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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ss110

Which statements about F-centres in alkali halides are correct?

  1. ANaCl heated in sodium vapour becomes blue.
  2. BThey are anion vacancies that trap electrons.
  3. CThe crystal shows colour because the trapped electrons absorb visible light.
  4. DThe crystal is paramagnetic because of unpaired electrons.

Answer: B, C, D

F-centres are anion sites occupied by electrons from excess metal atoms. These electrons absorb visible light (colour) and are unpaired (paramagnetism). NaCl with excess sodium turns yellow, not blue.
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