Formula of metal oxide with metal deficiency defect in its crystal is A0.8O. The crystal contains A2+ and A3+ ions. The fraction of metal existing as A2+ ions in the crystal is

Thus, if there are 100 O atoms, then number of A atoms = 80 Charge on 100O2− ions =100×(−2)=−200Suppose number of A atoms present as A2x = x∴   Number of A atoms present as A3+=80−xTotal charge on A2+ and A3+=x×(+2)+(80−x)×(+3)=2x+240−3x=240−xAs metal oxide is neutral,Total charge on cations = total charge on anions∴ 240−x=200 or x=40∴   Fraction of metal existing as A2+ ions in the crystal=4080=0.5