Quantum mechanics is essential to make sense out of the properties of solids. Some of the most important properties of solids were already discussed in chapter 6. It is a good idea to review these sections before reading this chapter.
The discussion will remain restricted to solids that have a
crystal structure. In a crystal the atoms are packed
together in a regular manner. Some important materials, like glass
and plastic, are amorphous, they do not have such a regular crystal
structure, and neither do liquids, so not all the ideas will apply to