The band model of solids is an extended of the molecular orbital (MO) method. The overlap of atomic orbitals in an extended solid produces rise to continuous bands of electronic energy levels associated with different degrees of bonding. In a simple monatomic solid the bottom of the band is build up of orbitals bonding between all neighboring atoms; orbitals at the top of the band are antibonding, and levels in the middle have an intermediate bonding character. Different atomic orbitals may, in principle, give rise to different bands, although they may overlap in energy.

Fig. 1. Band picture for (a) nonmetallic and (b) metallic solid; occupied electronic levels are shown shaded.

Increases in temperature approximately in accordance with the Arrhenius equation used in chemical kinetics.