We pass at once from gases to solids. In gases the atoms and/or molecules are almost completely free; in solids they are almost completely lacking in mobility. Indeed in a solid the thermal motion is so greatly reduced that individual atoms can move from their fixed positions only with the greatest difficulty. It is this which imparts to solids their most characteristic macroscopic property – they maintain whatever shape they are given, they have appreciable stiffness. Although the atoms are in fixed locations they possess some thermal energy – in some cases individual atoms can diffuse through the solid but in general the process is extremely slow. Their main thermal exercise consists in vibrating about an equilibrium position.

Types of solids

There are three main types of solids: crystalline, amorphous and polymeric. The first part of this chapter will deal with crystalline solids; amorphous solids will be described briefly at the end while a separate chapter (Chapter 13) will be devoted to polymers.

Crystalline solids

The main feature here is long-range order. The molecules or atoms are in regular array over extended regions within each individual grain or crystal. With large single crystals the regular array may extend over an enormous number of atoms or molecules. For example with a single crystal of copper of side 1 cm, along any one direction there will be 30 000 000 copper atoms in regular array, the arrangement of the last few being (in the ideal case) in perfect step with the first.