A metal crystallizes into two cubic phases, face centred cubic (FCC) and body-centred cubic (BCC) whose unit cell lengths are 3.5 and 3.0 A⁰ respectively. The ratio of the densities of FCC and BCC is ___

Given, Metal Crystallizes
We need to find the ratio of the densities of FCC and BCC is, from the given option.

$$\begin{gathered} \text{ Density }\left(\mathrm{\rho }\right)=\frac{{\mathrm{Z}}_{\text{eff }}\times \mathrm{Mw}}{{\mathrm{a}}^3\times {\mathrm{N}}_{\mathrm{A}}} \\ \text{fcc, }{\mathrm{Z}}_{\text{eff }}=4 \\ {\mathrm{\rho }}_{\mathrm{fcc}}=\frac{4\times \mathrm{Mw}}{{\left(3.5\times {10}^{-8}\right)}^3\times {\mathrm{N}}_{\mathrm{A}}} \\ \text{ For bcc, }{\mathrm{Z}}_{\text{eff }}=2 \\ \begin{array}{l}{\mathrm{\rho }}_{\mathrm{fcc}}=\frac{2\times \mathrm{Mw}}{{\left(3\times {10}^{-8}\right)}^3\times {\mathrm{N}}_{\mathrm{A}}}\\ \frac{{\mathrm{\rho }}_{\mathrm{fcc}}}{{\mathrm{\rho }}_{\mathrm{bcc}}}=\frac{4\times (3{)}^3}{(3.5{)}^3\times 2}=1.26\end{array} \end{gathered}$$

Therefore, the correct answer is 1.26