Question

Can somebody explain this? Metal excess defect due to anionic vacancies: Alkali halides
like NaCl and KCl show this type of 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
loss of 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 (from the German
word Farbenzenter for colour centre). They impart 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).