In corundum, oxide ions are arranged in h.c.p array and the aluminium ions occupy two thirds octahedral voids. What is the formula of corundum?

In corrundum only $\frac{2}{3}$ of octahedral voids are occupied. It means that corresponding to each oxide ion there are $\frac{2}{3}$ aluminium ions. The whole number ratio of oxide and aluminium ion in corrundum is therefore 3 : 2.

Hence, formula of corrundum is $A{l}_2{O}_3$