Ionic solids, which have anioninc vacancies due to metal excess defect, developed colour. Explain with the help of a suitalbe example.

Explanation

Taking the example of `NaCl`, when its crystals are heated in the pressure of sodium vapour, some chloride ions leave their sites to combine with sodium to form `NaCl`. For this reaction to occur. Na atoms lose electorns to form `Na^(o+)` ions. The electrons thus released diffuse into the crystal to occupy the anion vacanceis created by `Cl^(o+)` ions. The crystal how has excess of sodium. The sites occupied by upaired electrons are called F-centres. They impart yellow colour to the crystal becuase they absorb energy from the visible light and get excited.