The Solid State

In a fcc unit cell, there are 8 corners and 6 faces. Eight corners contribute 8 x 1/8 = 1 atom
six faces contribute 6 x 1/2 = 3 atoms.
So the formula of the compound is PQ₃.

In ionic solids, generally anions form close-packed structure and cations occupy interstitial sites. example NaCl, KCl. These are ionic solids

There are 1 mole of octhahedral voids in every one mole ofthe compound cotaining ccp structure.
since number of particles present in 1 mole compound =6.022 x 10²³ 
we know that number of octahedral voids=number of atoms in structure.

The coordination number is 4.