Valence Bond Theory (VBT) :
This approach was developed by Heitler and London in 1927 and further improved by Pauling and others.
1. Understanding VBT involves the knowledge of atomic orbitals, electronic configurations of elements, the overlap criteria of atomic orbitals and the principles of variation and superposition.
2. It is well known fact that all the mechanical system in this universe tend to lower their potential energy so that they can attain the greater stability. Same is the situation involved with the formation of bonds between the atoms; bonding occurs with decrease in energy.
3. In order to understand this concepts let us study the formation of H2 molecule on the basis of electrostatic interactions which lead to decreases of energy.
4. As we know that hydrogen atom has one electron which is present in its 1s-orbital. Let us assume two hydrogen atoms, HA and HB with their corresponding electrons eA and eB When the two atoms are at large distance from each other, there is no interaction between them.
The following new electrostatic interaction are set up:
(1) Attractive force between
-Electron eA and nucleus HB
-Electron eB and nucleus HA
(2) Repulsive force between
-Electron eA and electron eB
-Nucleus HA and nucleus HB
The magnitude of new attractive forces is more that the new repulsive forces. Two atoms start approaching each other and potential energy decreases. Ultimately, a state is reached where attractive forces, just balance the repulsive forces. This state is reached when the atoms are at critical distance r0. At a distance greater than r0, the attractive forces are dominant whereas at a distance smaller than r0, the repulsive forces dominate over the attractive forces which leads to increase of potential energy. maximum lowering of energy takes place at critical distance r0.
Therefore , the H atoms constitute a stable grouping called hydrogen molecule. The critical distance r0 corresponding to minimum energy is called bond length. Experiments have shown that the value of r0 is 74 pm. The energy corresponding to the minimum in the curve is called bond energy/enthalpy.