The attractive force which holds various constituents (atoms, ions, etc.) together in different chemical species is called a chemical bond. The formation of chemical compounds takes place as a result of combination of atoms in different ways. The atoms interact with each other on account of following reasons : (i) Lowering of energy of combining atoms : The process of bonding between the atoms decreases the energy of the combining atoms and gives rise to the formation of a system which has lower energy and hence has greater stability. (ii) Lewis octet rule : Octet theory of valency or electronic theory of valency states that in the formation of a chemical bond, atoms interact with each other by losing, gaining or sharing of electrons so as to acquire a stable outer shell of eight electrons. The tendency of atoms to achieve eight electrons in their outermost shell is known as Lewis octet rule.