In the fcc arrangement of `A` and `B` atoms whose `A` atoms are at corners of the unit cell and `B` are at the face centres one of the `A` atom is missing from one corner in each unit cell. What is the simplest formula of the compound?

Answer

`A_7B_(24)`

Explanation

No. of atoms of A at corners = 7 (because one A is missing).
Contribution of atoms A in a unit cell = `7 xx 1/8 = 7/8`
No. of atoms of B at face centres = 6
`:.` Contribution of aotms B in a unit cell = `6 xx 1/2 = 3`
`A : B = 7/8 : 3 " or " 7 : 24`
`:.` Simplest formula = `A_7B_(24)`