Topic: Ionization enthalpy and electron gain enthalpy· Classification of Elements and Periodicity in Properties

In which of the following options the order of arrangement does not agree with the variation of property indicated against it ?

1. \(Al^{3+} < Mg^{2+} < Na^{+} < F^{-}\) (increasing ionic size)

2. \(B < C < N < O\) (increasing first ionisation enthalpy)

3. \(I < Br < Cl < F\) (increasing electron gain enthalpy)

4. \(Li < Na < K < Rb\) (increasing metallic radius)

Doubtly Asked 30 Oct 2026 NEET UG 2016 Phase 1 Practice test neet Save WhatsApp Report

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Answer: 3. \(I < Br < Cl < F\) (increasing electron gain enthalpy)

Key idea: compare each series with the real trend.

Option 1: isoelectronic ions shrink as nuclear charge rises, so \(\mathrm{Al^{3+}<Mg^{2+}<Na^+<F^-}\). Agrees.
Option 4: metallic radius increases down group 1. Agrees.
Option 3: electron gain enthalpy of Cl is more negative than that of F (small F has strong inter-electronic repulsion), so the true order is I < Br < F < Cl. This does not agree.

The key answer is option 3. Honestly, option 2 is also imperfect since the real first ionisation enthalpy order is B < C < O < N, but the key follows the electron gain enthalpy anomaly of option 3.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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In which of the following options the order of arrangement does not agree with the variation of property indicated against it ?

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