In this section, we will study about another kind of bonding called covalent bonding. Covalent bonding is helpful in understanding the formation of molecules. In lesson 2, you studied that molecules having similar atoms such as H₂, Cl₂, O₂, N₂ etc. are constituents of elements whereas those containing different atoms like HCl, CO₂ etc. are constituents of compounds. Let us now see how are these molecules formed?
We will begin with the formation of hydrogen molecule (H₂). The hydrogen atom has one electron. It can attain the electronic configuration of the noble gas helium by sharing one electron of another hydrogen atom. When the two hydrogen atoms come closer, there is an attraction between the electrons of one atom and the proton of another and there are repulsions between the electrons as well as the protons of the two hydrogen atoms. In the beginning, when the two hydrogen atoms approach each other, the potential energy of the system decreases due to the force of attraction. The value of potential energy reaches a minimum at some particular distance between the two atoms. If the distance between the two atoms further decreases, the potential energy increases because of the forces of repulsion. The covalent bond forms when the forces of attraction and repulsion balance each other and the potential energy is minimum. It is this lowering of energy which leads to the formation of the covalent bond.

Potential energy diagram

We will next consider the formation of chlorine molecule (Cl₂). A molecule of chlorine contains two atoms of chlorine. Now how are these two chlorine atoms held together in a chlorine molecule?
You know that the electronic configuration of Cl atom is 2,8,7. Each chlorine atom needs one more electron to complete its octet. If the two chlorine atoms share one of their electrons as shown below, then both of them can attain the stable noble gas configuration of argon as shown below.