The radius of an octahedral void is$\text{'}\mathrm{r}\text{'}$' and the radius of an atom is $\text{'}\mathrm{R}\text{'}$ when these are in close packing. Derive the relation between the two.

Why is glass regarded as an amorphous solid?

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.