CBSE Class 12-science Answered

Band theory describes those ranges of energy called energy bands that an electron may have within the solids. It shows the behavior of electrons in solids by postulating the existence of energy bands. It is used to explain physical properties of solids such as electrical conductivity.
The band formed by a series of energy levels and contain valence electrons is known as valence band.
The band formed by the electrons which leave the valence band is known as conduction band. These electrons are responsible for the conduction of electricity.
The gap between the energies of conduction and valence band is known as forbidden energy gap.
In conductors, the conduction band and valence band overlap each other. It results in high electrical conductivity of metals (conductors). In insulators, the energy gap between conduction and valence band is large and the electrons in the valence band cannot reach the conduction band easily. This results in poor electrical conductivity.