Question

Which among the following solids show Frenkel defect?
(A) NaCl
(B) CsCl
(C) KCl
(D) AgCl

Answer 1

Hint: Before attempting this question, one should have prior knowledge of Stoichiometry and Stoichiometric defects. One should also focus on the Frenkel defect and the conditions that lead to this defect.

Complete answer:
> Frenkel defect is a type of Stoichiometric defect ( i.e. defects in the crystal lattice, in which the number of positive and negative ions in the molecule are consistent with what is indicated in their molecular formula.
> Frenkel effect arises when an ion is missing from its normal position and is occupying an interstitial site (i.e. a position in between the usual position of atoms and ions in a crystal lattice, and can be occupied by other atoms or ions)
- Some unique features of Frenkel defect are:-
1) The compounds that show Frenkel defects usually have low coordination numbers.
2) There is a large difference in size of anions and cations in the compound.
3) Alkali halides like CsCl, NaCL, etc. do not show the Frenkel defect.
> Since in NaCl, KCl the size of anions and cations are similar, they do not show Frenkel defects.
> In CsCl and AgCl, there is a large difference in the size of anions and cations , but CsCl being an alkali halide does not show Frenkel defect. So only AgCl shows Frenkel defect.
Hence, option D is the correct answer

Note: In Frenkel defect, it is normally the cation that is displaced to the interstitial site. Also Frenkel defect only involves displacement of ions so the Stoichiometry, electrical conductivity, and density of the compound does not change.