A compound formed by Mg, Al and  O, is found to have cubic close array of oxide ions, in which Mg2+ occupying 18th of tetrahedral voids and Al3+ ions  occupying 12 the octahedral voids. The formula for the compound is :

Oxide ions have ccp array∴    Number of Oxide ions/unit cell = 4Thus number of OV / unit cell = 4        number of TV / unit cell = 8Mg+2Occupied 18th  of TV ∴   Number of  Mg+2 ions/ unit cell  =  188=1Al+3 Occupied 12  of OV∴   Number of  Al+3  ions/ unit cell  =  124=2Formula :Mg+21    Al+32     O−24 MgAl2O4