Lowering of energy takes during bond formation. How does this happen?

i. When two combining atoms approach each other to form a covalent bond, the following interacting forces come into play.

• Forces of attraction : The nucleus of one atom attracts the electrons of the other atom and vice-versa.
• Forces of repulsion : The electron of one atom repels the electron of the other atom and vice-versa (as electrons are negatively charged). There is repulsion between the two nuclei (as the nuclei are positively charged).

ii. The balance between attractive and repulsive forces decide whether the bond will be formed or not.

iii. When the magnitude of attractive forces is more than the magnitude of repulsive forces, the energy of the system decreases and a covalent bond is formed.

iv. When the magnitude of repulsive forces becomes more than that of attraction, the total energy of the system increases, and a covalent bond is not formed.

Hence, 

Lowering of energy takes during bond formation.