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

Let the number of `O^(2-)` in hexagonal close packed structure = 1
Number of `TVs = 2//"atom"`, Number of `Ovs = 1 // "atom"`.
Out of these `OV s` only two `OV s` are occupied by `Fe^(3+)`
Number of `OV s` occupied `= (2)/(3)`, Number of `Fe^(3+) = (2)/(3)`
Formula. `Fe_((2)/(3))^(3+) O_(1)^(2-)`. On simplifying, `Fe_(2)O_(3)`.