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Question

The appearance of colour in solid alkali metal halides is generally due to:


A
f-centres
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B
Frenkel defect
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C
Schottky defect
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D
interstitial position
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Solution

The correct option is A f-centres
Alkali halides like $$NaCl$$ and $$KCl$$ show metal excess defect. When crystals of $$NaCl$$ are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The $$Cl^-$$ ions diffuse to the surface of the crystal and combine with $$Na$$ atoms to give $$NaCl$$. This happens by the loss of an electron by sodium atoms to form $$Na^+$$ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result, the crystal now has an excess of sodium. 

The anionic sites occupied by unpaired electrons are called F-centres. They impart a yellow colour to the crystals of $$NaCl$$. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals. 

Similarly, excess of lithium makes $$LiCl$$ crystals pink and excess of potassium makes $$KCl$$ crystals violet (or lilac).

Chemistry

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