An ionic solid A⁺B^– crystallizes as a body centred cubic structure. The distance between cation and anion in the lattice is 338 pm. Calculate the edge length of the unit cell (in pm).

In a body centred cubic unit cell, the distance
between the cation and anion (r⁺+r^–) is related
to the edge length (a) by
${\mathrm{r}}^{+}+{\mathrm{r}}^{-}\sqrt{3}\times \frac{\mathrm{a}}{2};\therefore \mathrm{a}=\frac{2}{\sqrt{3}}\left({\mathrm{r}}^{+}+{\mathrm{r}}^{-}\right)$
It is given that (r⁺+r^–) = 338 pm 
$\therefore \mathrm{a}=\frac{2}{\sqrt{3}}\times 338\mathrm{pm}=\frac{676}{1.732}=390.3\mathrm{pm}$