JEE Main · 2025 · 04 Apr 2025 · Shift 2 · Classification of Elements and Periodicity

The first ionisation enthalpy of group 13 does not decrease smoothly down the group, because the poor shielding provided by the intervening $d$ and $f$ electrons leaves the heavier atoms with a raised effective nuclear charge. Boron, being the smallest with the highest effective nuclear charge, has the greatest first ionisation enthalpy, and the values run $\mathrm{B}>\mathrm{Tl}>\mathrm{Ga}>\mathrm{Al}>\mathrm{In}$, so indium has the lowest. The elements with the highest and lowest first ionisation enthalpies are boron and indium respectively.