As for bcc structure $\mathrm{r}=\frac{\sqrt{3}}{4}\mathrm{a}$
So $2\mathrm{r}=\frac{\sqrt{3}}{2}\mathrm{a}$
As 2r represents the distance between the bcc atom and one of the corner atom so in a cube
the distance between the body centred atom and one corner atom is $\frac{\sqrt{3}}{2}\mathrm{a}$ .