Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : The ionic radii of \(O^{2–}\) and \(Mg^{2+}\) are same.
Reason (R) : Both \(O^{2–}\) and \(Mg^{2+}\) are isoelectronic species.
In the light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is not the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Answer 1

The Correct Option is D

To address the question, we need to analyze both the assertion and the reason provided, and determine the relationship between them in the context of ionic radii and isoelectronic species.

Assertion (A): The ionic radii of O^{2–} and Mg^{2+} are the same.

Reason (R): Both O^{2–} and Mg^{2+} are isoelectronic species.

Analysis:

1. Isoelectronic Species: Two species are isoelectronic when they have the same number of electrons. O^{2–} and Mg^{2+} both have 10 electrons, thus they are isoelectronic.

2. Ionic Radii:

• The size of an ion is affected by the nuclear charge (the number of protons). Despite having the same number of electrons, the nuclear charge of Mg^{2+} is greater than that of O^{2–} due to more protons in magnesium than oxygen.
• This increased nuclear charge results in a stronger attraction between the nucleus and the electron cloud in Mg^{2+}, making it smaller in size compared to O^{2–}.

Conclusion:

The statement in the assertion (A) that the ionic radii of O^{2–} and Mg^{2+} are the same is false. However, the reason (R) is true because both ions are indeed isoelectronic, having the same number of electrons (10), but this does not result in them having the same ionic radii.

Therefore, the correct answer is:

(A) is false but (R) is true.