A metal crystallizes in a body centred cubic lattice ($\mathrm{bcc}$) with the edge of the unit cell $5.2\text{Å}$ The distance between the next nearest neighbour is :

The metal crystallizes in a body centred cubic lattice ($\mathrm{bcc}$) with the edge of the unit cell $5.2\text{Å}$. Now, the next neighbour will be at the same distance of the edge. 

Thus, the distance between the next nearest neighbour is $5.2\text{Å}$.