Modern inorganic chemistry

164 GROUP IV

Diamonds

Diamonds are found in South Africa, India, South America and

Russia. The largest ever found was the Cullinan diamond which

weighed about 600 g. The structure is as shown in Figure 8.2. (There

are four possible crystalline arrangements all of which are found to

occur naturally.) The interatomic bonds are very strong (mean

thermochemical bond energy 356kJmol~^1 ). This high bond

strength is reflected in the great hardness and high melting point of

diamond. Diamond also has a high refractive index and is the

densest form of carbon (density 3.5gem"^3 ). The many uses of

diamond are largely dependent on its great hardness, for example

for cutting and grinding.

Very small synthetic diamonds have been made industrially by

subjecting graphite to pressures in the range 5.5-6.9 GNm~^2 , at

temperatures between 1500 and 2700 K. The diamonds produced

are very small but competitive with natural diamonds for use in

industrial cutting and grinding wheels.

Graphite

Graphite occurs naturally in Ceylon, Germany and the USA. It was
formerly mined in Cumberland. Its name (Greek, grapho = I write)
indicates its use in lead' pencils. The structure of graphite is indi-
cated in Figure 8.2. Each carbon atom is joined to three others by
six bonds, the arrangement being trigonal planar. The remaining
electron on each carbon atom is in a p orbital. A sideways overlap
of these orbitals occurs to give a delocalised n bond. It is this second
bond which reduces the C—C bond distance in graphite compared
with that found in diamond. The delocalised n bond readily explains
the conductivity and colour of graphite, properties absent in
diamond which has no such delocalised bonding. The planes of
carbon atoms are held together by van der Waal's forces which are
much weaker than either a or n bonding and allow the planes to
slide over each other. Graphite is consequently anisotropic and
much research has been carried out in attempts to produce large
single crystals. Graphite manufactured on a large scale by the
Acheson process, in which coke containing a little silica is heated in
an electric furnace in the absence of air for many hours, does not
produce large crystals. Single crystals of graphite, almost free from
defects, have been produced by striking an electric arc between
carbon rods. These "whiskers' have very high tensile strength along
the planes of carbon atoms but are very brittle.
So-called 'carbon fibres' have been produced by the controlled