In the sodium chloride structure, each ${\mathrm{Na}}^{+}$ ion is surrounded by six ${\mathrm{Cl}}^{-}$ ions nearest neighbours and  ${\mathrm{Na}}^{+}$ ions next nearest neighbours.

A unit cell of sodium chloride has four formula units. The edge length of the unit cell is $0.564 \mathrm{nm}$. What is the density of sodium chloride?

Chromium metal crystallises with a body-centred cubic lattice. The length of the unit cell edge is found to be $287 \mathrm{pm}$. Calculate the atomic radius. What would be the density of chromium in $\mathrm{g} {\mathrm{cm}}^{-3}$? 

Molybdenum forms body-centred cubic crystals and at $20 °\mathrm{C}$ the density is $10.3 \mathrm{g}/{\mathrm{cm}}^3$. Calculate the distance between the centres of the nearest molybdenum atoms.

In the cubic crystal of $\mathrm{CsCl} (\mathrm{d}=3.97 \mathrm{g}/{\mathrm{cm}}^3)$, the eight corners are occupied by ${\mathrm{Cl}}^{-}$ with a ${\mathrm{Cs}}^{+}$ at the centre and vice versa. Calculate the distance between the neighbouring ${\mathrm{Cs}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions. What is the radius ratio of the two ions? [At. Masses: $\mathrm{Cs}=132.91; \mathrm{Cl}=35.45$].