Ferric oxide crystalliizes 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.

Explanation

In a close packed arrangement, there is one octahedral site corresponding to each atom constituting the lattice . Therefore,
No. of oxide ions per unit cell in hcp arrangment = 6
No. of octahedral holes = `6 xx 1/2 = 6`
No. of iron ions = `6 xx 2/3 = 4`
Formula of the compound = `Fe_(4)O ` or `Fe_2 O_3`.