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Select the correct order of the magnitude of electron gain enthalpy (energy released when X(g) + e⁻ -> X^-(g)) for halogen atoms X:

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

Correct answer: Cl > F > Br > I

Solution

Despite F being the most electronegative element, its electron gain enthalpy is less than Cl's because F's very small 2p orbitals are already crowded, causing greater interelectronic repulsion when an extra electron is added. Cl has the highest magnitude electron gain enthalpy among halogens.

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