Question

In face centred unit cell each atom in the face is shared by two other adjacent atoms.Why?

Answer 1

Two unit cell share one face centre. And in FCC one atom is present at face centre and it is shared by two unit cell and that is why contribution of atom at face centre is half per face in unit cell. Total faces are 6 in unit cell. So 1/2×6=3. 3 atoms contribute from face centre per unit cell. 1 contribute from corners. So total no of atoms in FCC is 3+1=4. I hope you will be able to imagine from the following picture:-