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

$\begin{array}{1 1} 103.5 \\ 0.94 \\ 8.4 \\ 9.5 \end{array}$

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 center of cube and $CT^{-1}$ ions are present at the corners.
The coordination number of $Cs^{+}$ is eight .