Relating the unit-cell edge to the atomic radius of the atoms in a bcc cell is complicated by the face that atoms do not touch along the edges, nor do they touch along face diagonals. However, each corner atom does touch the atom in the center of the cell. This means that the body diagonal spans (1) the radius of a corner atom, (2) the diameter (2 radii) of the atom in the center, and (3) the radius of the atom in the opposite corner. This makes the body diagonal equivalent to 1+2+1=4 atomic radii(r).