GIVEN :-

r = 1.68 A⁰ = 1.68 x 10^-10 m.
Lattice system = SC
M = 209 kg / k. mol
N = 6.02 x 10²⁶ atoms / k. mol.

For SC, r = a/2
So a = 2r = 2 x 1.68 x 10^-10 m = 3.36 x 10^-10 m
and n = 1

So Density = nM / N (a)³
       = [( 1 atom) (209 kg / k. mol)] / [(6.02 x 10²⁶ atoms / k. mol) (3.36 x 10^-10m)³]
       = [(209 / 228.34)] x 10⁴ kg / m³
       = 9150 kg / m³