Valence Bond Theory

Valence Bond Theory

Valence Bond Theory is a qualitative method for predicting the behaviour of electrons in bonding. It focuses on the overlap of the outermost orbital where the value electrons reside. Electrons are thought to be concentrated along the intern clear axis, causing a density of negatively charged electrons between both atoms. Attracted by the nuclei, these shared electrons pull together their respective atoms to which the electrons belong, and the result is formation of a covalent bond. In the Following diagram, formation of a diatomic molecule is shown;

These hydrogen atoms come because of the electrostatic attractive between the nuclei and the electron density between them. Based on the principle of Valence Bond Theory, two electrons are required to make a single bond. Hydrogen has one valence electron, so two hydrogen atoms are needed to make a bond. This allows accurate prediction about the shaped of simple molecule. In case of H₂, the shape is simply linear. The valence bond theory was proposed by Heitler and London to explain of formation of covalent bond quantitatively using quantum mechanics. Later on, Linus Pauling improved this theory by introducing the concept of hydrogen of hybridisation.

The main postulates of this theory are following;

1.   A covalent bond is formed by the overlapping the two half filed valence atomic orbitals of two different atoms.

2.   The electrons in the overlapping orbitals get paired and confined between the nuclei of two atoms.

3.   The electron density between two bonded atoms increases due to overlapping. This confers stability to the molecule.

4.   Greater the extent of overlapping, stronger is the bond formed.

5.   The direction of the covalent bond is along is along the region of overlapping of the atomic orbitals so that it can be said that covalent is directional in nature.