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Question

The pink colour of lithium chloride crystal is due to:

A
Metal deficiency defect
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B
Impurity defect
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C
Metal excess defect
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D
Frenkel defect
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Solution

The correct option is C Metal excess defect
When a LiCl crystal is heated in an atmosphere of lithium vapour, Cl ions diffuse to the surface of the crystal and interact with the lithium atom to form Li+ and Cl ions. Electrons released by the reacted lithium atoms occupies the vacant sites of chloride ions that had moves to the surface to react with Li.
The vacant site of crystal lattice, which are occupied by electrons are called F-centres.
These free electrons gets excited to higher energy levels, give rise to absorption spectra. So, the compounds are often coloured. In KCl, F-centres imparts violet colour radiation. This is called metal excess defect.
Solids containing F-centres are paramagnetic, as the electrons occupying the vacant sites are unpaired

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