Valence bond theory ( V.B.T ).

Modern concept of covalency: Valence bond theory ( V.B.T ).

What is valence bond theory ?

In 1927,Heitler and London published their modern concept about covalent bond,depending on quantum mechanics,which is known as valence bond theory ( V.B.T ).

Later on, the valence bond theory has been developed mostly by Linus Pauling and J.C Slater.

 Postulates of valence bond theory.

( I) According to valence bond theory the formation of a covalent bond takes place due to the overlap of valence shell atomic orbitals containing unpaired electrons.

(II) The amount of energy due to overlapping of half filled atomic orbitals is a measure of stability of covalevt bond.

The strength of covalent bond depends on the extent of overlapping area of participating orbitals.

( III ) The direction of participating atomic orbitals predict the direction of covalent bond formed.

( IV ) The number of unpaired electrons in the valence shell of an atom is equal to valency of that element.

( V ) The potential energy decreases as the electron pair belongs to both nuclei and the molecule attains stability.

( VI ) Only the half filled atomic orbitals of one atom overlap with the half filled valevce shell atomic orbitals of other atom with similar symmetry and energy.

Formation of H2 molecule : Explanation by valence bond theory.

H2-molecule is formed by the interaction of two H-atom. Each H-atom contain one electron. The electrons are paired up during bonding.
When two H-atom approach to each other four probability may be happen.

(I) Attraction of nuclei with its own electron.

( II ) Attraction between nuclei of one H-atom with the electron of other H-atom.

(III) Repulsion between two nuclei of two H-atom.

 (IV) Repulsion between electrons of two H-atom.

It has been experimentally found that the value of summation of two attraction force is greater than the summation of two repulsive force.

As a result, the two H-atom gradually approaches to each other and their potential energy slowly decreases .

After a certain time, the attraction force is equal to repulsive force. At that time,the system attain lowest energy level.

Under this condition the two H-atom forms stable H2-molecule by releasing some amount of energy.This energy is called bond energy.

At that time, the distance between two nuclei is 0.74 Å . This is called bond length.

H(g) + H (g) ⟶ H2 (g) +435.8 k.j/mol 

Potential energy vs internuclear distance

Summary :

Postulates of valence bond theory .

Formation of hydrogen molecule and valence bond theory .

What is SNi reaction ?  Click here