Definition of Valence Bond Theory
Valence Bond theory states that the electrons which are present in a molecule tend to occupy atomic orbitals rather than molecular orbitals. During the formation of a bond, the atomic orbitals overlap each other. If the region over which the atomic orbitals are overlapping is large, the strength of the bond is very high and hence the stability of the molecule is increased.
The unpaired electrons present in the valence shell help in the formation of multiple bonds with other atoms. On the contrary, the paired ones do not participate in bonding. The bonds are directional i.e. the pi-bonds are formed from the sidewise overlapping while the sigma bond is formed due to the overlapping of orbitals along the axis.