Atomic radius is a measure of the size of an atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. There are various non-equivalent definitions of atomic radius, including Van der Waals radius, ionic radius, metallic radius, and covalent radius. Typically, atomic radius is measured in a chemically bonded state, but theoretical calculations are simpler when considering atoms in isolation. The value of the radius may depend on the atoms state and context. Under most definitions, the radii of isolated neutral atoms range between 30 and 300 picometers (trillionths of a meter), or between 0.3 and 3 ångströms. Therefore, the radius of an atom is more than 10,000 times the radius of its nucleus (1–10 femtometers), and less than 1/1000 of the wavelength of visible light (400–700 nanometers) .

Atomic radius is useful for determining many aspects of chemistry, such as various physical and chemical properties/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Atomic_Radii). The periodic table greatly assists in determining atomic radius and presents a number of trends/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Atomic_Radii). Atomic radius is generally stated as being the total distance from an atoms nucleus to the outermost orbital of electron/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Atomic_Radii). As you move across or down the periodic table, trends emerge that help explain how atomic radii change/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Atomic_Radii). For example, atomic radius generally decreases as you move from left to right across a period due to increasing nuclear charge, and increases as you move down a group due to the increasing number of electron shells. The covalent radius of an atom is the radius of an atom under the covalent bond with other atoms.