Explain the formation of H₂ molecule on the basis of valence bond theory.
The valence bond theory, introduced by Heitler and London and developed further by Pauling, explains covalent bond formation in terms of the overlap of atomic orbitals. To consider the formation of the simplest molecule, the H molecule, we look at two hydrogen atoms, each with a singly occupied 1s orbital.
As the two hydrogen atoms approach each other, the overlap of the 1s orbitals occurs. The potential energy of the system gets lowered as the two H atoms come near to each other. Because of this orbital overlap, the electron density between the nuclei increases, which helps in bringing them closer.
At the equilibrium inter-nuclear distance (bond distance), the energy touches a minimum. Any attempt to bring the nuclei still closer results in a sudden increase in energy and consequent destabilization of the molecule. Since energy gets released when the bond is formed, the hydrogen molecule is more stable than the isolated hydrogen atoms.
The energy released is called bond enthalpy, which corresponds to the minimum in the potential energy curve. For the H molecule, the bond enthalpy is and the bond length is . Conversely, of energy is required to dissociate one mole of the H molecule into isolated hydrogen atoms.
Explanation
The answer explains the formation of the H2 molecule using the valence bond theory as described in the textbook. It covers the overlap of singly occupied 1s orbitals, the decrease in potential energy as atoms approach, the equilibrium distance, the increase in electron density between nuclei, and the concept of bond enthalpy and bond length.