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

To determine the correctness of the statements given, we should evaluate each one based on known chemical principles and periodic trends.

1. Statement I: The second ionisation enthalpy of Na is larger than the corresponding ionisation enthalpy of Mg.
   • The first ionisation enthalpy removes one electron, and for sodium (Na), this electron is removed from a singly filled 3s orbital, resulting in the Na⁺ ion, which has a full 2s and 2p subshell, making it very stable (like the noble gas neon).
   • The second ionisation enthalpy involves removing an electron from this stable Na⁺ ion, making it exceedingly difficult because of the full subshell and stable electronic configuration. Therefore, the second ionisation enthalpy of Na is indeed very high.
   • For magnesium (Mg), the first ionisation removes an electron to form Mg⁺, which still has valence electrons left. The second ionisation removes one more electron to form Mg²⁺, which corresponds to the stable electronic configuration of neon.
   • Given these configurations, the second ionisation enthalpy of Na is indeed larger than that of Mg because the electrons removed from Mg⁺ are still within the same energy shell (3s), whereas removing an electron from Na⁺ disrupts a filled shell. Hence, Statement I is true.
2. Statement II: The ionic radius of O²⁻ is larger than that of F⁻.
   • Both O²⁻ and F⁻ are formed by gaining electrons to achieve stable configurations similar to the noble gas neon, but they originate from different atoms with different original electron counts.
   • Oxygen gains 2 electrons to form O²⁻, resulting in a configuration of [He] 2s² 2p⁶. Fluorine gains 1 electron to form F⁻, also achieving a configuration of [He] 2s² 2p⁶.
   • However, for O²⁻, there is increased electron-electron repulsion due to the additional electron compared to F⁻. This repulsion leads to a greater ionic radius for O²⁻ compared to F⁻. Hence, Statement II is true.

Therefore, both Statement I and Statement II are correct. The correct option is: Both Statement I and Statement II are true.