Chemistry - Solid State Question with Solution | TestHub

Solid alkali metal halides such as $NaCl$, $LiCl$, and $KCl$ exhibit characteristic colors due to a type of non-stoichiometric defect called a metal excess defect.

When crystals of $NaCl$ are heated in an atmosphere of sodium vapor, sodium atoms deposit on the crystal surface. Chloride ions ($C{l}^{-}$) diffuse to the surface and combine with sodium atoms to form $NaCl$. During this reaction, sodium atoms lose electrons to form $N{a}^{+}$ ions.

The released electrons diffuse into the crystal lattice and occupy the vacant anionic sites left behind by the chloride ions. These anionic vacancies trapped with unpaired electrons are known as F-centres.

The term F-centre is derived from the German word Farbenzentrum, which translates to color center. These trapped electrons absorb specific wavelengths of energy from visible light and undergo excitation, thereby imparting a characteristic color to the crystal. For example, $NaCl$ appears yellow, $LiCl$ appears pink, and $KCl$ appears violet.

Therefore, the F-centre defect is responsible for exhibiting color in solid alkali metal halides.

Answer: F-centre