Q. 625.0( 1 Vote )

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 :

The valence bond theory is based on the knowledge of atomic orbitals and electronic configurations of elements, overlap criteria of atomic orbitals and stability of the molecule.

The main points of the valence bond theory are:


(i) Atoms do not lose their identity even after the formation of the molecule.


(ii) The bond is formed due to the interaction of only the valence electrons as the two atoms come close to each other. The inner electrons do not participate in bond formation.


(iii) During the formation of a bond, only the valence electrons from each bonded atom lose their identity. The other electrons remain unaffected.


(iv) The stability of the bond is accounted for by the fact that the formation of the bond is accompanied by release of energy. The molecule has minimum energy at a certain distance between the atoms known as inter-nuclear distance. Larger the decrease in energy, stronger will be the bond formed.


Consider two hydrogen atoms A and B approaching each other having nuclei H and H and the corresponding electrons e- and e- respectively.


When atoms come closer to form molecules new forces begin to operate.


(a) The force of attraction between the nucleus of the atom and electron of another atom.


(b) The force of repulsion between two nuclei of the atom and electron of two atoms.


When two hydrogen atoms are at a farther distance, there is no force operating between them, when they start coming closer to each other, the force of attraction comes into play and their potential energy starts decreasing. As they come closer to each other potential goes on decreasing, but a point is reached when potential energy acquires minimum value.


(a) This distance corresponding to this minimum energy value is called the distance of maximum possible approach, i.e. the point which corresponds to minimum energy and maximum stability.


(b) If atoms come further closer than this distance of maximum possible approach, then potential energy starts increasing and the force of repulsion comes into play and molecules start becoming unstable.


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