Energy bands and band gaps

Different types of orbitals form separate bands. Therefore you can end up with an s band and a p band. Whether or not they form two distinct bands with a band gap, or overlap depends on the separation of the orbitals and how strong the interaction between the atoms is. Strong interaction means wide bands and a greater chance of overlap. (fig. 3)

The distinction between metallic and non-metallic solids comes from the way the orbitals are filled. Metallic behaviour arises from a partially full band as then there is no gap between the top filled level (Fermi level) and the lowest empty one. However, a non-metallic solid has a completely filled level (the valence band) and an empty one (the conduction band). These two bands are separated by a band gap. (fig. 4). In the filled band every electron is matched by another so you get no overall net motion of electric charge. Therefore, for conduction to occur electrons have to be excited up to the conduction band by overcoming an activation energy and hence, the conduction of these compounds increases with temperature.