Unit Cells

Po - Simple cubic lattice
Na - bcc
Cu - fcc

In face centred cubic lattice, the atoms are present at eight corners of faces and one each at 6 faces. Lattice points belonging to face centred cubic unit cell =

In the unit cell number of Cu atoms (fcc/ccp)

As Ag atoms occupying edge centred

and Au atoms are presents at the body centred = 1
formula,

Number of A ions in the unit cell Number of B ions in the unit cell Hence empirical formula of the compound

No.of Matoms
No.of Xatoms
So, formula =

Following generalization can be easily derived for various types of lattice arrangements in cubic cells between the edge length (a) of the cell and r the radius of the sphere.
For simple cubic:
For body centred cubic :
Thus the ratio of radii of spheres for these will be
simple: bcc: fcc

option (a) is correct answer.

There are two atoms in a bcc unit cell. So, number of atoms in
unit cells
atoms.

From the given data, we have
Number of Y atoms in a unit cell = 4
Number of X atoms in a unit cell

From the above we get the formula of the compound as or