An ionic compand made up of atoms A and B has a face- centred cubic arrangement in which atoms A are at the cornere and atoms B are at the face- centres. If one of the atoms is missing from the corrner, what is the simplest formula of the compound ?

Explanation

No. of atoms of A at the corners,= 7( because one A is missing )
Contributinon atoms of A towards unit cell =` 7 xx 1/8 = 7/8 `
No . Of atoms B at face -centres = 6 , Contribution of B towards unit cell = ` 6 xx 1/2 =3`
Ratio of A : B =` 7/8 : 3= 7: 24 ` : Formula is ` A_(7) B_(24)`