Copper crystallises in a face centred cubic $$(fcc)$$  lattice with a unit cell length of $$361\,pm.$$  What is the radius of copper atom in $$pm?$$

Solution :
In case of face centred cubic $$(fcc)$$  lattice,
radius $$ = \frac{{\sqrt 2 a}}{4}$$
∴ Radius of copper atom ( $$fcc$$  lattice )
$$\eqalign{ & = \frac{{\sqrt 2 \times 361}}{4} \cr & = 128\,pm \cr} $$