Ionic solids, which have anionic vacancies due to metal excess defect , develop colour. Explain with the help of a suitable example.

Ionic solids, which have anionic vacancies due to metal excess defect

1.	Ionic solids, which have anionic vacancies due to metal excess defect , develop colour. Explain with the help of a suitable example.
Answer» Solution :Taking the exampleof NaCl, when its crystals are heated in presence of sodium vapour , some chloride IONS leave their lattice sites to combine with sodium to from NaCl. For this reaction to occur, Na atoms lose electrons to form ` N^(+)`ions.The electrons thus released diffuse into the crystal to occupy the anion vanncies created by ` Cl^(-)`ions. The crystal now has EXCESS of soidium. The sites occupied by unpaired electrons are called F-centres.They impart yellowcolour to the crystal because they ABSORB energy from the visible LIGHT and get excited.