxide material.
Oxidation states.
Manganese atoms can exhibit oxidation states from !III through to +VII. The most
common and examples of their occurrence are listed below.
+II
+IV
+VI
+VII
MnCl2
MnO2
K2MnO4
KMnO4
manganese(II) chloride
manganese(IV) oxide or manganese dioxide
potassium manganate
potassium permanganate
pink
brown
green
purple
The range of oxidation states available to Mn atoms in compounds is the basis for
its use as an oxidant, especially as the permanganate ion and as manganese dioxide.
The oxidizing power of these compounds increases as the number of O atoms
bonded to the Mn atom increases.
Coloured compounds.
The d-block elements are remarkable for having many coloured compounds as
distinct from the compounds of the s and p blocks. This property is a consequence
of the large number of outer electrons and available orbitals. The colours of
solutions of the above compounds is listed on their right.
NICKEL (Z = 28).
Nickel is another metal that resists corrosion because a protective oxide layer forms
on its surface. Nickel plating is similar to chromium plating in that it affords a
corrosive metal such a iron a shiny and protective surface. Nickel is used as one
component of the common rechargeable batteries known as NICAD BATTERIES
(for nickel/cadmium) batteries as well as other types.
While on its own, nickel has many applications, its most widespread use is as a
component of many alloys (mixtures of metals), especially stainless steel which is
an alloy of nickel, iron and chromium. There are over 3000 different alloys of
nickel mixed with other metals, each tailored for specific purposes. A particularly
valuable alloy of mostly nickel with copper is monel which is very strong and highly
resistant to corrosion. It has particular uses in marine environments where it is very
resistant to corrosion in salt water.
An alloy of nickel with iron, aluminium and cobalt known as ALNICO can be
magnetised to form stronger and more lasting permanent magnets which have only
recently been superceded by rare earth magnets.
Some alloys of nickel are known as super alloys because of their strength and
extreme tolerance of heat. The turbine blades of jet engines make use of these
alloys which contain as many as ten other elements apart from nickel and are able to
resist the extreme temperature and high speeds of rotation experienced. Modern jet
engines would not be possible without various nickel alloys used in their
construction. The compositions of each alloy is designed for a specific purpose in
the engine.
XII - 49
Catalysts.
Apart from its role in alloys, nickel in a finely powdered form is used as a catalyst
for many chemical reactions which would be too slow without catalytic
participation. One common application as a catalyst is in the hydrogenation of
vegetable fats (which are usually liquids at room temperature) to form solid
margerine as a substitute for butter.
Shape memory alloys.
Some alloys of various metals have a shape memory whereby if the metal is
distorted from its initial shape such as a straight wire to a twisted or bent shape and
then warmed to a specific temperature, the wire returns to its previous exact state.
One such alloy contains a mixture of about 55% nickel and the remainder is
titanium, so it is known as nitinol. This self-expanding property of nitinol lends it to
a particularly useful application as stents to expand blocked arteries in the heart.
The stent can be made to the desired state at body temperature and then rolled into a
suitable volume in order to be inserted through an artery in the groin and into the
desired location in the heart. Once warmed to the original body temperature at
which the stent was made, it opens up to the required shape in situ. This type of
stent is replacing the older balloon expandable type previously used.
TECHNETIUM (Z= 43).
The existence of element 43 was predicted by Mendeleev in his Periodic Table
(1871). He named it as ekamanganese using his system for undiscovered elements,
in this case because its position is one place below manganese in the Table.
However technetium is not to be found in the normal manner of the elements
because it has no stable isotopes, the longest lived being 98Tc of half life 4.2 × 106
years. Instead, it can only be obtained from decay products of fuel rods in nuclear
reactors which gives it the name technetium and symbol Tc. There are numerous
other isotopes of Tc each with its own characteristic half life. Due to its relatively
short half life compared to the age of the earth, no significant amounts of technetium
can be found naturally occurring but bulk quantities are extracted from spent fuel
rods of nuclear reactors.
Nuclear medicine applications.
One isotope of technetium, 99Tc, can exist is what is called a meta stable state where
the nucleus has a higher energy and it is designated as 99mTc. This isotope has a
very short half life of 6 hours and when it decays to the more stable form, 99Tc, it
emits a gamma ray. Gamma rays (ã rays) are high energy electromagnetic radiation
like visible light but of much higher energy. Gamma rays can pass through
significant thicknesses of materials without being absorbed and in the case of
tissues, without doing significant damage to cells. Use is made of this property to
scan organs such as the heart by injecting a solution of a salt of 99mTc into
appropriate blood vessels and detecting the ã-rays as they emerge with a ã ray
camera. The resulting pictures show areas where for example, plaque has built up in
arteries or damage has occurred to the heart muscle. Having such a short half life
means the radioactive isotope is quickly cleared from the body so it does little
ongoing damage. This procedure is so widely used that millions of scans of this
type are conducted world wide ever year.
The problem with using 99mTc for diagnostic radiation testing is its half life is so
short that it must be used quickly after preparation. To overcome this to a certain
extent, it is made in situ from a radioactive isotope of the element molybdenum,
99
Mo, which decays to form 99mTc. The molybdenum isotope is mostly made in
XII - 50
older style nuclear reactors and it has a half life of 2.75 days so it can supply the
required 99mTc for about a week before a new supply is required. There is concern
that there may be world wide shortages of supply of 99mTc because most of the
reactors that are used to produce the molybdenum isotope are at the end of their
operating lives. In Australia, the Opal reactor at ANSTO in Sydney produces the
molybdenum isotope for distribution throughout the country and to nearby Pacific
nations.
TUNGSTEN (Z = 74).
The symbol for tungsten, W, comes from the name wolfram by which it was known
in some European countries. Tungsten is one of the hardest elements and it has the
highest melting point of any metal, 3422o C. Its density is almost twice that of lead
and nearly twenty times greater than water as a result of the large number of protons
and neutrons in its atoms.
Once a method of drawing tungsten into wires was developed, it found a widespread
use as the filament in incandescent electric light globes, replacing tantalum which
had been used earlier. The globe contains argon to prevent any tungsten oxide
forming which darkens the glass. Because of the very high melting point of
tungsten, an electric current passed through it causes the filament to glow without
melting. However, in the process, 95% of the electrical energy is converted to heat
rather than light so this type of globe is now rapidly being displaced by more
efficient sources. It is still used in X-ray tubes in which electrons from a heated
tungsten filament hit a target, itself often made from tungsten, and X-radiation is
released.
Tungsten is mainly used in various alloys which benefit from the increased hardness
and heat resistance, for example in high speed steel which contains as much as 20%
of tungsten.
Tungsten carbide.
This compound of formula WC is made by heating a mixture of tungsten and carbon
to temperatures up to 2000o C. It too has a very high melting point, 2870o C, and is
extremely hard. These properties make tungsten carbide an ideal material for
industrial grinding and cutting. Drill bits and saw blades are often tipped with
tungsten carbide.
It is also used in armoury such as bullets and protective sheeting for military
vehicles such as tanks.
The ball tip in the end of ball point pens is often made from tungsten carbide.
GOLD (Z = 79).
Gold is one of the few elements that occur as the free element in nature and its
symbol, Au, is from the Latin, aurum. It is a soft, dense metal and along with
copper, one of the two coloured metals. Gold occurs as nuggets in surface layers of
the ground and also as inclusions in rocks such as quartze. It also occurs free in
large veins underground - the largest piece found was about 1.5 m long discovered
at Hill End. Its limited supply and demand for jewelry and as a currency hedge and
industrial usage have all contributed to the high price gold commands.
Gold is extremely unreactive and does not corrode in air or water. It is an excellent
conductor of electricity and is used in electronics where reliable contacts are
essential. All mobile phones contain tiny amounts of gold. Because of this and
other valuable metals present, mobile phones are crushed and several elements
reclaimed. Despite the small amount of gold present, crushed phones contain a
much greater concentration than in the original ore from which they were extracted.
XII - 51
Medical applications.
Because gold is so inert, it has uses in medicine. Gold nanoparticles have the ability
to bind to antibodies and drugs and therapeutic agents and deliver them to the
desired part of the body, evading possible defence mechanisms. In cancer therapy,
gold nanoparticles have been found to be taken up more by tumours than healthy
tissue so anti-cancer agents can be concentrated near to the site where they are
needed. In some applications, adsorbed gold nanoparticles at the tumour sites can be
irradiated with near infra red light which then is converted to heat of sufficient
intensity to destroy malignant tissues.
Gold nanoparticles are also finding applications in the new area of gene therapy.
MERCURY (Z = 80).
Mercury, named after the god of speed, has the symbol which is derived from the
Greek name, hydragyrum. It has been known for thousands of years as it is easily
obtained by heating the ore cinnabar which is mercury(II) sulfide and which occurs
in highly concentrated deposits. Mercury is the only metal that exists as a liquid at
room conditions. Its shiny appearance is well known as are some of its common
uses such as in thermometers and blood pressure monitors. Less well known is that
the largest use of mercury today is for the extraction of gold by small scale miners
who mix mercury with low concentrations of gold such as from panning. Mercury
can dissolve a number of metals including gold to form an amalgam and then be
removed by heating to leave the concentrated gold, a process aided by mercury’s
relatively low boiling point of 630 o C. This process was also used by large scale
miners in the past but now combination of gold with cyanide is used commercially.
In past eras, mercury and its compounds were used as medicines and as cosmetics.
Mercury is used in tiny amounts in all fluorescent light tubes and the compact
fluorescent light bulbs which have replaced tungsten filament globes.
The topical antiseptic, mercurochrome, is found in most home medicine kits.
Until a few decades ago, the most common material used for dental fillings was an
amalgam of mercury with silver, tin and copper. About 50% of the filling is
mercury. Small amounts of mercury metal do leach out of these fillings so people
sometimes opt to have them replaced using modern materials which adhere to the
tooth rather than being wedged in as was the case with amalgams.
Health concerns.
Mercury ingested or inhaled is a deadly nerve toxin and in recent years, most of its
former uses have been supplanted by safer alternatives. Currently an effort is being
made to ban many of its traditional applications and to ultimately discontinue
mining it. To reduce pollution, fluorescent light tubes are recycled to remove the
mercury content. Coal fired power stations release mercury into the atmosphere and
efforts are now made to remove this source of pollution. Even thermometers
containing mercury are being replaced by alternatives.
Organo compounds of mercury such as methyl mercury are the most dangerous.
This danger was made very evident in Japan in 1956 when residents of a town called
Minamata fell ill in large numbers. The cause was a mystery but food poisoning of
some type was suspected as even local cats were dying. Ultimately it was traced to
the release of methyl mercury from an industrial plant into the nearby bay. The
mercury compound was concentrated by small aquatic organisms which in turn were
eaten by larger creatures up the food chain to fish and shellfish which contained
dangerous levels. Many thousands of people died or were severely affected by
eating the contaminated seafood. Despite the cause being known, the dumping of
XII - 52
methyl mercury into the bay there continued until 1968. Mercury poisoning by
organomercury compounds has subsequently been named as Minamata disease.
The f-block elements.
Block Overview.
The f-block is located near the bottom of the Table and contains 14 elements in each
of Periods 6 and 7 using the 4f and 5f orbitals respectively. The 14 elements using
the 4f orbitals following lanthanum are known as the lanthanides and those using the
5f orbitals following actinium are known as the actinides although neither
lanthanum nor actinium are f-block elements. They are all metals and many of the
actinides have unstable nuclei. Particularly important actinides are the elements
uranium (U) and plutonium (Pu) because of their relevance to the nuclear fuel cycle.
The many outer level electrons in the lanthanides impart unique magnetic properties
to these elements and colours to their compounds . The lanthanides are known as
the RARE EARTHS, a grouping which is frequently expanded to include the
elements scandium and yttrium from Group 3. The rare earths mostly are not rare
but they generally do not occur in sufficiently large concentrations to be mined
profitably. Mining and purifying these elements is a very polluting operation which
is now almost exclusively done in China where environmental controls are weak.
However, some of these rare earth elements have taken on great importance in
modern technology and have become indispensable. For example, rare earth
elements are vital in the production of lasers and tiny magnets used in computer
drives as well as the larger powerful magnets used in electric motors and generators.
(a) The lanthanides.
Lanthanum (La) - not strictly an f-block element as it is in the d-block in Group 3;
cerium (Ce); promethium (Pm) - not mined but synthesised in nuclear reactors;
praseodymium (Pr); neodymium (Nd); samarium (Sm); europium (Eu); gadolinium
(Gd); terbium (Tb); dysprosium (Dy); holmium (Ho); erbium (Er); thulium (Tm);
ytterbium (Yt); lutetium (Lu)
Among the many useful properties of these elements and their compounds are the
following:
Coloured compounds.
All of the elements in this row of the f-block have coloured compounds and when
their ions are excited they emit light of very specific wavelengths in their atomic
spectra. Erbium is particularly useful as an amplifier in the fibre optic cables used
for internet transmission. Transmission of light along fibre optic cables requires the
signal to be amplified at regular intervals and erbium compounds are the ideal
materials to do this. Erbium ions are incorporated into the walls of the cable at
intervals and are energised to an excited state with laser light. Light travelling along
the fibre stimulates the ions to release the stored energy which is of exactly the
required wavelength to boost the signal.
Small crystals of neodymium compounds are used to produce the green light from
lasers which for example are used in the familiar green pointers. Various mixtures
of rare earth elements and their compounds are the essential ingredients of
phosphors used in screens - for example phosphors based on terbium provide the
yellow-green colours while europium in conjunction with the rare earth from Group
3, yttrium, provides the red phosphor.
The amounts of these elements and their compounds used in the screens are tiny but
the quality of colour TV prior to their inclusion was very inferior.
XII - 53
Terbium compounds mixed with the element europium are also used in the long life
fluorescent light globes to provide the more favoured warm coloured light.
Magnetism.
A mixture of the elements neodymium, boron and iron allows the manufacture of
magnets which are 12 times stronger than those made from of the same weight of
iron. Consequently magnets made from neodymium can be much smaller than
comparable iron magnets and are essential for making the tiny motors used in the
hard drives of laptops. More significantly, the power of such magnets allows the
efficient production of electricity by wind turbines and has made electric vehicles
practical. The new generation of wind turbines is able to dispense with the gear
boxes which used to be needed to synchronise the blade revolutions with the
requirements of the electrical generators they drive by using 648 neodymium and
dysprosium magnets set in a doughnut shape directly on the rotating axle of the
blade.
Dysprosium in small amounts when added to neodymium alloy magnets prevents
the loss of magnetism which occurs when the temperature exceeds 300 o C.
Magnets made from samarium and cobalt resist demagnetising when exposed to
nuclear radiation which is particularly important in nuclear power stations.
Holmium is another 4f element which is used in high strength magnets.
Other applications.
Cerium is used for fine polishing of liquid crystal displays and as a catalyst in the
catalytic converters fitted to the exhaust systems of motor cars.
Gadolinium compounds are used in magnetic resonance imaging as a contrast dye.
Lanthanum and cerium mixed are used as one of the electrodes in the nickel metal
hydride rechargeable battery commonly used to power electric cars.
Yttrium is incorporated into high temperature resistant ceramics that are essential in
jet engine turbine blades and rocket exhaust systems and similar applications.
Promethium is used in atomic batteries for specialised purposes.
(b) The actinides.
Actinium (Ac); (like lanthanum, this element is not strictly a member of the f-block
and belongs to Group 3 in the d-block but is usually grouped with the adjacent fblock elements); thorium (Th); protactinium (Pa); uranium (U); neptunium (Np);
plutonium (Pu); americium (Am); curium (Cu); berkelium (Bk); californium (Cf);
einsteinium (Es); fermium (Fm); mendelevium (Md); nobelium(No); lawrencium
(Lr).
All the actinides are radioactive. Only uranium and thorium occur naturally in large
quantities so the others require nuclear reactors or particle accelerators to produce
them from smaller atoms. Plutonium is one such element produced in nuclear
reactors and it like uranium is used in both power stations and nuclear weapons.
Thorium is currently being used in a new prototype nuclear power plant where it
would have many advantages over the conventional fuels, especially from safety
aspects. Thorium reactors do not produce plutonium which was required for nuclear
bombs and it seems that this is the main reason thorium reactors were not the first
choice for power generation.
Americium is used as the ionizing source in most household smoke detectors.
XII - 54
FURTHER NOTES FOR LATER READING - uranium
URANIUM
Uranium occurs as two common isotopes, 235U and 238U. Both are slightly
radioactive but only 235U is fissile - its nucleus when subjected to impacting neutrons
breaks into two smaller nuclei such as 92Ba and 141Kr and releases 3 neutrons per
decaying nucleus. Accompanying this fission process is the release of much energy
as heat. The fission process occurs naturally in the ore but normally only at a very
slow rate. This property is harnessed in nuclear reactors and also atomic bombs. In
order to produce significant fission, it is necessary for neutrons to be available to
travel through the 235U nuclei in sufficient quantities for the reaction to be self
sustaining. Remember that neutrons carry no electrical charge so can penetrate the
positively charged nucleus without experiencing electrostatic repulsions. When a
235
U nucleus undergoes fission the three neutrons released can, if the uranium is
packed suitably, cause more nuclei to break apart and start a self-sustaining chain
reaction. In a nuclear reactor, the process is controlled by surrounding the fuel with
a moderator such as water to slow the neutrons to a suitable speed lest they travel
through nuclei without setting off fission. To stop the reaction, control rods of
material such as boron or rare earths which absorbs neutrons readily, are suspended
above the fuel rods and they can be lowered as needed. In an atomic bomb, two
sub-critical pieces of 235U are held well apart. To set off the reaction, they are
rammed together to form a critical mass in which the extra neutrons are enough to
provide an uncontrolled fission reaction.
In a nuclear power station, the heat from the fission reaction is captured and used to
convert water to steam at high pressure and this turns a generator to produce
electricity. One kilogram of uranium fuel can produce the same amount of
electricity as 1500 tonnes of coal.
The other common isotope, 238U, sometimes called depleted uranium, is not fissile
but has some applications resulting from its extremely high density which is 70%
greater than that of lead. Military uses include providing armour for tanks and
casing explosive shells.
Separating uranium isotopes.
Uranium ore contains 99.3% 238U and just 0.7% 235U which must be enriched in
order to have suitably concentrated fuel. Being isotopes, they are chemically
identical so physical methods are the only way to do this. The physical difference
between the isotopes which is exploited is their differing masses. Although only
small, by repeating mass separating processes many times, a suitable degree of
enrichment is obtained.
The first method used was gas diffusion in which the uranium is converted to
gaseous UF6 and the slightly more rapid diffusion of 235UF6 over many repeated
diffusion steps is able to be enriched to 3%. For weapons grade, a higher
concentration is needed. More recently, repeated sequential centrifuging of uranium
compounds has been developed as a more efficient method.
Dating.
The very long half life of 238U, 4.5 × 109 years, makes it ideal for dating old rocks as
uranium is reasonably prevalent in the earth’s crust. The isotope 235U has a half life
of 704 million years likewise is suitable for dating rocks of lesser age.
XII - 55
Hydrogen
Hydrogen does not fit into any Group of the Table due to its unique property of
having just one electron. Like Group 1 elements, it forms the +1 ion, but only in
association with a molecule such as water because H+ would be a naked proton,
incapable of free existence. Hydrogen is also similar to Group 17 elements
(halogens), existing as an H– ion in ionic compounds, e.g. sodium hydride (NaH).
However, ionic hydrides are unstable, reacting with water to form hydrogen gas and
OH– ions.
Metals vs non-metals - distribution throughout the table.
The metallic elements are located on the left hand section of the Table and the nonmetals are on the right. Although the Periodic Table was originally devised purely
on the basis of macroscopic properties, the true underlying basis for it is now
understood in terms of the structure of the atoms of the elements. Metallic
properties increase down each Group, even amongst those on the extreme left,
because the atomic radius increases down each Group and outer electrons are more
easily removed the further they are from the nucleus of the atom. The non-metals
are on the right because they have more electrons in the same outer orbit which in
turn are held more tightly by the increasing nuclear attraction from the greater
number of protons, causing the atomic radius to decrease from left to right contrary
to what one might have expected. Thus atoms of non-metals are more able to gain
electrons to form anions rather than lose them to form cations. At the conclusion of
each Period of the Table, the noble gases represent the most stable outer electron
arrangement as the next element has its last electron located in a new energy level
further from the nucleus and held less strongly, allowing that element to take on the
properties of a metal again. This sequence of metal v non-metal is repeated for
each Period of the Table. The following table gives a guide to the general
distribution of the metals compared with the non-metals in the Periodic Table.
Between the metals and non-metals, towards the middle of the Table, some of the
elements show properties of both and are called metalloids or semi-metals.
XII - 56
DISTRIBUTION OF METALS AND NON-METALS WITHIN THE PERIODIC TABLE
H
FF
Group
18
non-metal
Group 1
Group 2 Group
13
Group
14
Group
15
Group
16
Group
17
He
F
non-metal
Li
G
Be
G
B
:::
C
:::
N
FF
O
FF
F
FF
Ne
F
metal
metal
non-metal
non-metal
non-metal
non-metal
non-metal
non-metal
Na
G
Mg
G
Al
G
Si
:::
P
FF
S
FF
Cl
FF
Ar
F
metal
metal
metal
metalloid
non-metal
non-metal
non-metal
non-metal
K
G
Ca
G
Ga
G
Ge
:::
As
:::
Se
FF
Br
FF
Kr
F
metal
metal
metal
metalloid
metalloid
non-metal
non-metal
non-metal
Rb
G
Sr
G
In
G
Sn
G
Sb
:::
Te
:::
I
FF
Xe
F
metal
metal
metal
metal
metalloid
metalloid
non-metal
non-metal
Cs
G
Ba
G
Tl
G
Pb
G
Bi
G
Po
G
Rn
F
metal
metal
metal
metal
metal
metal
non-metal
Fr
G
Ra
G
metal
metal
Bond type in the element
G
metallic
FF molecular covalent
:::
F
network covalent
monatomic (noble gases only)
The predictive power of the Periodic Table.
When Mendeleyev devised his version of the Periodic Table, he was able to use it to
predict that a number of then unknown elements existed and further, was able to
make accurate predictions about the properties of those elements and some of their
compounds. However, the predictive power of the Periodic Table is by no means
limited to those initial triumphs. It has guided much successful chemical research
and still does today. The following is an example of where researchers have
employed a knowledge of the Periodic Table to guide their work.
The Nobel prize winner of 2010 in chemistry, Ei-ichi Negishi, won his prize for
work on palladium catalysis in such applications as the anti-cancer drug Taxol. In an
interview with New Scientist (16 October 2010) he said: “Years ago when I started
in chemistry, I was awed by the row of transition metals [containing palladium].
They are the most gifted bunch in the periodic table. I work with the Periodic Table
in front of me at all times and approach all challenges in terms of three particles:
positively charged protons, negatively charged electrons and neutrally charged
neutrons. That’s science.”
XII - 57
Objectives of this Topic.
When you have completed this Topic, including the tutorial questions, you should
have achieved the following goals:
1.
Understand that the Periodic Table was originally devised on the basis of
families of elements with similar properties, arranged in order of increasing
atomic weight (but subsequently found to be increasing atomic number).
2.
Know the electronic basis for the similarities and trends within Groups of
the Periodic Table.
3.
Commit to memory the elements of Groups 1, 2, 13-18.
4.
Know the general distribution of metals and non-metals within the Table.
5.
Have some familiarity with the main chemical properties of each Group.
Further resources.
Video
The video shown in the lecture can be viewed at
1. http://www.youtube.com/watch?v=M-lnauoORdA
on YouTube where there are many other videos relating to the Periodic Table also
available for viewing.
Audio
The BBC series titled “Elements” is available from their website as Elements
Podcasts.
Recommended follow up chemcal modules:
Section: Properties of Atoms
Module: Atomic Properties
Topic: The concept of core charge and its relationship to fundamental atomic
properties.
Module: Electronic Structure of Atoms and Ions
Topic: Trends in atomic properties in relation to the Periodic Table
Section: Properties of Molecules
Module: Electronegativity and Polar Molecules
Topic: Electronegativity in relation to periodic table; polar and non-polar
molecules.
XII - 58
SUMMARY
The classification of elements into groups with similar chemical and physical
properties was begun prior to any knowledge of the structure of atoms and when the
existence of undiscovered elements was overlooked. Purely on these bases, it was
apparent that some elements having similar properties could be grouped together as
families. With the measurement of atomic weights of elements came attempts to
find correlations of properties with atomic weight order, but these were hampered by
the many elements that had not been isolated at that time and also by inaccurate
atomic weight determinations. Mendeleev recognised these deficiencies and turned
them to advantage, still using atomic weight order but giving primacy to assigning
elements with similar properties to the same Group. He left gaps in the arrangement
of the elements where needed and predicted which elements were still to be
discovered and their likely properties. This arrangement of the elements, called the
Periodic Table, was further refined when the structure of the atom was elucidated
and it was then realised that the order of the elements in the Periodic Table should
be atomic number order rather than atomic weight order. From today’s knowledge
of atomic structure, the reason for similarities of chemical properties within any
Group of elements has been clearly established as a consequence of each element in
any Group having the same outer-shell electron arrangement.
However, gradations in properties within a Group are normally observed, with the
most metallic elements being at the bottom. Within any row (Period) of the Table,
the elements on the left are metals and on the right are non-metals, a gradual
increase in non-metallic properties from left to right being exhibited. Some
elements towards the middle of the Table show properties of both metals and nonmetals. These trends can also be related to aspects of atomic structure, in particular
the decreasing effective nuclear charge of atoms down each Group which leads to
less energy being required to remove electrons and thus form cations - a property of
metals. Elements to the right of the Table are non-metals because they have more
electrons contained within the same outer level of the atom and are accompanied by
a corresponding increase in the number of protons in the nucleus and thus an
increasing effective nuclear charge, resulting in all the electrons being held more
tightly. Thus more energy is required to remove electrons from non-metals instead, formation of anions or covalent bonding are their energetically preferred
options. The least reactive of all elements - the noble gases - have the largest
effective nuclear charge which explains their near complete lack of reactivity.
The Periodic Table is an elegant example of how observations and the collection of
data, development of hypotheses, predictions made from them and testing of each
hypothesis followed by discarding or refining it leads to increased understanding in
science.
Before commencing the questions associated with Topic 12, complete any
remaining questions from Topics 11, 7, 8, 9 and 10 in that order.
XII - 59
TUTORIAL QUESTIONS - TOPIC 12
1. List the physical properties of metals compared with non-metals.
2. How do the chemical properties differ for metals compared with non-metals with
regard to the following:
(a) Type of ion formed in salts
(b) Reaction with acids
3. Disregarding hydrogen and helium, for the Periodic Table Groups 1, 2, 13 - 18,
what electronic structural feature do all the atoms in any given Group have in
common?
4.
(a) Write the names and symbols for the elements of Group 1
(b) List some properties of Group 1 elements which indicate they are all
metals.
(c) What valence do all Group 1 elements exhibit in their compounds?
5.
(a) Write the names and symbols of the elements of Group 2.
(b) What properties would indicate that beryllium could in some respects be
more appropriately regarded as a non-metal?
(c) Aside from beryllium, list properties of the other members of Group 2
which indicate they are metals.
(d) What valence do all Group 2 elements show in their compounds?
6.
(a) Write the names and symbols of the elements of Group 13.
(b) Is boron a metal or a non-metal? Give reasons for your answer.
(c) Why is aluminium a useful structural material even though it reacts
readily with water and oxygen?
(d) What is the usual valence shown by elements of Group 13 in compounds?
7.
(a) Write the names and symbols of the elements of Group 14.
(b) Which elements of Group 14 could be best regarded as metals rather than
non-metals? Give evidence to support your answer.
(c) Diamond and graphite are both forms of pure carbon. Compare their
physical properties and explain the differences in terms of arrangement of
their atoms.
XII - 60
8.
(a) Write the names and symbols of the elements of Group 15.
(b) What type of bonding is present in elemental nitrogen?
(c) Give the names and formulas for some species to which nitrogen is
converted in the process of nitrogen fixation.
(d) Summarise the metallic/non-metallic properties of the Group 15 elements.
(e) In what way does bismuth differ from the other members of Group 15?
9.
(a) Write the names and symbols for the elements of Group 16.
(b) Summarise the metallic/non-metallic properties of Group 16.
properties which support your answer.
10.
List
(a) Write the names and symbols for Group 17 elements (halogens).
(b) Explain why the halogens all have low melting and boiling points.
(c) Why are halogens strong oxidising agents?
(d) List all the elements that occur naturally as diatomic molecules.
(e) What properties confirm that Group 17 elements are all non-metals?
11.
(a) Write the names and symbols for Group18 elements (noble gases).
(b) Why do all the elements of Group 18, the noble gases, occur as
monatomic gases in nature?
12.
(a) Where in the Table do the elements of the d-block appear?
(b) While the d-block elements are all metals, these elements show some
properties in general that differ from the metals of Groups 1 and 2. What are
these different properties?
13.
Summarise the occurrence of metals and non-metals in the Periodic Table.
14.
Where in the Periodic Table are those elements which are (a) most easily
oxidised and (b) those most easily reduced located? Explain why this is so.
15.
Define the terms “effective nuclear charge” and “screening” in the context of
atomic structure. How does each relate to the properties of the elements as
arranged in the Periodic Table?
XII - 61
ANSWERS TO TUTORIAL TOPIC 12
1.
Metals: shiny when freshly cut, malleable, ductile, good conductors of heat
and electricity
Non-metals: dull solids, powders or gases, brittle, poor conductors of heat
and electricity
2.
(a) Metals form cations in reactions that produce salts while non-metals form
anions in those reactions.
(b) Many metals react with acids forming cations as part of a salt while nonmetals do not react with acids.
3.
All the elements in any Periodic Table Group have the same arrangement of
electrons in their outer level.
4.
(a) lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs)
(b) They all have the physical properties listed for metals above; they all form
cations in reactions with acids to produce salts.
(c) Group 1 elements always have a valency of 1 in their compounds.
5.
(a) beryllium (Be) magnesium (Mg) calcium (Ca) strontium (Sr) barium (Ba)
(b) Beryllium does not form cations in its compounds, instead it is usually
covalently bonded.
(c) They all display the physical properties of metals given above; they all
form cations in reactions with acids to produce salts.
(d) Group2 elements always have a valency of 2 in their compounds.
6.
(a) boron (B), aluminium (Al), gallium (Ga), indium (In), thallium (Tl)
(b) Boron is a non-metal as it is a black powder, does not form salts with
acids and is only covalently bonded in its compounds.
(c) Aluminium reacts with oxygen in the air to produce an oxide which
adheres strongly to the surface of the metal and protects it from further
corrosion in the atmosphere.
(d) Group 13 elements mostly have a valency of 3 in their compounds.
XII - 62
7.
(a) carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb)
(b) tin and lead behave more like metals than the rest of Group 14 because
they have the physical properties of metals and form cations in some of their
compounds with non-metals.
(c) Carbon atoms in diamond are arranged in a very stable tetrahedral
structure with each C atom bonded to four other C atoms. The very stable
structure of diamond imparts the properties of considerable hardness and nonconduction of electricity. In graphite, each C atom is bonded to just three
other C atoms in a planar arrangement, leaving one unused valence electron
on each atom. These unused electrons form weak partial bonds to C atoms in
the planes above and below. Consequently graphite is soft and the planes of
C atoms are easily peeled apart (as in lead pencils). Also graphite can
conduct electricity because the electrons between the planes are so weakly
held that they are mobile under the influence of an electrical voltage.
8.
(a) nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi)
(b) Elemental nitrogen consists of molecules consisting of two N atoms
covalently bonded by a triple bond. This is termed molecular covalent
bonding.
(c) ammonia (NH3), ammonium ion (NH4+), nitrogen dioxide (NO2), nitrate
ion (NO3–), nitrite ion (NO2–)
(d) Nitrogen and phosphorus are clearly non-metals. Arsenic and antimony
have properties of both metals and non-metals. Bismuth is more metallic
than non-metallic.
(e) Bismuth forms many ionic compounds containing the Bi3+ cation and it
has metallic bonding in the elemental state.
9.
(a) oxygen (O), sulfur (S), selenium (Se), tellurium (Te)
(b) All the elements of Group 16 are non-metals. They have the usual
physical properties of non-metals, do not dissolve in acids and they form
anions when in compounds with metals. Their compounds with other nonmetals are covalently bonded.
10.
(a) fluorine (F), chlorine (Cl), bromine (Br), iodine (I)
(b) The halogens occur as diatomic molecules which only have weak forces
of attraction between their molecules. Consequently it requires relatively
little energy (thus lower temperature) for the molecules to separate into the
liquid phase from the solid (melt) or from the liquid phase into the gas (boil).
XII - 63
(c) The atoms of halogens only need to gain one extra electron to become
isoelectronic with a noble gas. Species which readily gain electrons are good
oxidants.
(d) H2, N2, O2, F2, Cl2, Br2, I2
(e) Apart from their physical properties, the halogens all form anions in salts
and covalent compounds with other non-metals.
11.
(a) helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn)
(b) The outer electron level of the atoms of all the noble gases is filled with 8
electrons (or 2 electrons for helium), the arrangement that gives the noble
gases their stability. To form diatomic molecules for example, more
electrons would have to enter this level in forming covalent bonds between
the atoms and this is not possible as the level is already filled.
12.
(a) The d-block elements are located in the Periodic Table between Group 2
and Group 13. Where they occur, there are 10 d-block elements in each
Period.
(b) Among the properties that distinguish d-block elements from the metals of
Groups 1 and 2 are that they frequently show a range of valencies and often
have coloured compounds.
13.
Metals are located from the left hand side starting with Group 1 and
extending to include the d-block. The non-metals are located on the right
hand side of the table extending to boron in Group 13 and carbon in Group
14. Metallic properties increase down each Group of the Table.
14.
(a) The most easily oxidised (i.e. the strongest reductants) are located in the
bottom, left hand part of the Table where the outer electrons of elements are
further out from the nucleus and not held so tightly by attraction to it.
Consequently relatively small amounts of energy are required for an oxidant
to remove an electron. Group 1 elements are the most easily oxidised as they
only need to lose one electron to become isoelectronic with the nearest noble
gas.
(b) The most easily reduced (i.e. the strongest oxidants) are located in the top,
right hand corner of the Periodic Table, excluding the noble gases. Fluorine
is the strongest oxidant followed by oxygen and chlorine. Elements in this
region of the table are only one or two electrons short of having the stable
structure of the nearest noble gas, so energy is released when the atom
captures an electron in the process of oxidizing another atom and itself being
reduced in the process.
XII - 64
15.
Effective nuclear charge is the actual force experienced by electrons in the
outer energy level of an atom. Outer electrons are said to be screened from
the full attraction they would otherwise feel from the positively charged
nucleus because of those electrons which occupy orbits closer to the nucleus.
Effective nuclear charge increases from left to right across each period of the
Table because, as electrons are added progressively to the outer orbit, all the
outer electrons experience increased attraction to the larger number of
protons in the nuclei. This results in a steadily reducing atomic radius and
increasing energy requirements to remove electrons from atoms to form
cations. Thus cations with greater that +3 charge are rarely formed. In each
Period, the last element (a noble gas) has the maximum effective nuclear
charge and so reactions in which electrons are removed from noble gas atoms
require a prohibitively large energy input.
The outer electron from elements containing one more electron than a noble
gas (always Group 1) must have that electron in the next highest energy orbit
which is now screened from the nucleus by the inner electrons. The
consequence of this is not only a larger atomic radius but also much less
energy being required to remove that electron to form a +1 cation.
Effective nuclear charge decreases down each Group due to increased
screening by the ever larger number of inner electrons and so electrons can be
removed from atoms more readily, i.e. elements become more metallic the
lower they are in a Group.
The large effective nuclear charge on atoms to the right of each Period also
explains why elements such as the halogens form anions so easily, actually
releasing energy in the process. Once the noble gas structure is attained, any
additional electrons would have to occupy the next outer orbit and be
screened from the nucleus to such an extent that they would be unstable.
This is also the reason that noble gases do not form anions.
Nuclear
reactors
Apparatus to
m e a s u r e
radioactivity
9
8
High energy
particle
accelerators
Bunsen burner
and
spectroscope
9
9
8
Liquid air
Electrolysis
9
Apparatus to
handle gases
9
8....... Improvements in analytical ....... 8
methods
Prehistoric
Use of furnaces
to smelt ores
9
8..... Methods for analysing minerals - ... 8
especially use of the blowpipe
DISCOVERY OF THE ELEMENTS - NUMBER OF KNOWN ELEMENTS vs
YEAR
this file won’t print the labels - use the file called 04bridgingcourse append1.