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In which of the following arrangements the order is NOT according to the property indicated against it?

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

Correct answer: Al3+ < Mg2+ < Na+ < F− Increasing ionic size

Solution

The order of ionic sizes for Al3+, Mg2+, Na+, and F− is incorrect because Al3+, being a cation with a higher positive charge, is actually smaller than Mg2+, which is smaller than Na+ and larger than the anion F−. Thus, the arrangement does not reflect the expected trend of increasing ionic size.

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