ZnS , AgCl , AgBr and AgI show which of the following defect

The correct answer is Option (1) → Frenkel Defect

A Frenkel defect is shown when the cation is much smaller than the anion, so it can slip into an interstitial site. In ZnS, AgCl, AgBr and AgI the cations \( \ce{Zn^{2+}} \) and \( \ce{Ag^{+}} \) are small and the anions are large, so these solids show the Frenkel defect.

Why the other options are incorrect

• Option (2) Schottky Defect — Incorrect. Equal numbers of cations and anions are missing; it is shown by ionic solids whose cation and anion are of almost the same size, such as NaCl, KCl, CsCl. Of the four solids, only AgBr shows it as well.

• Option (3) Impurity Defect — Incorrect. It arises when a foreign ion takes a lattice site, for example \( \ce{Sr^{2+}} \) in NaCl; it is not a property of these pure solids.

• Option (4) Metal Excess Defect — Incorrect. It arises from anion vacancies holding electrons (F-centres, as in NaCl heated in sodium vapour) or from extra cations in interstitial sites (ZnO on heating); it is not the defect named for this group of solids.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Stoichiometric Defects – “Frenkel Defect: This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and AgI due to small size of Zn²⁺ and Ag⁺ ions.”