We envisage a solid as a collection of atoms whose average positions are fixed with respect to one another. When we studied the properties of gases we essentially ignored the potential energy of interaction of the atoms, but in solids we cannot do this: solids exist because of the potential energy of interaction between atoms. The atoms of a solid can vibrate about their average position, but can only exceptionally change their position with respect to their neighbours. This picture will probably be familiar to you, but just in case it is not, Figure 6.1 illustrates how we imagine the motion of the atoms in a solid.

Figure 6.1 An illustration of the motion of atoms in a solid. The arrows indicate the direction of atomic motion. Notice the small separation between the atoms and the random orientation of their vibrations. The atoms themselves are shown as a central darkly-shaded region, where the electron charge density is high, and a peripheral lightly-shaded region. The electric field in this peripheral region significantly affects the motion of neighbouring atoms, and disturbs the electronic charge density of neighbouring atoms.

When we discussed the properties of gases we were able to arrive at the theory of a ‘ideal gas’, which for many purposes was a good approximation to the properties of real gases. Solids, however have many fewer properties that can be explained in terms of a theory of an ‘ideal solid’. The diversity of properties exhibited by solids calls for us to make several simple models to serve as starting points for attempts to understand the behaviour of real solids. Despite the diversity in the properties of solids, it is important to realise that in all the materials, the only force acting between atoms is the electrostatic coulomb force. The coulomb force, coupled with the different configurations of electrons in the outer parts of the 100 or so different types of atoms, is sufficient to produce solids with the diversity that you find around you. In this chapter, we will discuss four simplified models solids which represent idealised categories. However, most real solids do not fall neatly into one category or another. Our hope is that by looking at a few (rather rare) ‘simple solids’ which do fit this categorisation scheme, we will be able to shed light on what is happening in more common, but more complex, solids.