JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Classification of Elements and Periodicity

Electron gain enthalpy becomes less negative down a group because the added electron enters a larger, more diffuse shell and is held less tightly. Fluorine breaks the trend: its $2p$ subshell is very compact, so the incoming electron meets strong repulsion from the electrons already crowded there and less energy is released than its small size alone would suggest. Chlorine accepts the electron into a roomier $3p$ subshell with much less repulsion and so releases the most energy of all the halogens. The order of the magnitudes is $\mathrm{Cl}>\mathrm{F}>\mathrm{Br}>\mathrm{I}$.