Explain the formation `H_(2)` molecule on the basis of Valence Bond theory.

Explanation

This theory explains the shapes of covalent molecules as well as the directions of the bonds in them.
Important postulates :
1) Covalent bond is formed by the overlap of two atomic orbitals.
2) The overlapping orbitals contain unpaired electrons with opposite spins.
3) Each of the bonded atoms retains its own atomic orbitals. But the electron pair in the overlapping orbitals is shared by both the atoms.
4) The greater the extent of overlap of orbitals, the stronger is the covalent bond formed.
5) Except 's' orbitals, the remaining all atomic orbitals are directional. So the bonds formed due to their overlap are also directional. This determines the shape of the molecule.
6) When the atomic orbitals of the two atoms overlap, then electron density increases on their internuclear axis. This binds the two atoms, firmly. Thus the molecule is stabilised.
Ex : Formation of Hydrogen molecule (s - s overlap) :
Hydrogen molecule is formed due to overlap of 1 s - orbitals of two hydrogen atoms. The bond formed between the two H - atoms is called `sigma_(s-s)` bond and the type of overlapping is s - s overlap.