2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 31 – Group 16
1. Group 15 – B-N Compounds.
2. Group 16 – O, S, Se, Te, P

The Elements

Allotropes of O and S

Oxides and Acids

Halides

Sulfides
1. Group 15- A Survey of Oxidation States

We discussed in the previous lecture that nitrogen displays each of nine
oxidation states from -3 to +5.

It is this variety which makes it unique in the group.

Phosphorous displays the same range but only -3, 0, +3, +4 and +5 are of any
major
j iimportance.
t

Arsenic and Antimony are restricted to -3, 0, +3, and +5.

Bismuth shows only 0, +3 and +5.
Nitrogen (-3) Compounds: Nitrides and Ammonia

The nitrides are a class of compounds in which nitrogen has a -3 oxidation state.

Major covalent nitrides include those of boron
boron, sulfur and phosphorus
phosphorus.

Boron nitrides involve p-p B=N bonds.
1




We have seen the structure of Borazine in a previous lecture, it is sometimes
referred to as inorganic benzene.
Note that two resonance structures are necessary to describe this molecule
using valence bond concepts.
Experimentally, all the B-N bonds have the same bond length, intermediate
between the characteristic of single and double bonds.
In this case B-N is isoelectronic to carbon (benzene), it exists in graphite and
diamond structures.

Consider also P bonding to N

This affords Cyclic Phosphazenes: P3N3 a six membered ring.

In this case though remember it is a pp-dp interaction
since P has available 3d orbitals to form P=N bonds.

One of the best known cyclic phosphazenes is the trimer (a). In analogy with the
borazines, these molecules are pictured with alternating single and double p-d
P-N bonds, but in fact the P-N distances are about 1.5 Å shorter than P-N
bonds and longer than P=N bonds.

The N atoms are sp2 hybridized with the third unhybridized p orbital forming
the
h d
double
bl b
bonds.
d

The P atoms are sp3 hybridized with a 3d orbital forming double bonds.
P
N
P
N
p-dbonding in phosphazanes
2
2. Group 16 (6A) - The Chalcogens
O, S, Se- Selenium (all non metals), Te- Tellurium (metalloid), Po- Polonium
(metal, radioactive).







Chalcogens “copper producing” appropriate because copper sulfide is a primary
copper
pp ore.
Selenium (greek for moon) and tellurium (earth) are found in conjunction with
the coinage metals (copper, silver, gold).
Oxygen was discovered by Priestly in the early 1770’s
Tellurium in the early 1780’s and polonium at the very end of the 19th Century by
Pierre and Marie Curie.
Polonium-210 has a half life of 138.4 days compared to radium-226 which is 1600
years.
Polonium is named after Marie’s homeland.
Twenty seven polonium isotopes, none of them stable are known, but only
polonium-210 has been produced in sufficient quantities (mg’s) for chemical
investigations.
Marie Curie
Through her discovery of radium, Marie Curie paved the
way for nuclear physics and cancer therapy. Born of Polish
parents, she was a woman of science and courage,
compassionate yet stubbornly determined. Her research
work was to cost her her life.

In her pioneering way, Marie Curie decided, in 1897, to take a physics doctorate. Henri
Becquerel, who was studying X-rays, had recently observed that uranium salt left an
impression on a photographic plate in spite of its protective envelope. What better subject
could there have been for Marie than to try and understand the effect, the energy of these
uranic rays? Pierre consented. And so his frail wife set about her work, handling tons of
minerals; she noted that another substance, thorium, was "radioactive", a term she herself
had coined. Together, they demonstrated in a major discovery that radioactivity was not the
result
lt of
f a chemical
h i l reaction
ti b
butt a property
t of
f th
the element
l
t or, more specifically,
ifi ll of
f th
the
atom. Marie then studied pitchblende, a uranic mineral in which she measured a much more
intense activity than is present in uranium alone. She deduced that there were other
substances besides uranium that were very radioactive, such as polonium and radium, which
she discovered in 1898.
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
In their experiments, Pierre observed the properties of the radiation while
Marie, for her part, purified the radioactive elements.

Both shared the same, uncanny tenacity, which was all the more admirable given
their deplorable living conditions. Their laboratory was nothing more than a
miserable hangar,
g , where in winter the temperature
p
dropped
pp to around six
degrees.

One chemist commented that "it looked more like a stable or a potato cellar".
And yet, Marie admitted that "one of our pleasures was to enter our workshop at
night; then, all around us, we would see the luminous silhouettes of the beakers
and capsules that contained our products".

D
Despite
it th
their
i diffi
difficulty
lt att obtaining
bt i i any advances
d
or lloans, M
Marie
i and
d Pi
Pierre
Curie refused to file a patent application that would have secured them
financially; in their eyes, enabling any scientist, French or foreign, to find
applications for radioactivity took priority.

Pierre tested radium on his skin. It caused a burn, and then a wound: its effect
on man was thus proven. Soon radium was being used to treat malign tumours:
Curietherapy was born.

In 1903, Marie defended her thesis. Together with Becquerel, the Curies were
awarded the Nobel Prize for Physics for their discovery of natural radioactivity.
Their
h
happiness
h
was short
h
lived.
l d In 1906,
1 06 Pierre,
P
weakened
k
d by
b radiation
d
and
d
overworked, was run over by a car. Marie was forced to continue alone. She took
charge of educating her two children; she took up the position which her
husband had finally obtained at the Sorbonne, and thus became the first woman
to be appointed professor there.

She also had to fight the prejudices of her day: hatred of foreigners and sexism
which, in 1911, p
prevented her from entering
g the Academy
y of Science. And yet,
y
soon after, she was honored with a Nobel Prize for Chemistry for determining
the atomic weight of radium. But her real joy was "easing human suffering". The
founding of the Radium Institute by the University of Paris and the Pasteur
Institute in 1914 would enable her to fulfill her humanitarian wish.
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
Group 16 – tend to have the negative oxidation states.

Stable oxidation states from -2 to +6 (all have -2 and S and Po only have +6).

The two most common elements are O and S.
Allotropes of O and S
Oxygen has two allotropes:
1.
O2 molecular oxygen.
yg
2.
O3 ozone.
O

O
O
O O
O=O
unstable form it is a strong oxidizer
it kills any life form in the water (e.g. bacteria)
in the ozone layer the O2 molecules are under low pressure
so it breaks up into O atoms that recombine to give O3 molecules
which we call the ozone layer.
Allotropes of Sulfur - the stable ones at room temperature have no double
bonds.

S8 is the stable allotrope at room temperature, it is mined in “salt mines” and
it is yellow and non-metallic.
Sulfur was known to ancient civilisations, and referred
to in Genesis as brimstone.
Sulfur exists as several allotropes, of which
orthorhombic sulfur S8 is the most stable at room temp.
It is a pale yellow, brittle, odorless solid.
In the crystals comprised of these molecules, S
form two S-S bonds. The lone pairs of electrons
make the S-S-S bend (108 deg), resulting in S8
having the shape of a crown. At 298 K, rhombic
sulfur is stable. On heating, sulfur vapor, S8, S6, and
S2 molecules are present.
What happens at when the solid sulfur melts? The
S8 molecules break up. When suddenly cooled, long
chain molecules are formed in the plastic sulfur
which, behave as rubber. Plastic sulfur transforms
into rhombic sulfur over time.
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S
S
S
S
S
S
S
melts at 1200C
S8 (liquid)
via monoclinic
opaque yellow
solid
S
continue heating
S8 solid
rhombic translucent
yellow, solid
S6, S12, ((etc))
1600C
long chains of S causes the viscous liquid - intertwining chains
viscosity of the liquid to
S
S
S
S S
increase
S4, S3, S2 (gas)
S
S
S
S
S=S-S
ice water
continue
heating
S=S
rubbery substance
Plastic sulfur (dark, blackish)
stretchable long fibers
rings predominate
only at very high temperatures (6000C) do double bonds
form, that is the allotropes are more stable without double
bonds at room temperature.
Sulfides and Sulfur Compounds


Sulfur - a commodity
Sulfur is recovered by the Frasch process. This process has made sulfur a high
purity (up to 99.9 percent pure) chemical commodity in large quantities.

Natural Sources of Sulfur

Most sulfur containing minerals are metal sulfides,
sulfides and the best known is
perhaps pyrite, (FeS2, known as fools gold because of its golden color). The most
common sulfate containing mineral is gypsum, CaSO4.2H2O, also known as plaster
of paris.
Mining Method - Frasch process
Frasch process force (99.5% pure) sulfur out by using
hot water and air. In this process, superheated water is
forced down the outer most of three concentric pipes.
pipes
Compressed air is pumped down the center tube, and a
mixture of elemental sulfur, hot water, and air comes up
the middle pipe. Sulfur is melted with superheated water
(at 170 degrees C under high pressure) and forced to the
surface of the earth as a slurry.
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Applications

Sulfur is mostly used for the production of sulfuric acid, H2SO4.

Most sulfur mined by Frasch process is used in industry for the manufacture of
sulfuric acid. Sulfuric acid, the most abundantly produced chemical in the United
States, is manufactured by the Contact process.

Most (about 70%) of the sulfuric acid produced in the world is used in the
fertilizer industry.

Sulfuric acid can act as a strong acid, a dehydrating agent, and an oxidizing
agent. It's applications use these properties.

Sulfur is an essential element of life in sulfur-containing proteins.
Reactions of Sulfur

Reading the following reactions, figure out and notice the change of the oxidation
state of S in the reactants and products. Common oxidation states of sulfur are 2, 0, 4, and 6.

Sulfur (brimstone, stone that burns) reacts with O2 giving a blue flame:

S + O2

SO2 is produced whenever metalsulfide is oxidized. It is recovered and oxidized
SO2 (anhydride of H2SO3)
further to give SO3, for production of H2SO4. SO2 reacts with H2S to form H2O
and S.

2SO2 + O2
2SO3 (anhydride of H2SO4)
SO3 + H2O
H2SO4 <- a valuable commodity
SO3 + H2SO4
H2S2O7 <- pyrosulfuric
lf i acid
id

Sulfur reacts with sulfite ions in solution to form thiosulfate,

S + SO32-

but the reaction is reversed in an acidic solution.
S2O32-,
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S-N Compounds

S2N2 –disulfur dinitride.

The nature of the bonding in N2S2 (and other N-S compounds) is far from
S
N
N
S
obvious.

Is 'N2S2 is said to be aromatic.
aromatic Discuss'. The answer is meant to be that the
molecule is planar and possesses six electrons in orbitals of p symmetry, which
implies some similarity to the benzene molecule. In actual fact, nothing could be
further from the truth.

So what is the correct bonding and structure of N2S2? According to Greenwood
and Earnshaw, the geometry of the N2S2 molecule is, indeed, almost exactly a
square, in spite of the disparity in the sizes of the S and N atoms (b): The S-N
bond lengths
lengths, determined from X-ray
X ray diffraction studies,
studies are 165
165.11 and 165
165.7
7 pm
pm,
the S-N-S bond angle is 90.4°, while the N-S-N angle is 89.9°.

At room temperature, N2S2 readily polymerizes to form (SN)x, which is metallic
(c).


At very low temperatures (0.33K), the polymer becomes superconducting.
This polymer is comprised of adjacent N2S2 units un which bonds have been
rearranged to form a nearly flat chain.

Again the S-N bonds are intermediate in length, implying complete
delocalization of the electron density along the axis of the polymer.

This material acts as a one dimensional material, i.e. it is a golden bronze
metallic color when viewed from one side but flat black when viewed end on. In
addition it conducts electricity as well as mercury but only along the polymeric
axis.

When it is cooled to absolute zero it loses its electrical resistance entirely and
becomes a superconductor.

These compounds are currently the subject of intense research.
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
Various derivatives of these compounds have been prepared in great numbers in
the last few years. For example cyclothiazenes (analogous to the
cyclophosphenes) (NSCl)3
p-d overlap between N and S creates
the double bond
Cl
S
Cl
N
N
S
S
N
sp3 hybridized
Cl
sp2 hybridized
The Cl derivative has delocalized S-N bonds,
but the F analogue has unequal bond lengths implying more localized S
S-N
N and S=N bonds..
bonds

Tetrasulfur tetranitride S4N4, is an easily detonated orange-yellow solid at room
temperature (a).

Its structure remained unknown for more than a century after it was first
prepared.

It has a structure reminiscent of S8 (see earlier)
earlier). All the S-N
S N bonds are
intermediate in length between single (1.74Å) and double (1.54Å) bonds, the latter
involving a p-d bond, implying considerable delocalization around the ring.

If S4N4 is heated in a vacuum and passed over silver wool, colorless explosive
crystals of S2N2 can be trapped by rapidly cooling the vapor.
2S4N4 + 8 Ag
250-300 0C
4Ag2S(s) +2N2(g) + 2S2N2(s)
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