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# Lil occurs as cubical - closest packing . If the edge length of a unit cell is $624\;pm$ , determine the ionic radii of $Li^{+}$ and $I^{-}$ ions.

$\begin{array}{1 1} 91.35 \\ 1.143 \\ 8.4 \\ 9.5 \end{array}$

Answer :$91.35$
The cubical closet packing has a face- centered cubic unit cell.
$I^{-}$ ions occupy the corners and the face centers.
These ions touch each other along the face diagonal of the cube. Hence
$4r_1- =\sqrt 2 a$
or $r_{1-}=\large\frac{a}{2 \sqrt 2}$