<P> Ionic bonding is common between pairs of atoms, where one of the pair is a metal of low electronegativity (such as sodium) and the second a nonmetal of high electronegativity (such as chlorine). </P> <P> A chlorine atom has seven electrons in its 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) is + 328.8 kJ per mole of chlorine atoms . Adding a second electron to chlorine requires energy, energy that cannot be recovered by 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). </P> <P> A sodium atom has a single electron in its outermost electron shell, the first and second shells again being full with two and eight electrons respectively . To remove this outer electron requires only the first ionization energy, which is + 495.8 kJ per mole of sodium atoms, a small amount of energy . By contrast, the second electron resides in the deeper second electron shell, and the second ionization energy required for its removal is much larger: + 4562.4 kJ per mole . Thus sodium will, in most cases, form a compound in which it has lost a single electron and have a full outer shell of eight electrons, or octet . </P> <P> The energy required to transfer an electron from a sodium atom to a chlorine atom (the difference of the 1st ionization energy of sodium and the electron affinity of chlorine) is small: + 495.8 − 328.8 = + 167 kJ mol . This energy is easily offset by the lattice energy of sodium chloride: − 787.3 kJ mol . This completes the explanation of the octet rule in this case . </P>

Sodium has one electron in its outer energy level. according to the octet rule