Hi

According to the question For an FCC crystal, atoms are in touch along the diagonal of the unit .

4r= 2

a, where a is that the edge length of unit and r is that the radius of atom.

Substituting values within the above expression, we get

r= 42 ×407

=143.9 pm

Number of atoms per unit cell (FCC) =4

Mass of 1 unit cell =197×4=788amu=1.66×10 −27 ×788kg=1.30808×10 −24 kg

Volume of 1 unit cell =(407×10 −12 ) 3 m 3 =6.741×10 −29 m 3 =2.824×10 −28 m 3

Density= 6.741×10 −29

1.30808×10 −24

=19404.83kg/m 3 =19.4g/cm 3

Hope helpful.