We know, that covalent bond between the two atoms is formed by mutual sharing of their valence electrons so as to complete their octet. According to orbital concept when two atoms approach each other for bonding their half-filled atomic orbitals overlap. Overlapping of atomic orbitals means that a part of electron cloud of each of the two half-filled atomic orbitals become common. The probability ·of finding the shared electrons in the region of overlap is maximum than at other places.
Let us sum up various postulates of orbital concept regarding the formation of covalent bonds.
• Covalent bonds are formed by the overlapping of half filled atomic orbitals present in the valence ‘shell of the atoms participating in bonding.
• The orbitals undergoing overlapping must have electrons with opposite spins.
• The strength of a covalent bond depends upon the extent of overlapping. The greater the overlapping, more is the energy released and consequently, stronger will be the covalent bond.