The compounds producing Schottky defect, when added to crystals AgCl…

The correct answer is Option (3) → B and C only

When an ionic crystal is grown with a little of a salt whose cation has a higher charge, each such cation takes the place of two host cations: it occupies the site of one and the other site stays empty. The cation vacancies so produced are equal in number to the added divalent ions. The book calls this an impurity defect; because it creates vacant cation sites, it has the same effect as a Schottky-type vacancy, and that is what the question asks for.

In AgCl the host cation is \( \ce{Ag^{+}} \) (charge +1). So only a chloride of a divalent cation can create cation vacancies:

• A. NaCl — Does not produce the defect. \( \ce{Na+} \) has the same charge (+1) as \( \ce{Ag+} \); one \( \ce{Na+} \) replaces one \( \ce{Ag+} \), so no vacancy forms.

• B. \( \ce{CdCl2} \) — Produces the defect. Each \( \ce{Cd^2+} \) replaces two \( \ce{Ag+} \) ions and leaves one cation site vacant; the book gives the solid solution of \( \ce{CdCl2} \) and AgCl as an example.

• C. \( \ce{CaCl2} \) — Produces the defect. \( \ce{Ca^2+} \) also carries +2 charge, so each \( \ce{Ca^2+} \) creates one cation vacancy, in the same way as \( \ce{Sr^2+} \) in NaCl.

• D. KCl — Does not produce the defect. \( \ce{K+} \) is univalent like \( \ce{Ag+} \); no vacancy is needed to keep the crystal neutral.

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):
Impurity Defects