CsCl crystallises in a cubic and that has a Cl– at each corner and $Cs^+$ at the centre of unit cell. If $r_{Cs^+} = 1.69A^{\large\circ}$ and $r_{Cl^-}= 1.81A^{\large\circ}$, what is the value of edge-length ‘a’ of the cube? Compare this value of ‘a’ (calculated) from the observed density of CsCl, $3.97 gm/cm^3$ [M(CsCl) = 168.5]