According to Valence bond theory , covalent bond formation between the atoms when they have half-filled valence atomic orbitals, each containing a single unpaired electron.
By this theory a covalent bond is formed between two atom by the overlapping of half filled valence atomic orbital..

Valence bond (VB) theory is one of basic theories, developed to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed.According to this theory a covalent bond is formed between the two atoms by the overlap of half filled valence atomic orbitals of each atom containing one unpaired electron.