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

There is one octahedral hole for each atom in hexagonal close packed arrangement.
If the number of oxide ions `(O^(2-))` per unit cell is 1, then the number of `Fe^(3+)` ions `=2//3xx"octahedral holes"=2//3xx1=2//3`.
Thus, the formula of the compound `=Fe_(2//3)O_(1)or Fe_(2)O_(3)`.