According to Mulliken's definition, the atomic electronegativity is closely allied to ionicity. So the electronegativity of an atom in the stable molecule may be considered as the electric potential at the ionic boundary. In other words, the electronegativity of an atom is directly proportional to its effective nuclear charge and inversely proportional to its ionic radius. By plotting this electric potential against Pauling's electronegativity scale, a linear relationship between them is obtained.