Covalent or Network Solids

When covalent bonds form continuously between adjacent atoms throughout a crystal, the whole crystal becomes effectively one enormous molecule. Such covalent (network) solids are therefore also called giant molecules.

Because covalent bonds are strong and directional, the atoms are locked firmly in position. This makes network solids:

• Very hard and brittle,
• possessing extremely high melting points (some even decompose before melting),
• and behaving as insulators that do not conduct electricity.

Diamond (as in Fig. 1.3, a rigid three-dimensional network in which each carbon is covalently bonded to four neighbours in a tetrahedral arrangement) and silicon carbide are typical examples.

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Graphite — the exception

Graphite (Fig. 1.4) belongs to the same class, yet it is soft and a conductor of electricity — the opposite of what we expect. This is due to its special layered structure. The carbon atoms lie in flat layers, and within a layer each atom is covalently bonded to only three of its neighbours, forming hexagonal sheets. The fourth valence electron of each atom lies between the layers and is free to move, and it is these mobile electrons that make graphite a good conductor. Moreover, the layers can slide over one another, which makes graphite soft and an excellent solid lubricant. …