What is VB theory

Valence bond theory was proposed by Heitler and London in 1927 based on atomic orbitals and their configuration.

A covalent bond is formed when pure, valence atomic orbital of one atom overlaps with another pure, valence atomic orbital of another atom. During covalent bond formation energy releases that means stability increases.

Each of the overlapping orbitals contains the unpaired electron of opposite spin.

The electron pair is shared by both the atoms. The strongest bond is formed when the orbitals of the two atoms overlap to the maximum extent.

Based on the overlapping of orbitals, two types of covalent bonds are formed. These are known as sigma and pi bonds.

Sigma bonds are formed by the end-to-end overlap of atomic orbitals along the inter-nuclear axis known as a head-on or axial overlap.

End on over lapping is of three types, they are s-s overlapping, s-p overlapping and p-p overlapping.