# In CsCl with cubic structure $Cl^-$ ions are located at each corner and $Cs^+$ ions at the centre of the unit cell. If $r_{cs^+}$ and $r_{cl^-}$ are $1.69A^{\large\circ}$ and $1.81A^{\large\circ}$ respectively . Find the value of edge length of cube.

$(a)\;4\qquad(b)\;5\qquad(c)\;6\qquad(d)\;8$

$r_{cs^+} + r_{Cl^-}$ = 1.69 +1.81
$\;\;\;\;\;\;\;\;\;\;\;\;=3.5A^{\large\circ}$
Diagonal length = $\large\frac{a\sqrt3}{2}$
$\therefore \large\frac{a\sqrt3}{2} = 3.5$
a = $\large\frac{3.5\times2}{\sqrt3}$
a = 4