Question

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

Answer 1

electronic configuration of Na=1s2,2s2, 2p6, 3s1.

electronic configuration of Mg=1s2,2s2, 2p6, 3s2

s subshell of na is half filled whereas s subshell of mg is full filled.
full filled subshells are more stable than half filled subshells
hence it is easy to remove 1 electron from Na.

therefore its first ionisation enthalpy is low.

after losing 1 electron

electronic configuration of Na=1s2,2s2, 2p6

electronic configuration of Mg=1s2,2s2, 2p6, 3s1

as mentioned above full filled are more stable than half filled.

so it is easy to remove 1 electron from mg as compared to Na.

hence second ionisation enthalpy of Na is greater than Mg.

hope it helps you