Vacancy and interstitial defects in solids are examples of:

The correct answer is Option (2) → thermodynamic defect

Point defects are irregularities in the arrangement of particles around a point or an atom in a crystal. They are of three types: stoichiometric, impurity and non-stoichiometric defects.

Stoichiometric defects do not disturb the ratio of the particles, so the formula of the solid stays the same. They are also called intrinsic or thermodynamic defects, since they arise on their own at any temperature above 0 K. In non-ionic solids they are of two kinds:

• vacancy defect: some lattice sites are empty, which lowers the density;

• interstitial defect: some particles occupy interstitial sites, which raises the density.

In ionic solids the same ideas appear as the Schottky defect (pairs of cation and anion vacancies) and the Frenkel defect (a cation moved to an interstitial site).

Why the other options are incorrect

• Option (1) non-stoichiometric defect — Incorrect. These change the ratio of the particles (metal excess or metal deficiency), for example Fe_0.95O; vacancy and interstitial defects leave the ratio unchanged.

• Option (3) impurity defect — Incorrect. It is caused by a foreign ion in the lattice, for example \( \ce{Sr^{2+}} \) in NaCl creating cation vacancies.

• Option (4) Metal deficiency defect — Incorrect. It is a non-stoichiometric defect in which the solid has less metal than the ideal formula, as in FeO.

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