What are molecular crystals? Name the forces which are holding the constituent particles in molecular crystals.

Show that the density of a cubic crystal is given by the expression:

$\mathrm{d}=\frac{\mathrm{ZM}}{{\mathrm{a}}^3{ \mathrm{N}}_{\mathrm{A}}}$

where $\mathrm{Z}$ is the number of atoms/molecules/ions present per unit cell, $\mathrm{M}$ is molecular weight, ${\mathrm{N}}_{\mathrm{A}}$ is Avogadro's number and ${\mathrm{a}}^3$ is the volume of unit cell.