58. 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.
58. 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.
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1 Answer
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58. Options (i) and (iv) are correct since an excess of K+ ions make KCl crystals appear violet due to the defect called metal excess defect . The anionic vacancies are found to be created due to the movement of potassium atoms to the surface of the crystal when KCl is heated in an atmosphere of potassium vapours and the Cl-ions then diffuse to the surface.
This happens due to the loss of the electron by the potassium atom to form K+ ions. In this case the electrons released diffuse to the crystal and occupy anionic sites being created. The anionic sites occupied by unpaired electrons are known as F-centres. These F-centres impart
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