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

Explanation

Electronic configuration of Na is `1s^(2),2s^(2)2p^(6)3s^(1)` . After losing one electron from its outermost shell sodium easily attains stable electronic configuration ( `1s^(2),2s^(2)2p^(6)` ) while magnesium does not lose its electron easily due to presence of two electrops in s-orbital (`1s^(2),2s^(2)2p^(6) 3s^(2)`). Hence first ionisation energy of sodium is less than magnesium. When one electron is removed from Na and Mg their configurations become`1s^(2),2s^(2)2p^(6)` and `1s^(2),2s^(2)2p^(6) 3 s^(1)` respectively. Now it is casicr to remove one electron from 3s of `Mg^(+)` than `2p^(6)` of `Na^(+)` . Hence, second ionisation energy of Mg is less than Na.