Atomic radius is the distance from the atomic nucleus to the outmost stable electron orbital in a atom that is at equilibrium and is measured in picometers or Angstroms.
Atomic radii are called covalent radii 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.