Ferric oxide crystallises in a hexagonal close

packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions. Derive the formula of the ferric oxide

There is one octahedral hole for each atom in

hexagonal closed packed arrangement. If the number of oxides ions per unit cell is x, then

Number of Fe3+ ions = 2/3 X octahedral holes

2/3 x=2x/3

.•. Ratio of Fe3+: O2- = 2x/3 : x = 2:3

Thus, formula of compound = Fe203