(a) Metallic Conductors :
In case with metallic conductors, the lowest level in the conduction band happens to be lower than the highest level of the valence band, the electrons from the valence band can easily move into the conduction band. Normally the conduction band is empty. But when it overlaps on the valence band electrons can move freely into it. Some conductors, the conduction band is partially filled and the valence band is partially empty which allows electrons from its lower level to move to higher level making conduction possible.
(b) Insulators :
If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are available in the conduction band. This makes the material an insulator (Fig. (b)).
