In corundum, O2-ions form hcp and Al3+ occupy two third of octahedral voids. Determine the formula of corundum.
Only 23 of the octahedral gaps are filled in corundum. It indicates that for every oxide, there are two-thirds of aluminium ions. Corundum has a 3:2 ratio of oxide to aluminium ions. Therefore, corudum's formula is Al2O3.
