Question

Define
(A) Metallic radius
(B) Van der waals radius

Answer 1

Hint: Metallic radius is defined as one-half of the distance between the nuclei of two atoms in a crystal or between two adjacent metal ions in the metallic lattice. The Van der Waals radius of an atom is the radius of an imaginary hard sphere representing the distance of closest approach for another atom.

Complete answer:
Metallic radius: is the radius of an atom joined by a metallic bond. The metallic radius is half of the total distance between the nuclei of two adjacent atoms in a metallic cluster. Since a metal will be a group of atoms of the same element, the distance of each atom will be the same. The metallic radius follows the given trends in the periodic table.
Metallic radii decrease across the period due to an increase in the effective nuclear charge.
Metallic radii increases down the group due to increase in principal quantum number.
Van der Waals radius: is a measure for the size of an atom that is not chemically bound. In general a van der Waals radius is defined as half the closest distance of two equal, non-covalently bound, atoms. Van der Waals radii may be determined from the mechanical properties of gases, from the critical point, from measurements of atomic spacing between pairs of unbonded atoms in crystals or from measurements of electrical or optical properties. These various methods give values for the Van der Waals radius which are similar but not identical.

Note:
Remember the van der Waals radius is longer than covalent radius because 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.