4.2.2 The significance of the average energy
The average thermal energy of the atoms in a solid indicates how much they are 'rattling' or vibrating around their mean positions. Since the atoms are close together, virtually touching, and because atoms are almost incompressible, they cannot get much closer. But they can get further apart. So, since thermal energy is manifested in the vibrations of the atoms, bigger vibrations mean that the atoms must spend more time further apart. On average then, there is a tendency for a solid to expand when it is heated, as we have already discussed. It follows too that as thermal energy is removed, atoms on average vibrate less and spend more time closer together, and bulk material can then be expected to contract on cooling.