(1) Metals are good conductors of heat and electricity. This is explained on the basis of band theory which involves the presence of free electrons.
(2) According to band theory, the atomic orbitals of metal atoms overlap to form molecular orbitals which are spread all over the crystal structure.
(3) The energy difference between adjacent molecular orbitals decreases as the number of molecular orbitals increases and when it becomes very less, the orbitals merge into one another forming continuous bands which extent over the entire crystal.
(4) There are two types of bands of molecular orbitals as follows :
- Valence band : The atomic orbitals with filled electrons from the inner shells form valence bands, where there are no free mobile electrons since they are involved in bonding.
- Conduction band : Atomic orbitals which are partially filled or empty on overlapping form closely placed molecular orbitals giving conduction bands where electrons are delocalised and can conduct, heat and electricity.
(5) Band gap :
- The energy difference between valence band and conduction band is called band gap.
- Band gap decides whether electrons from valence band can be promoted to vacant conduction band or not.
- The conductors like metals have very small or no band gap and electron can be promoted by thermal energy. The nonconductors have large band gap. The insulators have very large band gap.
