Atomic radii are called covalent radii (a reference to the types of bonds formed) when referring to non-metallic elements and metallic radii when referring to metals. Technically, the atomic radius is one half of the equilibrium internuclear distance between two adjacent atoms (which may either bonded covalently or present in a closely packed crystal lattice) of an element.
A covalent radius is one-half the distance between nuclei of two of the same atoms that are bonded to each other. Covalent radii for elements whose atoms cannot bond to each another can be estimated by combining radii of those that do with the distances between unlike atoms in various molecules. A metallic radius is one-half of the closest internuclear distance in a metallic crystal.
In the Periodic Table, atomic radii increase down a group as new electron shells are added, and decrease left-to-right as the nuclear charge (or number of protons) is increased.
- Periodic Table of the Elements Atomic Radius