Question

Assertion: Frenkel defect is also called dislocation effect.
Reason: Frenkel defect is shown by ionic substances in which cation and anion are almost similar sizes.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Both Assertion and Reason are incorrect

Answer 1

Hint: To answer this question, you must recall the Frenkel defects arising in an ionic crystal. An imperfection or defect in an ionic crystal is caused by the departure of any ion from the perfectly ordered arrangement of the ions of the crystal. Frenkel defect is a type of stoichiometric defect, where the ion leaves its position and occupies interstitial void.

Complete step by step solution:
Stoichiometric defects are the defects in an ionic crystal in which the ratio of the anions to the cations remains the same throughout as is represented by the molecular formula, i.e. the number of anions and cations lost are equal. Stoichiometric defects can be broadly classified into three types, namely. Frenkel defects, Schottky defects and impurity defects.
Frenkel defects is the defect in which one of the ions leaves its lattice position and moves into an interstitial void, creating a vacancy at the lattice point. Since the ion is dislocated from its lattice site, the Frenkel defect is also known as dislocation defect. Such a defect is possible only if the size of the ion dislocated is very small as compared to the other so as to fit easily in the interstitial sites.
Thus, we can conclude from the above statements that the Assertion is a correct statement while the Reason is an incorrect statement.
The correct answer is C.

Note:
Defects arising in an ionic crystal are broadly classified into two types, namely atomic or pint imperfections and electronic imperfections. Atomic or point imperfections are those defects which arise due to some irregularities in the arrangement of the ions in the crystal lattice. Point imperfections are further classified into impurity defects, stoichiometric defects and non- stoichiometric defects. In the above question, we have discussed only the stoichiometric defects.