The ionic radii of Cs⁺ and Cl^– are 0.165 nm and 0.181 nm respectively. Their atomic weights are 133 and 35.5. Then,

a) Lattice parameter $=\frac{4\times \mathrm{r}}{\sqrt{3}};\mathrm{r}={\mathrm{r}}^{+}+{\mathrm{r}}^{-}$
$=\frac{4\times 0.346}{\sqrt{3}}=0.399\approx 0.4$
c) Density $=\frac{\mathrm{Z}\times \mathrm{M}.\mathrm{W}}{{\mathrm{N}}_{\mathrm{A}}\times {\mathrm{a}}^3}$
$=\frac{1\times 168.5}{6.023\times {10}^{23}\times {\left(0.4\times {10}^{-7}\right)}^3}$
$=4.37\mathrm{g}/\mathrm{cm}=4.37\times {10}^3\times \mathrm{kg}/{\mathrm{m}}^3$
4.37 × 103 × kg/m3