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?

Explanation

Valence bond theory (VBT) was introduced by Heitler and London (1927) and developed further by Pauling and other. VBT is based on the knowledge of atomic orbitals, electronic configuration of elements, the overlap criteria of atomic orbitals, the hybridisation of atomic orbitals and the principles of variation and superposition.
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 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) electrons of two atoms like `e_(A)-e_(B)`
ii) nuclei of two atoms like `N_(A) - N_(B)`
Attractive forces tend to bring the two atoms close to each other whereas repulsive forces tentot push them apart.
Experimentally, we have 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.
Hence, a stage is reached, where hte net force of attraction balances the force of repulsion and system acquires minimum energy. At this stage, two H-atoms are said to be bonded together to form a stable molecule having the 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 so released is called as bond enthalpy, which is corresponding to minimum in the curve depicted in the given figure. Conversely `435.8` kJ of energy is required to dissociate one mole of `H_(2)` molecule.
`H_(2)(g) + 435.8kJmol^(-1) to H(g) + H(g)`

the potential energy curve for the formation of `H_(2)` molecule as a function of internuclear distance of the H-atoms. The minimum in the curve corresponds to the most stable state or `H_(2)`.