Energy bands and band gaps

The underlying idea behind the electronic structure of solids is that the valence electrons from the atoms involved spread throughout the entire structure, ie molecular orbitals are generally extended over all the constituent atoms. A large number of overlapping atomic orbitals lead to molecular orbitals very similar in energy over a certain range. This forms an almost continuous band. These bands are separated by band gaps, which are the energy values where there are no orbitals. (fig. 1)

This continuous band arises from the molecular orbitals being slightly different in energy, which arises from the different degrees of bonding in each. At the bottom of the band you have the lowest energy MO which has all bonding character. At the top, with highest energy is an MO with all anti-bonding character. Therefore the rest of the band is formed from all the MO’s with intermediate bonding character between the two extremes. (fig. 2)