Question

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.

Answer 1

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)`.