JEE Main · 2020 · 03 Sep 2020 · Shift 2 · Classification of Elements and Periodicity

Successive ionisation enthalpies climb gently while electrons come from the same outermost shell, then jump sharply once the next electron must be pulled from an inner, tightly held shell. Here the first three values, $800$, $2427$ and $3658{\text{kJ mol}}^{-1}$, rise steadily, but the fourth leaps to $25024{\text{kJ mol}}^{-1}$. The large gap after the third removal shows that only three electrons occupy the valence shell, so the element has $3$ valence electrons.