In corundum, oxide ions are arranged in hexagonal close packing and aluminium ionsa occpy tow-third of the octaheral voids. What is the formula of corrundum ? .

Answer

`Al_2O_3`

Explanation

Suppose oxide ions =n. Then octahedral voids= n. Hence, `Al^(3+)` ions `=2/3xxn="2n"/3`
`therefore` Ratio `Al^(3+) : O^(2-)="2n"/3:n =2:3` , i.e., formula is `Al_2O_3`