atomic orbitals, the hybridization of atomic orbitals and the principles of variation and superposition. A rigorous treatment of the VB theory in terms of these aspects is beyond the scope of this book. Therefore, for the sake of convenience, valence bond theory has been discussed in terms of qualitative and non-mathematical treatment only. To start with, let us consider the formation of 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 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) nucleus of one atom and its own electron that is N A – e A and N B – e B .
(ii) nucleus of one atom and electron of other atom i.e., N A – e B , 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 (Fig. .
). 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 system acquires minimum energy.
At this stage two hydrogen atoms are said to be bonded Fig. . Forces of attraction and repulsion during the formation of H molecule together to form a stable molecule having the bond length of 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