# If the radius of $Cs^+$ is 169pm and that of $Cl^-$ is 181pm,predict the structure of CsCl.What is the coordination number of $Cs^+$ ion?

$\begin{array}{1 1}\text{Eight}\\\text{Six}\\\text{Four}\\\text{One}\end{array}$

We have
$\large\frac{r_c}{r_a}=\frac{169}{181}=$$0.94$
Since this value is greater than 0.732,the structure of CsCl involves body-centred cubic unit cell,where $Cs^+$ is present at the body centre of cube and $Cl^-$ ions are present at the corners.The coordination number of $Cs^+$ is eight.