12. FINDING PATTERNS: THE START OF THE PERIODIC TABLE
12.1. INTRODUCTION
In the first decade of the nineteenth century, fourteen new elements were discovered. By 1820, over
40 elements had been discovered and fifty-five elements had been recognized by 1830. Scientists
started to wonder how many elements there might be and how the different elements were related to
each other.
12.2. GOALS
• Explain the evolution of the periodic table, specifically
ƒ Explain the models and the order in which they were developed;
ƒ Explain how they accounted for the existing data of the time; and
ƒ Explain what new data was found that allowed the development of the new model.
• Explain how the periodic table is periodic.
12.3. DÖBEREINER’S LAW OF TRIADS
Scientists started recognizing that some of the elements behaved very much like other elements
and looked for a relationship between them. For example, Lithium, sodium and potassium all react
strongly with water and all three are soft shiny solid metals. The atomic weights of lithium, sodium
and potassium, however, are 6.9 u, 23.0 u and 39.1 u respectively. The three elements formed a
triad.
In 1817, Johann Döbereiner found an interesting relationship: If you add the atomic weights of
the lightest and heaviest elements in the triad, and then divide by two, the result is equal to the
atomic weight of the middle element. Döbereiner found the same relationship held for calcium,
strontium, and barium; and for chlorine, bromine, and iodine. His work was published in 1829.
Other scientists found groups of similar elements that were more than triads: Fl (fluorine) was
added to Döbereiner’s chlorine/bromine/iodine group (making a tetrad). Sulfur, oxygen, selenium
and tellurium were grouped and nitrogen, phosphorus, arsenic, antimony and bismuth were also
classified together. These groupings suggested that elements might be arranged into rows and
columns; however, some of the available data still was inaccurate, especially in terms of atomic
weights. This worked both ways. Some patterns we now know to be true were rejected because the
data weren’t accurate enough and some patterns were wrongly identified.
When Leopold Gmelin published the first edition of his famous Handbuch der Chemie in 1843,
he included three tetrads and even the pentad - nitrogen, phosphorus, arsenic, antimony and bismuth
- which we now recognize as group 15 of the periodic table.
12.4. THE TELLURIC HELIX
In 1862, Alexandre Beguyer de Chancourtois (1820-1886) published a list of all known elements.
De Chancourtois took the atomic weight of oxygen as 16 and used this as the standard against which
the atomic weights of the other elements were compared. His list was constructed as a helical graph
with 16 columns wrapped around a cylinder. Chancourtois noticed that certain of Döbereiner’s
triads appeared below one another in his spiral. Elements with similar properties occupied positions
on the same vertical line of cylinder (the list also included some ions and compounds). The cylinder
was called ‘the telluric helix’. His work was ignored by other scientists, in part because he was a
geologist and in part due to the difficulty of describing his idea in the paper without a diagram.
Despite the scientific community’s lack of excitement, it is significant that De Chancourtois was the
first person to notice the periodicity of the elements, that is, when the elements were arranged
according to their atomic weights, similar elements occured at regular intervals.
In particular, the tetrad that included oxygen, sulfur, selenium and tellurium fell together. The
atomic weights of these elements are 16, 32, 79 and 128, respectively, which are multiples or near
multiples of 16. Other parts of the telluric helix were less successful. Boron was near aluminum, but
they were followed by nickel, arsenic, lanthanum and palladium. Chancourtois had discovered
periodicity, but had the frequency wrong.
12.5. THE LAW OF OCTAVES
John Alexander Reina Newlands benefited greatly from Cannizzaro’s reform of atomic weights
in 1860. The improved atomic weights were much closer to the values we accept today. Newlands
organized the elements in a table of seven columns and entered his elements in order of increasing
atomic weight. His tables, done in 1864 and 1865 (Figure 12.1), followed what he called the Law of
Octaves. When ordered by atomic weight, every eighth element showed similar chemical properties.
His arrangement produced some misalignments, but Newlands was sufficiently secure in his
chemical knowledge to put similar elements in the same column even if it meant squashing two
elements into some boxes. Newlands recognized silicon and tin as part of a triad and predicted that
there would be a missing element intermediate between these, with atomic weight of about 73;
however, he didn’t leave space in the published version of his table in 1865. (Germanium, the
predicted element, has an atomic weight of about 72.6) Newlands left no vacant slots, nor did he
allow that there might be periods longer than eight. Newlands introduced the term atomic number
for the first time. His table of 1865
F
Cl
Co/Ni
Br
Pd
I
Pt/Ir
shows no atomic weights and simply H
numbers the elements in order from Li
Na
K
Cu
Rb
Ag Cs
Tl
one to fifty six.
G
Mg Ca
Zn
Sr
Cd
Ba/V
Pb
Like De Chancourtois, Newlands’
table was not received well. His Bo Al
Cr
Y
Ce/La
U
Ta
Th
efforts were criticized and even
Si
Ti
In
Zn
Sn
W
Hg
publicly ridiculed by chemists. In C
March, 1866 an audience member at N
P
Mn As
Di/Mo
Sb
Nb
Bi
his talk facetiously asked whether
S
Fe
Se
Ro/Ru
Te
Au
Os
Newlands had ever attempted to O
classify the elements in alphabetical Figure 12.1: Newland’s proposed ‘Law of Octaves’ Table
order. It was not until in 1887 that
Newlands’
contribution
was
recognized by the Royal Society, which awarded him the Davy medal.
12.6. MENDELEEV
The next milestone in the development of the periodic table was due to the Russian scientist
Dmitri Mendeleyev (1834-1907) during the years 1869-1871. Mendeleyev wrote the names of the
(by then) sixty-three known elements, along with their atomic weights and other properties, on cards.
He then started arranging the cards into rows and columns. When the elements were ordered
according to atomic weight, Mendeleyev, like de Chancourtois and Newlands, saw that certain
chemical properties were repeated periodically; however he was the first to point out that not all the
elements fit the pattern exactly. Mendeleyev moved some elements to new positions – despite their
accepted atomic weights – so that they would be in the same column as elements with similar
properties. Mendeleev had periods of eight (like Newlands), but also allowed for longer periods in
the transition and rare-earth elements. He made a number of correct predictions for missing
elements and he had Co/Ni and Te/I in their correct chemical order. If you check a modern periodic
table, you’ll find that Ni comes after Co, even though Co has a larger atomic weight.
H=1
Li=7
Ti=50
Zr=90
?=180
V=51
Nb=94
Ta=182
Cr=52
Mo=96
W=186
Mn=55
Rh=104.4
Pt=197.4
Fe=56
Ru=104.4
Ir=198
Ni=Co=59
Pd=106.6
Os=199
Cu=63.4
Ag=108
Hg=200
Be=9.4
Mg=24
Zn=65.2
Cd=112
B=11
Al=27.4
?=68
Ur=116
C=12
Si=28
?=70
Sn=118
N=14
P=31
As=75
Sb=122
O=16
S=32
Se=79.4
Te=128?
F=19
Cl=35.5
Br=80
J=127
Na=23
K=39
Rb=85.4
Cs=133
Tl=204
Ca=40
Sr=87.6
Ba=137
Pb=207
?=45
Ce=92
?Er=56
La=94
?Yt=60
Di=95
?In=75.6
Th=118?
Au=197?
Bi=210?
Figure 12.2: Mendeleyev’s Periodic Table. The element symbols are followed by their atomic
weights.
12.6.1. Features of Mendeleev’s Periodic Table. Figure 12.2 shows Mendeleev’s organization of
the elements known at the time. It is a little different from the periodic table with which you are
familiar because what you are used to as rows are columns in this version of the table. Figure 12.3
shows the same data in a horizontal format.
Returning to Figure 12.2: Each horizontal row contains analogous elements, still ordered by
increasing atomic weight. This is the table that Mendeleev found, along with some comments from
his original paper. The triads and tetrads of Döbereiner can be seen. Mendeleev made the following
comments regarding his arrangement.
1. The elements, if arranged according to their atomic weights, exhibit a periodicity of properties.
2. Chemically analogous elements have either similar atomic weights (Pt, Ir, Os), or weights that
increase by equal increments (K, Rb, Cs).
3. The arrangement according to atomic weight corresponds to the valence of the element and to a
certain extent the difference in chemical behavior, for example Li, Be, B, C, N, O, F.
4. The elements distributed most widely in nature have small atomic weights, and all such elements
are marked by the distinctness of their behavior. They are, therefore, the representative elements;
and so the lightest element H is rightly chosen as the most representative.
5. The magnitude of the atomic weight determines the properties of the element. In the study of
compounds, not only the quantities and properties of the elements and their behavior must be taken
into consideration, but also the atomic weight of the elements. Thus the compounds of S and Te, Cl
and J, display not only many analogies, but also striking differences.
H
1
Ti V Cr Mn
50 51 52 55
Fe
56
Zr Nb Mo Rh Ru
90 94 96 104.4 104.4
? Ta W Pt
Ir
180 182 186 197.4 198
Li
7
Be
9.4
B
11
C
12
N
14
O
16
F
19
Na
23
Mg
24
Al
27.4
Si
28
P
31
S
32
Cl
35.5
K
39
Ni=Co=59 Cu Zn
63.4 65.2
?
68
?
70
As
75
Se
79.4
Br
80
Pd=106.6 Ag
108
Ur
116
Sn Sb
Te
Cs Ba
J
118 122 128? 127 133 137
Os=199
Hg
200
Cd
112
Au
197?
Bi
210?
Ca
40
? ?Er ?Yt ?In
45 56 60 75.6
Rb Sr Ce La
85.4 87.6 92 94
Di Th
95 118?
Tl Pb
204 207
Figure 12.3: Mendeleyev’s Periodic Table transposed – compare it with the current periodic table.
6. One can predict the discovery of many new elements, for example analogues of Si and Al with
atomic weights of 65-75.
7. A few atomic weights will probably require correction; for example Te cannot have the atomic
weight 128, but rather 123-126.
8. From the above table, some new analogies between elements are revealed. Ur appears as an
analogue of Bo and Al, as is well known to have been long established experimentally.
12.6.2. Strengths. Mendeleev’s model had a number of features that previous models had either
overlooked or ignored. Mendeleev’s table predicted not only that there should be new elements, but
also the properties of these missing elements. The gap in atomic weights between cerium (140) and
tantalum (182) suggested to Mendeleev that a whole period of the table remained to be discovered.
Many of these elements, which we now call the lanthanides, were isolated later in the century.
Mendeleev’s most notable successes were with eka aluminum (which now is called gallium) and
eka-silicon (which now is called germanium). Lecoq de Boisbaudran discovered gallium in 1875 and
reported its density as 4.7 cmg 3 , which did not Dmitri Mendeleev was born in Tobolsk in Western
agree with Mendeleev’s prediction of 5.9 cmg 3 . Siberia in 1834, the youngest of 14 children,
whose father became blind and died of
When de Boisbaudran was told that his new tuberculosis the year Dimitri finished school. His
element was Mendeleev’s eka-aluminium, and father taught Russian literature and his mother
that Mendeleev had predicted most of its owned and operated a glassworks. His early
properties accurately, de Boisbaudran re- contacts with political exiles gave him a lifelong
determined its density more accurately and love of liberal causes, and his freedom to roam the
found it to be as predicted, 5.956 cmg 3 .
glassworks stimulated an interest in business and
A second strength was that Mendeleev industrial chemistry. His mother—after her
allowed for the possibility that the ‘known’ husband's death and shortly before her own—took
atomic weights might be wrong, so some the 15-year-old Dmitri to St. Petersburg. There he
elements are not strictly in order of atomic attended the Main Pedagogical Institute and the
weight. Mendeleev was not correct in all University of St. Petersburg, where he pursued a
cases. For example, he presumed that the doctorate in chemistry. During his graduate
atomic weight of Te had been determined studies he traveled to Heidelberg to work with
incorrectly; however, new analyses confirmed Bunsen. He returned 1861 to St Petersburg, where
the original value. This anomaly remained a he eventually became professor of general
chemistry in 1867.
puzzle for chemists until the discovery of isotopes (which we will discover for ourselves in unit III).
Mendeleev’s notes and a page from his published paper are shown in Figure 12.4.
Figure 12.4: Mendeleev’s original notes (www.chemheritage.org/.../ chemach/ppt/lm01.html) in the
leftmost figure and his original paper (www.promotega.org/fld30059/ originaltablepage.html) at
right
Closer inspection of Mendeleev's periodic table of 1869 reveals some of the difficulties he had in
putting the elements into groups. He spotted that the titanium group needed a final heavy metal
member, and predicted that it would be found among titanium ores. This missing element (hafnium)
was discovered by the Hungarian George de Hevesy in 1923. Its atomic weight of 178 is very close
to that of 180 predicted by Mendeleev.
12.6.3. Weaknesses. Although Mendeleyev arranged elements in order of increasing atomic weight,
atomic weight alone cannot explain the periodicity. Why do all the group I elements burn when
exposed to air, even though they have very different atomic masses? The elements with atomic
numbers 3 and 4 have similar atomic masses, but their chemical properties are very different from
each other. The periodicity suggested that atoms might have additional internal structure – there
might be smaller pieces that are gradually added on as one moves across the periodic table. One (or
more) pieces would be added as you moved from atomic number 3 to 10, then something would start
over again from atomic numbers 11 to 18.
Mendeleev also put mercury in the same group as copper and silver, even though this meant
mercury would have to come before gold. Gold has a lower atomic weight in Mendeleev’s table.
Consequently, Mendeleev put gold in the boron group under uranium. Uranium was wrongly placed
because its atomic weight was erroneously thought to be 116, half of what it should be. Mendeleev
also struggled unsuccessfully to accommodate those other f-block elements which were known at the
time: erbium, yttrium, cerium, thorium and didymium, most of which had incorrectly determined
atomic weights at the time. Didymium turned
Julius Lothar Meyer (1830–1895) worked with
out to be a mixture of praseodymium and
Robert Bunsen in Heidelberg only five years apart
neodymium.
from Mendeleev, but they arrived there with
12.7. MEYER AND MENDELEEV
significantly different backgrounds. Meyer came
Although Mendeleev was the person who from a medical family of Oldenburg, Germany,
got credit for the periodic table, he was not and first pursued a medical degree. In medical
the only person to be thinking of this school he became interested in chemistry,
organization scheme. The German Julius especially physiological topics like gases in the
Lothar Meyer also used Cannizzaro’s atomic blood.
weights to draw up a primitive table in 1864,
but the more sophisticated version he produced in 1868 for the second edition of his textbook was
not used and remained among his papers to be published only after his death in 1895. Meyer
published too late to claim priority, but his work served as confirmation that Mendeleev’s periodic
table was based on sound chemical principles. Although Mendeleev published his tables in the new
and obscure Journal of the Russian Chemical Society, his paper was abstracted within weeks of its
appearance into the more widely read German journal Zeitschrift für Chemie.
12.7.1. Atomic Volumes. Since we
know that one mole of any element
contains the same number of atoms, we
can compare the volume of different
elements that have the same number of
atoms. The ratio of the volumes of one
mole of each element was equal to the
ratio of the volumes of single atoms of
each element. Meyer published in 1870
a graph of atomic volume vs. atomic
weight.
If the atomic volumes of the
elements are plotted against their atomic
weight, a series of waves is produced,
with sharp peaks at the alkali metals:
sodium, potassium, rubidium, and
cesium. Each fall and rise corresponds
to a period in the periodic table, as Figure 12.5: Molar Volume vs. atomic number.8
shown in Figure 12.5. Similar patterns
are observed in other physical properties, such as density, as shown in Figure 12.6. Meyer’s graph
clearly showed periodic changes in the atomic volume at intervals of 7, 7, 14 and 15. If more
elements had been known at the time, this graph would have revealed the observed intervals of 8, 8,
18 and 18 of the first four rows of the modern table.
Hydrogen, the first in the list of elements (it has the lowest atomic weight) is a special case and
can be considered as making up the first period all by itself. The second and third period in Meyer's
table included seven elements each, and duplicated Newlands' law of octaves. You can see in
Figure 12.5 that the next wave following included more than seven elements, which showed where
Newlands had been mistaken. The later periods had to be longer than the earlier periods.
8 www.webelements.com
12.7.2. Valence. In the first edition of
his 1864 textbook Die Modernen
Theorien der Chemie, Meyer used
atomic weight to arrange 28 elements
into 6 families that bore similar
chemical and physical characteristics,
leaving a blank for an as-of-yetundiscovered element. Meyer made an
important conceptual advance over his
predecessors, which was considering
valence. Valence was at that time a
number representing the combining
power of an element (for example,
how readily is combined with atoms of
hydrogen).
Meyer realized that
valence was the link among members Figure 12.6: Density vs. atomic number.9
of each family of elements and was the
pattern for the order in which the families were themselves organized.
12.8. DISCOVERY OF THE NOBLE GASES
12.8.1. The Missing Group. Compare the periodic table of Mendeleev to a current periodic table
and you will notice that there is an entire group (vertical row) missing. Neither Mendeleev nor
Meyer predicted the existence of the additional group. Lord Rayleigh (1842-1919) and William
Ramsey (1852-1916) discovered the inert gases in the early 1900’s. They initially were working
independently, but eventually started communicating with each other and working together.
Rayleigh was trying to make highly precise measurements of the atomic weights of oxygen,
hydrogen, and nitrogen. He found nitrogen from the air seemed to have a slightly higher atomic
weight than nitrogen from chemicals in the soil. At that time, the common belief was that air was
made of oxygen and nitrogen; however, they found gas left over after removing the oxygen and
nitrogen.
12.8.2. Discovery of Argon. Ramsay recalled that Cavendish had tried to combine nitrogen in the
air with oxygen. He had found that there always was a final bubble of gas left over that could not be
made to combine with oxygen in any circumstances. Perhaps the gas that was left was responsible
for the apparently larger atomic weight of nitrogen in the air. In 1894, Ramsay repeated Cavendish's
experiment; however, he had the advantage of an analytical instrument not available to Cavendish.
The technique, called spectroscopy, will be discussed in Unit III. Ramsay found that the final
bubble Cavendish saw was a new gas denser than nitrogen and making up about 1 per cent of the
volume of the atmosphere. It was chemically inert, meaning that it could not be made to react with
any other element. It was named argon from a Greek word meaning "inert". The atomic mass was
about 40 u, which would put it somewhere in the region of sulfur (atomic weight 32), chlorine
(atomic weight 35.5), potassium (atomic weight 39), and calcium (atomic weight, just over 40);
however, the chemical behavior was so different from those elements that it didn’t readily fit in any
of the existing groups.
12.8.3. Discovery of Other Inert Gases. Ramsey discovered the remainder of the inert gases using
newly-discovered techniques for cooling and liquefying gases, by separating other gases from air,
including neon, krypton, xenon and radon. They had discovered a whole group of elements, none of
which had been known to Mendeleev. The final member of the group, helium, had been discovered
9 www.webelements.com
a few years earlier. At first, Mendeleev did not believe that a new gas had been discovered because
it didn’t fit into his Periodic Table; however, he saw later that these elements actually confirmed the
basic idea behind the Table.
The inert gases initially were curiosities with no apparent use; however, in 1910, Georges Claude
(1870-1960) showed that an electric current forced through certain gases such as neon produced a
soft, colored light. By the 1940’s neon signs lined the Great White Way of New York City.
12.9. THE MODERN PERIODIC TABLE
12.9.1. Atomic Number. Atomic number at this point was merely a number without physical
meaning. The atomic number was simply the numbering of the elements after they had been placed
in order by atomic weight. Atomic number is represented by Z. We will find in upcoming chapters
that atomic weight doesn’t tell the whole story, and that atomic number does have a rather important
physical meaning. There were more than 90 elements discovered by 1990. Figure 12.7 shows the
number of elements known at the end of each decade.
120
100
Total No Elements
80
60
40
20
16
20
16
40
16
60
16
80
17
00
17
20
17
40
17
60
17
80
18
00
18
20
18
40
18
60
18
80
19
00
19
20
19
40
19
60
19
80
20
00
pr
e
0
Year
Figure 12.7: Number of elements known as a function of year.
Figure 12.8: Current Periodic Table (from www.webelements.com)
We now know that there are only 92 naturally occurring elements in nature. The rest are manmade and most exist only for fractions of seconds. The `modern' periodic table, shown in
Figure 12.8, is a result of reordering by Mosely according to atomic number. We will see in Units II
and III why this reordering made sense. We also will examine how the heaviest elements were
discovered and how elements that are not found in nature are made. Elements 113, 115, and 117 are
not known, but are included in the table to show their expected positions. Elements 114, 116, and
118 were reported in the last few years; however, the team of Berkeley scientists that announced in
1999 the observation of what appeared to be Element 118 (the heaviest undiscovered element at the
time) has retracted its original paper after several confirmation experiments failed to reproduce the
results. The discrepancy was attributed to scientific fraud and element 116 also went back to being
‘predicted’.
12.10. READING THE PERIODIC TABLE
This is just a reminder about where you find
information
on
the
periodic
table.
(See
http://www.chem.qmul.ac.uk/iupac/AtWt/table.html
for the periodic table we will use in this class. You
can also access www.webelements.com for more
information about any element.)
12.11. SUMMARIZE
12.11.1. Definitions: Define the following in your
own words. Write the symbol used to represent the
quantity where appropriate.
1. Triads, Tetrads and Pentads
2.
Periodicity
3.
Telluric helix
Atomic number
64
Gd
tomic mass (in u)
157.25
Figure 12.9: The element Gadolinium
4.
Valence
5.
Inert gas
12.11.2. Concepts: Answer the following briefly in your own words.
1. What is the telluric helix? Why was the telluric helix ignored by other scientists?
2.
What are triads? Why were they important?
3.
What is the law of octaves?
4.
What did Mendeleyev do differently that made his work more convincing than de
Chancourtois, Newland and Meyer’s work?
5.
What was the most significant thing about de Chancourtois’ telluric helix?
6.
What does it mean for something to be periodic?
7.
Why didn’t Meyer get credit for his periodic table?
8.
What types of errors were responsible for some of the incorrect placements of elements in
Mendeleev’s periodic table?
12.11.3. Your Understanding
1. What are the three most important points in this chapter?
2.
Write three questions you have about the material in this chapter.
12.11.4. Questions to Think About
1. Why were the inert gases the last group of elements discovered?
2. Explain the idea of valence and how it unites elements within the same family.
3. Is the periodic table really periodic since the periods are not the same length?
4. See the article at: http://www.aip.org/pt/vol-55/iss-9/p15.html regarding the retraction of the
discovery of element 118. Answer the following questions.
a) How was the alleged fraud discovered?
b) Ninov, the scientist who allegedly perpetrated the fraud, claims he is innocent. Should the
researchers have retracted their paper before they had a chance to investigate the situation
and determine whether Ninov actually did fabricate the data?
c) How did this alleged fraud occur? From the article, what can researchers do to make sure
that it doesn’t happen again?
You might also look at:
http://www-ccec.ece.ucsb.edu/people/smith/classnotes/ethics/ninov_nytimes_15Oct2002.pdf
for
more information.
5. Look at http://www.aip.org/tip/INPHFA/vol-8/iss-6/p12.html for information on another
alleged fraud case (in the same year as the Berkeley one, alas!).
Also
http://www.siliconvalley.com/mld/siliconvalley/news/editorial/4149200.htm
a) How was the alleged fraud discovered?
b) What does Schon claim as far as his guilt or innocence?
c) How did this alleged fraud occur? From the article, what can researchers do to make sure
that it doesn’t happen again?
PHYS 261 Spring 2007
HW 13
HW Covers Class 12 and is due February 7th, 2007
1.
2.
3.
What did Mendeleyev do differently that made his work more convincing than de
Chancourtois, Newland and Meyer’s work?
What types of errors were responsible for some of the incorrect placements of elements in
Mendeleev’s periodic table?
See the article at: http://www.aip.org/pt/vol-55/iss-9/p15.html regarding the retraction of the
discovery of element 118. Answer the following questions.
a) How was the alleged fraud discovered?
b) What does the article say that researchers can do to make sure that this type of fraud
doesn’t happen again?