Question

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?

Answer 1

Definition of Valence bond theory

• It is based on the knowledge of atomic orbitals, electronic configurations of elements, overlapping of atomic orbitals and hybridization of orbitals.

Explanation of Valence bond theory

• Consider two hydrogen atoms A and B approaching each other having nuclei NA and NB and electrons present in them are represented by eA and eB. When two atoms are at 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:

1. Nucleus of one atom and its own electron i.e., $NA-eA\text{and}NB-eB$.
2. Nucleus of one atom and electrons of another atom i.e., $NA-eB,NB-eA$.

• Similarly, repulsive forces arise between:
  

1. Electrons of two atoms i.e., $eA-eB$
2. Nuclei of two atoms i.e., $NA-NB$.

Forces of attraction and repulsion during formation of $H_2$ molecule.

• Attractive forces tend to bring the two atoms close to each other whereas repulsive forces tend to push them apart. Experimentally it has been found that the magnitude of the new attractive forces 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 the bond length of $74pm$.

The potential energy curve for the formation of $H_2$ molecules as a function of the inter-nuclear distance of the $H$ atoms. The minimum in the curve corresponds to the most stable state of $H_2$.