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Arrange the halogens F (Z=9), Cl (Z=17), Br (Z=35), and I (Z=53) in decreasing order of their electron gain enthalpy (magnitude).

1. F > Cl > Br > I
2. Cl > F > Br > I
3. Br > Cl > I > F
4. I > Br > Cl > F

Correct answer: Cl > F > Br > I

Solution

Cl has the highest electron gain enthalpy (most negative) because F's small 2p shell causes excessive repulsion upon gaining an electron. Beyond Cl, electron gain enthalpy decreases down the group: Cl > F > Br > I.

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