Which of the following defects are shown by \(KCl\) Crystal?
(A) Schottky defect
(B) Frenkel defect
(C) Metal excess defect
(D) Metal deficiency defect
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
KCl is an ionic solid in which \( \ce{K^{+}} \) (133 pm) and \( \ce{Cl^{-}} \) (181 pm) are of comparable size and the coordination number is high (6:6).
(A) Schottky defect — True. It is shown by ionic solids whose cation and anion are of almost similar size (NaCl, KCl, CsCl, AgBr): equal numbers of \( \ce{K^{+}} \) and \( \ce{Cl^{-}} \) are missing, and the density decreases.
(B) Frenkel defect — False. It needs a large difference in ion sizes so that the small cation can move into an interstitial site (ZnS, AgCl); \( \ce{K^{+}} \) is too large for that.
(C) Metal excess defect — True. When KCl is heated in potassium vapour, K atoms deposit on the surface, \( \ce{Cl^{-}} \) ions diffuse out to combine with \( \ce{K^{+}} \), and the electrons released occupy the anion vacancies (F-centres). Excess potassium makes KCl violet (lilac).
(D) Metal deficiency defect — False. It occurs in compounds of metals with variable oxidation states, such as FeO (Fe0.95O); potassium shows only +1.
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 – “… (iv) Schottky Defect: It is basically a vacancy defect in ionic solids. … Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects.”
Non-Stoichiometric Defects – …
