The Foundations of Chemistry

mobile. As a result, covalent solids are usuallypoor thermal and electrical conductors at
ordinary temperatures. (Diamond, however, is a good conductor of heat; jewelers use this
property to distinguish diamonds from imitations.)
An important exception to these generalizations about properties is graphite, an
allotropic form of carbon. It has the layer structure shown in Figure 13-32c. The overlap
of an extended -electron network in each plane makes graphite an excellent conductor.
The very weak attraction between layers allows these layers to slide over one another
easily. Graphite is used as a lubricant, as an additive for motor oil, and in pencil “lead”
(combined with clay and other fillers to control hardness).

It is interesting to note that the

allotropes of carbon include one very

hard substance and one very soft

substance. They differ only in the

arrangement and bonding of the C

atoms.

13-16 Bonding in Solids 527

Figure 13-31 The packing
arrangement in a molecular crystal
depends on the shape of the
molecule as well as on the
electrostatic interactions of any
regions of excess positive and
negative charge in the molecules.
The arrangements in some
molecular crystals are shown here:
(a) carbon dioxide, CO 2 ; (b)
benzene, C 6 H 6.

(a)

(b)