Chemistry, Third edition

5 · MORE ABOUT BONDING

Diamond

Diamond is a transparent solid which does not conduct electricity. In diamond, each

carbon atom forms single covalent bonds to four other carbon atoms, which are at

the corners of a tetrahedron. The four carbon atoms are, in turn, covalently bonded

to four other carbon atoms and so on. This bonding exists throughout the entire

crystal. A part of this arrangement is shown in Fig. 5.8. When all the atoms in a crystal

are covalently bonded to one another throughout the whole crystal, the solid is

termed a network solid.

Diamond is a very hard substance because the carbon–carbon single bonds are

very strong and extend throughout the whole structure. In order to melt diamond,

many of these bonds need to be broken and this requires a high temperature. This

explains why diamond has a very high melting point (3823 K).

Graphite

Graphite is another allotrope of carbon and another example of a

network solid. Graphite is a soft, blackish solid that conducts

electricity (unusual for a covalently bonded substance!). In

graphite, there are layers of carbon atoms. Within each layer,

each carbon atom is covalently bonded to three others in a trigo-

nal planar arrangement (Fig. 5.9).

Although the covalent bonds between the carbon atoms within

the layers are strong, the layers are held together by weak forces

(London dispersion forces) and can slide over one another easily.

Graphite therefore has a ‘slippery’ feel and is used as a lubricant

and as the so-called ‘lead’ in pencils. The pencil marks paper

because the layers of graphite are easily rubbed off. Each carbon

atom uses only three of its valency electrons in bonding to three

other carbon atoms and therefore has one valency electron left

over. These electrons form a cloud above and below the layers,

rather like that in the metallic bond. The free electrons allow

graphite to conduct electricity and account for its shiny appear-

ance. Soot consists of small crystals of graphite.

Buckminsterfullerene

Unlike diamond and graphite, which have been known for many

centuries, this third allotrope of carbon has been discovered only

recently (1985). The allotrope has the formula C 60 and, although it cannot be classi-

fied as a giant molecule in the same way as diamond and graphite are giant

molecules, it is still much larger than other molecular solids such as S 8 or P 4. The

shape of C 60 resembles domes designed by the American architect Buckminster

Fuller, hence its name. The arrangement of carbon atoms also resembles a football

(Fig. 5.10), with twenty hexagons and twelve pentagons; because of this a more

informal name for the allotrope is sometimes used – bucky ball. Buckminster-

fullerene is a dark solid at room temperature and dissolves in covalent solvents, such

as benzene. Research still continues into its properties. Chemical derivatives of the

allotrope are believed to have great potential for a number of applications, including

lubricants, batteries and semiconductors. Three scientists – Harry Kroto, Richard

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Fig. 5.8The arrangement of
carbon atoms in diamond.

Fig. 5.10The arrangement of carbon atoms in
buckminsterfullerene: (a) is the arrangement of the
carbon atoms; (b) is a football for comparison.

Fig. 5.9The arrangements of carbon atoms in
graphite.