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.
Let the number of oxide (O2−) ions be x.
So, number of octahedral voids = x
It is given that two out of every three octahedral holes are occupied by ferric ions.
So, number of ferric (Fe3+) ions`=2/3x`
Therefore, ratio of the number of Fe3+ ions to the number of O2− ions,
Fe3+ : = `O^(2-) = 2/3x:x`
`= 2/3 :1`
= 2 : 3
Hence, the formula of the ferric oxide is Fe2O3.