VALENCE BOND THEORY

The valence bond theory was proposed by Heitler and London in 1927 to explain the nature of covalent bond which was later modified by Pauling and Slater in 1931 to explain the directional characteristics of covalent bond.
The various postulates of valence bond theory are:

1. A covalent bond is formed when an atom of half filled orbital overlaps with the other atom which is also half filled.
2. The combining orbitals merge together to form a localised molecular orbital in which both the electrons occupy the place.Two types of covalent bond include-sigma(σ) and pi(π) bond.
3. The two electrons which occupy the same localised molecular orbital must have opposite spins.
4. The localised molecular orbital is said to belong to the electrons of both the combining atoms.
5. Maximum electron density lies some where in the overlapped region.The electrostatic force binds together the covalently bonded atoms which is attractive in nature.
6. Another force responsible for the formation of covalent bond is neutralisation of spin magnetic moment of the two electrons which take part in bond formation.

Limitations of valence bond theory;

1. There certain case like formation of hydronium ion(H₃O+) in which filled 2p-orbital of the central oxygen atom of H₂O overlaps with the vacant 1s-orbital of H+ ion to form [H₂O➡O]+ ion.
2. According to valence bond theory B₂ and O₂ are diamagnetic but in real they are paramagnetic because there are two unpaired electrons.
3. This theory does not explain bond length,bond angle etc, For example-In methane (CH₄) molecule all bond length are found to be equal experimentally.According to valence bond theory ,three C-H bond length are equal (s-p σ bond) and fourth C-H bond length is not same(s-s σ bond).                                                                    Tarbej Ansari