A chlorine atom has seven electrons in its third and outer electron shell, the first and second shells being filled with two and eight electrons respectively. The first electron affinity of chlorine (the energy release when chlorine gains an electron to form Cl−) is 349 kJ per mole of chlorine atoms. Adding a second electron to form a hypothetical Cl2- would require energy, energy that cannot be recovered by the formation of a chemical bond. The result is that chlorine will very often form a compound in which it has eight electrons in its outer shell (a complete octet), as in Cl−.