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JEE Main levelSingle correct optionDifficulty 2 of 3ss032

Heating a crystal of KCl in potassium vapour gives it a violet colour. The colour is due to

Aexcess $Cl X^{-}$ ions in interstitial sites
Belectrons trapped in anion vacancies (F-centres)
Ccation vacancies filled by $Cl X^{-}$
D $K X^{+}$ ions occupying octahedral voids

Answer: B

Potassium atoms deposit on the surface, ionise, and

Cl X^{-}

ions diffuse there to form KCl; the freed electrons diffuse into anion vacancies. These trapped electrons (F-centres) absorb visible light and cause the violet colour.

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