Question

How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?

Answer 1

Electronic configuration of Na and Mg are 

Na = 1s² 2s² 2p⁶ 3s¹ 

Mg = 1s² 2s² 2p⁶ 3s² 

First electron in both cases has to be removed from 3s-orbital but the nuclear charge of Na (+11) is lower than that of Mg (+12) therefore first ionization energy of sodium is lower than that of magnesium.

After the loss of first electron, the electronic configuration of 

Na = 1s² 2s² 2p⁶ 

Mg = 1s² 2s² 2p⁶ 3s¹ 

Here electron is to be removed from inert (neon) gas configuration which is very stable and hence removal of second electron requires more energy when compared to Mg. 

Therefore, second ionization enthalpy of sodium is higher than that of magnesium.