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F-centres are responsible for the colour of crystals such as NaCl heated in sodium vapour because they consist of
Aelectrons trapped in anion vacancies
Bextra cations occupying interstices
Choles in the cation lattice
Dmolecular substitutions
Answer & Solution
Correct answer: A. electrons trapped in anion vacancies
1. Heating NaCl in Na vapour deposits sodium on the crystal surface, leaving Cl⁻ vacancies behind.
2. Electrons released by sodium migrate to these vacancies to maintain neutrality.
3. These trapped electrons (F-centres) absorb visible light, giving the crystal a yellow colour.
_Source: Maharashtra Balbharati Std XII Chemistry, Ch 1 "Solid State" §1.11_
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