Question

Given below are two statements:
Statement I: The second ionisation enthalpy of Na is larger than the corresponding ionisation enthalpy of Mg.
Statement II: The ionic radius of $O^{2-$ is larger than that of $F^-$.}

• A. Statement I is true but Statement II is false
• B. Statement I is false but Statement II is true
• C. Both Statement I and Statement II are true
• D. Both Statement I and Statement II are false

Hint:

For isoelectronic species, the anion with the smallest nuclear charge (Z) has the largest ionic radius.

Answer 1

The Correct Option is C

Statement I: $IE_2(Na)$ involves removing an electron from the stable $2p^6$ noble gas configuration ($\text{Na}^+$). $IE_2(Mg)$ involves removing a $3s^1$ electron from $\text{Mg}^+$. $IE_2(Na)>IE_2(Mg)$. True.
Statement II: $O^{2-}$ (Z=8) and $F^-$ (Z=9) are isoelectronic (10 electrons).
Ionic radius decreases as effective nuclear charge ($Z_{eff}$) increases. Since $Z_{eff}(O^{2-})<Z_{eff}(F^-)$, $r(O^{2-})>r(F^-)$. True.