Band theory

Band theory explains the electrical properties of solid metals, nonmetals and metalloids in terms of bands -- vast numbers of closely spaced electronic energy levels. Just as the interaction of two atomic orbitals produces two molecular orbitals (MOs), the interaction of the enormous number of atomic orbitals of the atoms in a solid produces an equally enormous number of MOs, so closely spaced in energy that they merge into a continuous band. Band theory rests on three ideas:

i. Conduction band : The highest-energy band containing electrons is the conduction band. It is formed from the outermost energy levels of the solid's atoms and may be partially filled or entirely vacant. Electrons in the conduction band are delocalised over the whole solid -- they are the mobile charge carriers.

ii. Valence band : The band of energies lower than the conduction band is the valence band. Its electrons remain bound to their respective nuclei and are not free to move. …