The first ionisation energy of magnesium is greater
than that of sodium, whereas the reverse is true for
second ionisation energy. Explain.

Explanation

Magnesium has two valence electrons, so like all other alkaline earth metals magnesium also forms stable bivalent ions. The second ionization energy of Mg is higher than the first but close to it as loss of both electrons will result in inert gas configuration. As for sodium there is only one valence electron. After loss of one electron, the next electron lies in a completely filled `2p^6` subshell. Looking at the electronic configurations of Mg and Na to make things clearer: Mg: `1s^2 2s^2 2p^6 3s^2" and Na: " 1s^2 2s^2 2p^6 3s^1`