JEE Main 2021 — Classification of Elements and Periodicity in Properties Question with Solution

The ionization energy of the elements increases from left to right in the periodic table. The first ionization of magnesium is greater than that of aluminium because magnesium has a stable complete electronic configuration $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^2$, so it requires more energy to remove the first electron from it, whereas aluminium has electronic configuration $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^1$ which need lesser energy than that of magnesium. Similarly, phosphorus has more ionisation energy than sulphur due to the half filled electronic configuration in the phosphorus.

$\mathrm{Mg} \mathrm{Al} \mathrm{P} \mathrm{S}\to \mathrm{IE} .\mathrm{order} \Rightarrow \mathrm{Al}<\mathrm{Mg}<\mathrm{S}<\mathrm{P}$