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.

Given,

Ferric oxide crystalizes in a hexagonal close packed array of oxide ions.

Two out of every three octahedral holes are occupied by ferric ions.

Let the number of oxide ions = x

Since, two out of every three octahedral holes are fitted by ferric ions.

Thus, voids filled by ferric ions

(Fe³⁺)

Therefore, number of ferric ions

(Fe³⁺)

Now, the ratio of ferric ions to the oxide ions

i.e. Fe³⁺ : O^2- =

= Fe³⁺ : O^2- = = 2 : 3

Thus, formula of ferric oxide is Fe₂O_{3 .}