Substances are composed of bands. A band is a group of molecular orbitals, the energy differences between them are so small that the system behaves as if a continuous, non-quantization of energy within the barrier is possible.
Materials consists of a valence band and a conduction band separated by a band gap. A band gap occurs when the energy difference between two bands is significant.
The magnitude of band gap determines whether a material will be a metal, nonmetal or metalloid.
Metals have a very little band gap hence they are able to conduct electricity more effectively than other materials, such as ionic and covalent substances.
