Question

The first ionisation enthalpy of sodium is lower than that of magnesium but it's second ionisation enthalpy is higher than that of magnesium. explain?

Answer 1

This is because after losing 1 electron, sodium attains the stable noble gas configuration of neon (1s2 2s2 2p6) . On the other hand, magnesium, after losing 1 electron still has one electron in the 3s-orbital(1s2 2s2 p6 3s1). In order to attain the stable noble gas configuration, it still has to lose one more electron. Thus, the energy required to remove the second electron in case of sodium is much higher than that required in case of magnesium. Hence, the second ionization enthalpy of sodium is higher than that of magnesium.