Question

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.

Answer 1

O^2- ions form cubic close packed lattice

Number of O^2- = 8 x \(\frac{1}{8}\) = 1 

Number of Fe³⁺ = \(\frac{2}{3}\) octahedral voids = \(\frac{2}{3}\)

Formula of the compound = \(Fe^{3+} + \frac{2}{3}O^{2-} = Fe_2O_3\)