Explain the following term with a suitable example:
$(iv)$ $F$-centres

(N/A) $(iv)$ $F$-centres: When the anionic sites of a crystal are occupied by unpaired electrons,these sites are called $F$-centres (from the German word $Farbenzentrum$,meaning colour centre).
These unpaired electrons absorb energy from visible light and get excited,which imparts colour to the crystals.
For example,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 from the crystal to its surface and combine with $Na$ atoms,forming $NaCl$.
During this process,the $Na$ atoms on the surface of the crystal lose electrons.
These released electrons diffuse into the crystal and occupy the vacant anionic sites,creating $F$-centres,which turn the $NaCl$ crystal yellow.