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 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 yellow colour to the crystal because they absorb energy from the visible light and get excited.