False statement about Frenkel defect is:

The correct answer is Option (3) → Non-ionic solids show this defect

In a Frenkel defect the smaller ion (usually the cation) leaves its lattice site and moves into an interstitial site. It is a defect of ionic solids in which the two ions differ greatly in size.

• Option (1) It is a combination of vacancy defect and interstitial defect. — A true statement about the Frenkel defect. The moved ion leaves a vacancy at its original site and creates an interstitial defect at its new site; so it is also called a dislocation defect.

• Option (2) Density remains unchanged — A true statement about the Frenkel defect. No ion leaves the crystal, so the mass and volume stay the same.

• Option (3) Non-ionic solids show this defect — The false statement. The Frenkel defect is shown by ionic solids only, because it is the displacement of an ion.

• Option (4) Shown by solids with large difference in size of cations and anions. — A true statement about the Frenkel defect. Examples are ZnS, AgCl, AgBr and AgI, with small \( \ce{Zn^2+} \) and \( \ce{Ag+} \) ions that fit into the interstitial sites.

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):
Frenkel Defect – “This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site (Fig. 1.25). 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.”