Physical Chemistry/Valence Bond Theory

Introduction

The Valence Bond Theory was proposed by Heitler and London (1927) and further explained by Pauling et. al..

According to this theory, atoms with unpaired electrons tend to combine with other atoms which also have unpaired electrons. In this way, the unpaired electrons are paired up and the atoms involved attain a stable electronic arrangement. The spins on two electrons however, must be opposite (antiparallel) because of the Pauli Exclusion Principle.

The basic idea of the theory is that a bond between two atoms is formed when two electrons with their spins paired are shared by two overlapping atomic orbitals, one orbital from each of the atoms joined by the bond. This electron pair becomes concentrated in the region of overlap and help the nucleus bind together. The amount of decrease in potential energy when the bond is formed is determined in part by the extent to which the orbitals overlap.

Orbital Overlap Concept

According to the above theory, a covalent bond is formed between two atoms in a molecule, when a half-filled valence atomic orbitals (AOs) of the two atoms containing unpaired electron overlap with one another and the electrons pair up in the overlapping region. Due to this overlapping process, electrons are localized between the atoms in the bond region. When the attractive forces (electron-nucleus of two different atoms) are stronger than the repulsive forces (electron-electron or nucleus-nucleus), energy is released, lowering the energy of the bonded atoms and increasing the molecule's stability.

Types of Overlapping and Nature of Covalent Bonds

The following bonds are observed based on type of overlap. Covalent bonds are classified into two main types: sigma ($\sigma$) bonds and pi ($\pi$) bonds.