NEET 2026 Chemistry Question 53 Solution

Electron gain enthalpy (Δ_eg H) is the enthalpy change when an isolated gaseous atom in its ground state gains an electron to form a gaseous uninegative ion. For halogens, this process is highly exothermic.

• • Negative Δ_eg H indicates energy release.
• • Trend question compares magnitude (exothermicity).

Although electronegativity decreases uniformly from F to I, the electron gain enthalpy of fluorine is unexpectedly less negative than that of chlorine. In fluorine (n=2), the incoming electron enters the compact 2p subshell where it experiences strong repulsive forces from the other seven valence electrons. In chlorine (n=3), the incoming electron enters the larger 3p subshell where interelectronic repulsions are substantially weaker.

• • Δ_eg H of Cl = -349 kJ mol⁻¹.
• • Δ_eg H of F = -328 kJ mol⁻¹.

Beyond chlorine, as we descend to bromine and iodine, the increase in atomic size and nuclear shielding causes electron affinity to steadily decrease.

• • Trend: Cl > F > Br > I.
• • Matches Option (3).