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  • Briefly describe the valence bond theory of covalent bond formation by taking an example of hydrogen. How can you interpret energy changes taking place in the formation of dihydrogen?

Briefly describe the valence bond theory of covalent bond formation by taking an example of hydrogen. How can you interpret energy changes taking place in the formation of dihydrogen?

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Valence bond theory was introduced by Heilter and London in 1927. It was developed by Pauling. It is based on knowledge of atomic orbitals, electronic configuration of elements, overlapping of atomic orbitals and hybridization of orbitals.

Let us consider two hydrogen atoms A and B. Let’s assume that they are approaching each other and let their nuclei be NA and NB and electrons present in them be eA and eB.

Now let us imagine that the two atoms are at a large distance from each other, hence, there is no interaction between them. Now think that they are coming close to each other, and new attractive and repulsive forces begin to operate.

Now we know that attractive forces arises between:

  1. Nucleus of one atom and its own electron that is N_{A}-e_{A} and N_{B}-e_{b}.
  2. Nucleus of one atom and electron of other atom, i.e., N_{A}-e_{B},N_{B}-e_{A}, NB-eA. Similarly, repulsive forces arise between

(a) Electrons of two atoms like e_{A}-e_{B}

(b) Nuclei of two atoms N_{A}-N_{B}.

It has been found that the magnitude of the new attractive force is more than the new repulsive forces. As a result, two atoms approach each other and potential energy decreases. Eventually the net force of attraction balances the force of repulsion and system acquires minimum energy. At this point two hydrogen atoms are said to be bonded together to form a stable molecule having bond length 74pm.

Since the energy is released when the bond is formed between two hydrogen atoms, the hydrogen molecule is more stable than that of the isolated hydrogen atoms. The energy so released is called the bond enthalpy, which is corresponding to minimum in the curve depicted in figure. Conversely, 435.8kJ of energy is required to dissociate one mole of H2 molecule.

H_{2}(g)+435.8 \; kJ\; mol^{-1}\rightarrow H(g)+H(g).

The potential energy curve for the formation of H2 molecule as a function of internuclear distance of H atoms. The minimum in the curve corresponds to the most stable state of H2.

 

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