CsBr crystallises in a body centred cubic lattice. The unit cell...

Density of $\mathrm{CsBr}=\frac{\mathrm{Z} \times \mathrm{M}}{a^3 \times \mathrm{N}_0}\mathrm{Z} \rightarrow$ No. of atoms in the bec unit cell $=2 \mathrm{M} \rightarrow$ Molar mass of $\mathrm{CaBr}=133+ 80=213 a \rightarrow$ Edge length of unit cell $=436.6 \mathrm{pm}=436.6 \times 10^{-10} \mathrm{~cm} \mathrm{N}_0$ is Avogadro number $$\begin{aligned} & \therefore \text{ Density } \\ & =\frac{2 \times 213}{\left(436.6 \times 10^{-10}\right)^3 \times 6.02 \times 10^{23}} \\ & =8.50 \mathrm{~g} / \mathrm{cm}^3 \end{aligned}$$ For a unit cell =$\frac{8.50}{2}=$4.25 $\mathrm{~g}$ / $\mathrm{cm}^3$ Related Theory When particles are closed packed resulting in either cpp or hcp structure, two types of voids are generated are Tetrahedral voids are holes or voids. (Coordination number of a tetrahedral void is 4.) Octahedral voids (Coordination number of octahedral void is 6.)