Coordination number and number of carbon atoms per unit cell of diamond respectively are

$\mathrm{In} \mathrm{diamond} \mathrm{each} \mathrm{carbon} \mathrm{atom} \mathrm{is} \mathrm{surrounded} \mathrm{by} \mathrm{four} \mathrm{other} \mathrm{carbon} \mathrm{atoms}. \mathrm{Unit} \mathrm{cell} \mathrm{of} \mathrm{diamond} \mathrm{contains} 8 \mathrm{carbon} \mathrm{atoms} \mathrm{effectively}.$

The co-ordination number of carbon in diamond is 4.It has F.C.C structure similar to ZnS.

4 carbons occupy lattice sites while 4 carbons occupy alternate tetrahedral voids .

Thus one unit cell of diamond has 8 carbon atoms.