If three elements P, Q and R crystallise in a cubic unit cell with P atoms at the corners, Q atoms at the cubic centre and R atoms at the centre of each face of the cube, then write the formula of the compound.

Explanation

As P atoms are present at the 8 corners of the cube. Therefore,
No. of P atoms in the unit cell = `8 xx 1/8 = 1`
No. of Q atoms in the unit cell = 1
R atoms are present at the edges. Since there are 12 edges and atom at each is shared by four atoms.
Therefore,
No. of R atoms in the unit cell = `12 xx 1/4 = 3`
`:.` The formula of the compound = `PQR_3`.