Question: 63: Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A):
In a particular point defect, an ionic solid is electrically neutral, even if few of its cations are missing from its unit cells.
Reason (R):
In an ionic solid, Frenkel defect arises due to dislocation of cation from its lattice site to interstitial site, maintaining overall electrical neutrality.
In the light of the above statements, choose the most appropriate answer from the options given below:
(1) (A) is correct but (R) is not correct
(2) (A) is not correct but (R) is correct
(3) Both (A) and (R) are correct and (R) is the correct explanation of (A)
(4) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
Answer: Option (4)
Explanation:
The assertion refers to a point defect in which some cations are missing from their lattice sites,
yet the ionic solid remains electrically neutral.
This describes a Schottky defect, where equal numbers of cations and anions are missing from the lattice, maintaining overall electrical neutrality.
The reason correctly describes a Frenkel defect, in which a cation leaves its normal lattice position and occupies an interstitial site.
Since the cation is not lost but only displaced, the solid remains electrically neutral.
Although both Assertion (A) and Reason (R) are correct individually, the reason explains a Frenkel defect, whereas the assertion refers to a Schottky defect.
Therefore, Reason (R) is not the correct explanation of Assertion (A),
making option (4) the correct answer.