Valence Bond Theory | Physical Chemistry for NEET PDF Download

• As we know that Lewis approach helps in writing the structure of molecules but it fails to explain the formation of the chemical bond. It also does not give any reason for the difference in bond dissociation enthalpies and bond lengths in molecules like H₂ (435.8 kJ mol^-1, 74 pm) and F₂ (155 kJ mol^-1, 144 pm), although in both cases a single covalent bond is formed by the sharing of an electron pair between the respective atoms. It also gives no idea about the shapes of polyatomic molecules.

• Similarly, the VSEPR theory gives the geometry of simple molecules but theoretically, it does not explain them and also it has limited applications. To overcome these limitations the two important theories based on quantum mechanical principles are introduced. These are the valence bond (VB) theory and molecular orbital (MO) theory.

Valence Bond Theory

• Valence bond theory was introduced by Heitler and London (1927) and developed further by Pauling and others.  

VB Theory for formation of Hydrogen Molecule

• A discussion of the valence bond theory is based on the knowledge of atomic orbitals, electronic configurations of elements (Units 2), the overlap criteria of atomic orbitals, the hybridization of atomic orbitals and the principles of variation and superposition. 

To start with, let us consider the formation of a hydrogen molecule which is the simplest of all molecules:

• Consider two hydrogen atoms A and B approaching each other having nuclei N_A  and N_B and electrons present in them are represented by e_A and e_B. 
• When the two atoms are at a large distance from each other, there is no interaction between them. 
• As these two atoms approach each other, new attractive and repulsive forces begin to operate.
• Attractive forces arise between:
  (i) The nucleus of one atom and its own electron, i.e. N_A – e_A and N_B– e_B
  (ii) The nucleus of one atom and electron of the other atom, i.e. N_A– e_B and N_B– e_A
• Similarly, repulsive forces arise between:
  (i) electrons of two atoms like e_A – e_B
  (ii) nuclei of two atoms N_A – N_B
• Attractive forces tend to bring the two atoms close to each other whereas repulsive forces tend to push them apart.
  Forces of attraction and repulsion during the formation of H₂ molecule
• Experimentally it has been found that the magnitude of new attractive force is more than the new repulsive forces. 
• As a result, two atoms approach each other and potential energy decreases. Ultimately a stage is reached where the net force of attraction balances the force of repulsion and the system acquires minimum energy. 
• At this stage, two hydrogen atoms are said to be bonded together to form a stable molecule having a bond length of 74 pm.
• Since the energy gets released when the bond is formed between two hydrogen atoms, the hydrogen molecule is more stable than that of isolated hydrogen atoms. The energy released is known as bond enthalpy, which is corresponding to the minimum in the curve depicted in the graph below. 
• Conversely, 435.8 kJ of energy is required to dissociate one mole of H₂ molecule.

H₂(g) + 435.8 kJ mol^–1 → H(g) + H(g)

Potential energy curve for the formation of H₂ molecule as a function of the internuclear distance of the H atoms