Among the following, the correct trend in electron gain enthalpy (most negative first) is:

Solution & Explanation

Concept — Electron gain enthalpy (Δ_egH) is the energy released when an atom in the gas phase gains an electron; "most negative" means most energy released. Down a group it normally becomes less negative (larger atom, weaker attraction), but the smallest member of period 2 is anomalous. For the halogens (Group 17): on adding an electron to F, the incoming electron enters the very small, compact 2p subshell where electron–electron repulsion is unusually high. This repulsion offsets much of the energy released, so fluorine's electron gain enthalpy is LESS negative than chlorine's. Chlorine (3p) is larger, so the added electron feels less repulsion yet still strong nuclear attraction → Cl has the MOST negative electron gain enthalpy of the group. Beyond Cl, size keeps increasing and attraction weakens, so the trend resumes its normal decrease: Cl > F > Br > I. Hence the correct order (most negative first) is Cl > F > Br > I — option C. (Trap: the "expected" group trend F > Cl > Br > I ignores fluorine's small-size anomaly and is wrong.)