The electronic configuration of chlorine is 2, 8, 7. So the atomic number of the chlorine is 17. chlorine requires 1 electron to complete its octet as chlorine has 7 electrons in its outer most shell. It gains this electron by sharing with other chlorine atom. so each atom share there one free electron with each other to complete there octet. So two chlorine atom share there one electron so the fore of attraction between them is very high. This fore of attraction is known as covalent bond. Now, each chlorine atom in the chlorine molecule has the electronic configuration 2,8,8 resembling its nearest inert gas argon. Since the chlorine atoms in a chlorine molecule have inert gas electron arrangements, therefore, a chlorine molecule is more stable than two separate chlorine atoms.