1. Covalent radius – One half of the distance between the nuclei of two covalently bonded atoms of same element in a molecule (used for non metals).
2. Van der waals radius – One half of the distance between nuclei of two identical atoms.
3. Metallic radius – One half of the internuclear distance between the two adjacent metal ions in the metallic lattice.
Van der waals radius > metallic radius > covalent radius
The size of the cations or anions having the same charge increases with atomic number. The 3+ cations of lanthanoides (Z = 58 to Z = 71) however, show a steady decrease with atomic number, due to inefficient shielding by the f subshells (lanthanoid contraction).
