JEE Main · 2021 · 26 Feb 2021 · Shift 2 · Classification of Elements and Periodicity

Down group 16 the atom grows larger, the incoming electron is added further from the nucleus and less energy is released, which puts $\mathrm{S}$ above $\mathrm{Se}$ above $\mathrm{Te}$. Oxygen breaks the trend because its $2p$ subshell is compact: the added electron enters a small region already crowded with electrons and the resulting repulsion cancels much of the energy released, so oxygen has the least negative electron gain enthalpy in the group. The order, from most negative to least negative, is $\mathrm{S}>\mathrm{Se}>\mathrm{Te}>\mathrm{O}$.