Question

Which is the correct order a covalent radius metallic radius Van der Waal's radius?

Answer 1

One half of the distance between the nuclei (internuclear distance) of two covalently bonded atoms in a homodiatomic molecule is called the covalent radius of that atom. 

The molecules of non-metal atoms are generally gases. On cooling, the gaseous state changes to solid state. In the solid state, the nonmetallic elements usually exist as aggregations of molecules are held together by van der wall forces. One half of the distance between the nuclei of two adjacent atoms belonging to two neighboring molecules of a compound in the solid state is called van der walls radius.

Van der wall's radius  > Metallic radius  > Covalent radius

Metal atoms are assumed to be closely packed spheres in the metallic crystal. These metal atom spheres are considered to touch one another in the crystal. One half of the internuclear distance between the two closest metal atoms in the metallic crystal is called metallic radius.

Metallic > Covalent radius

So Metallic radii are smaller than the van der Waals' radii, since the bonding forces in the metallic crystal lattice are much stronger than the van der Waal's forces and Values of van der Waals' radii are larger than those of covalent radii because the van der Waals' forces are much weaker than the forces operating between atoms in a covalently bonded molecule.