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.

Explanation

Number of octahedral holes in hcp structure of oxide ions `(O^(2-))` = Number of packed oxide ions = x(say).
Since two-thirds of octahedral voids are occupied by `Fe^(3+)` ions, the number of `Fe^(3+)` ions = `2/3x`.
Therefore, the ratio of `Fe^(3+)andO^(2-)` ions in the unit cell of ferric oxide = `2/3x:x=2:3`
`therefore" "` Formula of ferric oxide: `Fe_(2)O_(3)`