Question

Given below are two statements:
Statement (I): The radius of isoelectronic species increases in the order: \[ \text{Mg}^{2+} < \text{Na}^{+} < \text{F}^{-} < \text{O}^{2-} \] Statement (II): The magnitude of electron gain enthalpy of halogens decreases in the order: \[ \text{Cl} > \text{F} > \text{Br} > \text{I} \] In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement I and Statement II are correct.
• B. Statement I is incorrect but Statement II is correct.
• C. Statement I is correct but Statement II is incorrect.
• D. Both Statement I and Statement II are incorrect.

Hint:

For isoelectronic species, the size decreases as the nuclear charge increases. Electron gain enthalpy becomes more negative as the atomic size increases.

Answer 1

The Correct Option is C

The provided problem requires the evaluation of two statements concerning Chemistry, specifically focusing on isoelectronic species and the trends in electron gain enthalpy.

Statement I Analysis:

Isoelectronic species share an identical electron count but exhibit distinct nuclear charges. The radius within an isoelectronic series is modulated by the effective nuclear charge, which escalates with an increment in protons. Consequently, atomic size diminishes as nuclear charge increases. For the series \(\text{Mg}^{2+}, \text{Na}^{+}, \text{F}^{-}, \text{O}^{2-}\), the stated order of increasing size is accurate, attributable to the variable nuclear charge's influence on electron contraction:

\[\text{Nuclear charge}: \text{Mg}^{2+} > \text{Na}^{+} > \text{F}^{-} > \text{O}^{2-} \]

Conclusion: Statement I is validated.

Statement II Analysis:

Electron gain enthalpy denotes the energy transformation upon the addition of an electron to a neutral atom. For halogens, the typical trend, influenced by increasing atomic size, electron-electron repulsion, and effective nuclear attraction, is \(\text{F} > \text{Cl} > \text{Br} > \text{I}\). However, despite fluorine's high electronegativity, chlorine possesses a more favorable electron gain enthalpy due to reduced electron repulsion at its larger atomic radius:

\[\text{Cl} > \text{F} > \text{Br} > \text{I} \]

Conclusion: Statement II is invalidated as it contradicts the actual trend.

Final Assessment: Statement I is correct, while Statement II is incorrect.