This theory was put forwarded by Hitler and London in 1931 A.D. This theory is based on the orbital picture of wave mechanics.
Assumption of Valance bond theory:
- A covalent bond is formed by the overlapping of half filled orbital of one atom with the half filled orbital of another atom.
- The half filled orbital should have their electron with opposite spin.
- During the overlapping of atomic orbital, pairing of the electrons takes place and there is also decrease in the energy of the bonded atom.
- The greater is the extent of overlapping, the stronger is the covalent bond formation.
- Depending upon the nature of atomic orbital involved and the extent of overlapping, the covalent bond can be divided into two types: Sigma bond and pi bond.
Limitation of valence bond theory:
- This theory does not take into account the effect of non-bonding electrons on the overlap between the two orbitals.
- This theory does not explain the diamagnetic and paramagnetic properties of a compound.