Explanation based on valence bond theory:
According to valence bond theory (VBT), a covalent bond is formed by the overlap of half-filled atomic orbitals of two atoms, with the paired electrons having opposite spins.
Electronic configuration of hydrogen atom:
Each hydrogen atom has one electron in its 1s orbital:
H = 1s1
Thus, each hydrogen atom has one half-filled orbital.
Orbital overlap and bond formation:
When two hydrogen atoms approach each other, their 1s atomic orbitals overlap axially. This overlap leads to the pairing of the two electrons with opposite spins.
The overlap of two 1s orbitals forms a sigma (σ) bond, which is the strongest type of covalent bond.
Energy consideration:
During overlap, the potential energy of the system decreases due to attraction between the nuclei and the shared electron pair. At an optimum internuclear distance, the energy becomes minimum, resulting in a stable H2 molecule.
Nature of bond:
• The bond formed is a single covalent σ-bond
• Both hydrogen atoms attain a stable duplet configuration
Conclusion:
Thus, according to valence bond theory, the H2 molecule is formed by the axial overlap of half-filled 1s orbitals of two hydrogen atoms, resulting in a stable covalent bond.