van der Waals' radius is half of the internuclear separation of two non-bonded atoms of the same element on their closest possible approach while covalent radius is refers to half of the distance two singly bonded atom of the same element (Homonuclear X−X bond). Overlapping of orbitals takes place thus covalent radii is least while van der Waals' radius is most and metallic radii is between van der Waals' radius and covalent radii.