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  • An excess of potassium ions makes KCl crystals appear violet or lilac in colour since__________

An excess of potassium ions makes KCl crystals appear violet or lilac in colour since ________.

(i) some of the anionic sites are occupied by an unpaired electron.

(ii) some of the anionic sites are occupied by a pair of electrons.

(iii) there are vacancies at some anionic sites.

(iv) F-centres are created which impart colour to the crystals.

Answers (1)

The answer is the option (i, iv).

When KCl is heated in vapour of K, anionic vacancies are created as some of the Cl^{-}  ions leave their lattice site. This chloride ion wants to combine with K vapour to form KCl. For this to happen, loss of electron by potassium atom to form potassium ion occurs. The unpaired free electron gets entrapped in the anion vacancy called F-centre that are responsible for imparting color to crystals. This entrapped electron gains energy due to the visible light falling on the crystal goes to the higher level and when it comes back to the ground state, energy is released in the form of light.

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