ABSTRACT
Solids 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. This chapter discusses simplified models solids which represent idealised categories. In ionic solids, the ‘entities’ that make up the solid are essentially ions, atoms stripped of one or more of their electrons, or with one or two more added. In covalent solids, electrons, which in isolated atoms occupied quantum states localised around a single atom, occupy orbitals that are distributed around at least one other atom in addition to their ‘parent’ atom: literally ‘co-valence’. The directionality of the covalent bond means that solids with covalent bonds tend to form crystals with quite different structures from those formed by molecular solids.
