A measure of the size of an atom. As an atom does not have a sharp boundary in quantum mechanics, it is only an approximation to regard an atom as a hard sphere characterized by an exact radius. The size of an atomic radius depends on whether it is a free atom or bound, with the atomic radius depending on the environment it is bound in. General trends of atomic radii in the periodic table are that they decrease going along each row (L–R) and increase going down each column.