Write the definition of valence band and conduction band.

Explanation

Some solids have a crystalline structure means there is a systematic arrangement of atoms or molecules in it.
When atoms are arranged close to each other the atom gets interact with neighbouring atoms and other atoms as a result energy of electrons of atoms are changed.
The energy level of inner shells electrons are not affected, but the energy levels of the outer shells electrons (valence electron) are changed, since these electrons are shared, by more than one atom in the crystal.
The electrons of the atoms in the crystal has closely spaced different energy levels instead of widely separated energy level of electrons of isolated atom. Such a band of close energy levels is called energy band.
The valence electrons (in last counter orbit) are called as valence electrons in incomplete orbit. The energy band which includes the energy levels of the valence electrons is called the valence band.
The energy band above the valence band is called the conduction band.
Usually a valence band has a valence electron while no electron in conduction band.
There is some gap between the conduction band and the valence band. The energy difference of this empty space is called band gap energy `(E_(g))`.
When the minimum energy level of the conduction band is lower than the higher energy level of the valence band, the electrons in the valence band can easily move to the conduction band. This happens in metals (conductors).
If there is some gap between the conduction band and the valence band, electrons in the valence band all remain bound and no free electrons are available in the conduction band. This makes the material an insulator.
If the distance between the conduction band and the valence band is relatively small and if some of the electrons in the valence band get external energy crosses band gap energy and goes into the conduction band then there is a possibility of flow in conduction band as well as the electron going from the valence band in the conduction band there is a possibility of conduction due to empty spaces in the valence band. This is only possible in the semiconductor.