The actual density of a crystal of NaCl is observed to be lower than the expected value. It is caused
due to the presence of:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Schottky Defect
In a Schottky defect equal numbers of cations and anions are missing from their lattice sites (so the crystal stays neutral). The missing ions leave the crystal with less mass in the same volume, so its density decreases. NaCl, with cations and anions of similar size and a high coordination number, shows this defect, which explains a measured density lower than the calculated value.
Why the other options are incorrect
Option (1) Interstitial Defect — Incorrect. Extra particles occupy interstitial sites; this increases the density.
Option (2) Frenkel Defect — Incorrect. An ion (usually the smaller cation) only moves from its site to an interstitial site; no mass is lost, so the density does not change. It is shown by ZnS, AgCl, not by NaCl.
Option (4) Impurity Defect — Incorrect. e.g. \( \ce{Sr^{2+}} \) in NaCl creates cation vacancies, but it is not what makes the density of a pure NaCl crystal low; the observation describes the Schottky defect.
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):
Schottky Defect – “It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal (Fig. 1.26). Like simple vacancy defect, Schottky defect also decreases the density of the substance.”
