Which of the following defect in solid is a vacancy defect as well as interstitial defect?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Frenkel defect
In a Frenkel defect the smaller ion of an ionic solid (usually the cation) leaves its normal site and moves to an interstitial site. This creates a vacancy defect at its original site and an interstitial defect at its new location, which is why it is also called a dislocation defect. The density of the solid does not change. It is shown by solids with a large difference in the sizes of the ions, such as ZnS, AgCl, AgBr and AgI.
Why the other options are incorrect
Option (2) Impurity defect — Incorrect. Foreign ions take the place of the host ions (e.g. \( \ce{Sr^{2+}} \) in NaCl replaces \( \ce{Na^{+}} \) and leaves a cation vacancy); no ion of the crystal is moved into an interstitial site.
Option (3) Schottky defect — Incorrect. Equal numbers of cations and anions are missing from the lattice; it is only a vacancy defect and it lowers the density.
Option (4) Metal deficiency defect — Incorrect. Some cations are missing and the charge is balanced by other cations in a higher oxidation state (e.g. Fe0.95O); there are vacancies but no interstitial ions.
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.”
