The five successive ionisation energies of an element $\mathrm{X}$ are 800, 1427, 2658, 25024 and $32824 \mathrm{kJ} {\mathrm{mol}}^{-1}$ respectively. The valency of $\mathrm{X}$ is

The element ($\mathrm{X}$) has three valence electrons because after losing three electrons, it gains a stable inert gas configuration, and this is because the difference between ${\mathrm{I}}_3$ and ${\mathrm{I}}_4$ is maximum, i.e. there is a severe change in the ionisation from third to fourth.