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 closed packed arrangement there is one octahedral site corresponding to each atom constituting the lattice. Therefore number of oxide ions per unit cell in hcp arrangement is 6.
Number of octahedral voids `= 6xx1=6`
Number of `Fe^(+3)` ions `= 6xx(2)/(3)=4`
`therefore` Formula of the compound `Fe^(3+): O^(-2)` or is `Fe_(2)O_(3)`.
4 : 6
2 : 3