A metal crystallizes in two cubic phases, face centred cubic (fcc) and body centred cubic (bcc) whose unit cell lengths are 3.5A0 and 3.0A0  respectively. Calculate the ratio of density of fcc and bcc.

ρ = Z×MNA×a3For fcc centred cubic cell Z=4, a=3.5A0∴ρ(fcc)=4×MNA×(3.5)3                                       …… (i) For bcc lattice Z=2,a=3.0A0ρ(bcc)=2×MmNA×(3.0)3                                              …… (ii) From equations (i) and (ii) ∴ρ(fcc)ρ(bcc)=42×33(3.5)3=4×3×3×32×3.5×3.5×3.5=1.259