A chemical element, symbol Sc, atomic number 21, atomic weight 44.9559. The only naturally occurring isotope is ⁴⁵Sc. The electronic configuration of the ground-state, gaseous atom consists of the argon rare-gas core plus three more electrons in the 3d¹4s² levels. It is the first element in the periodic table to have an unfilled inner shell and thus is the first transition metal. It is one of the elements of the rare-earth group as defined by the International Union of Pure and Applied Chemistry.