Sodium metal crystallizes in a body centred cubic lattice with a unit cell edge of 4.29 A. The radius of sodium atom is approximately

In a body centred cubic lattice, atoms touch each other along the body diagonal of the cube. Hence

$4r=\sqrt{3}a$ or $r=(\sqrt{3}/4)$ $a=(\sqrt{3}/4)(4.29 \text{Å})=1.86 \text{Å}$