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Which of the following sequences is incorrectly arranged with respect to the property mentioned alongside it?

1. Li < Na < K < Rb: Increasing metallic radius
2. I < Br < F < Cl: Increasing electron gain enthalpy (with negative sign)
3. B < C < N < O: Increasing first ionization enthalpy
4. Al3+ < Mg2+ < Na+ < F−: Increasing ionic size

Correct answer: I < Br < F < Cl: Increasing electron gain enthalpy (with negative sign)

Solution

The sequence for increasing electron gain enthalpy with negative sign is not correctly written as given. Halogens show an exception: chlorine has a more negative electron gain enthalpy than fluorine due to greater electron-electron repulsion in the compact 2p subshell of fluorine.

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