Yellow colour in NaCl is due to

The correct answer is Option (2) → Metal excess defect due to anionic vacancies

When NaCl crystals are heated in sodium vapour, Na atoms deposit on the surface; Cl^– ions diffuse out to combine with them, leaving anionic vacancies. The electrons released by the sodium atoms occupy these vacancies, called F-centres. The crystal now has an excess of sodium, and excitation of the F-centre electrons by visible light gives the yellow colour.

Why the other options are incorrect

• Option (1) Schottky defect — Incorrect. Equal numbers of cations and anions are missing; it lowers the density but does not colour the crystal.

• Option (3) Frenkel defect — Incorrect. An ion (usually the smaller cation) moves to an interstitial site; NaCl does not show it, and it produces no colour.

• Option (4) Metal deficiency defect — Incorrect. Seen in compounds like FeO, where some \( \ce{Fe^{2+}} \) is replaced by \( \ce{Fe^{3+}} \); it is not the cause of the yellow colour of NaCl.

Note 1: 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):
Non-Stoichiometric Defects – “… Metal excess defect due to anionic vacancies: Alkali halides like NaCl and KCl show this type of defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl^– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na⁺ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl.”

Note 2: The same/similar question has been asked in different years by NTA as per the details given below: