International audienceAtomic symbol: Sc Atomic number: 21 Atomic weight: 44.95591 Isotopes and abundances: 45 Sc 100 % 1 Atm melting point: 1541 C 1 Atm boiling point: 2836 C Common valences: 3+ Ionic radii: 6-fold, 74.5 pm, 8-fold, 87 pm Pauling electronegativity: 1.36 First ionization energy: 633.1 kJ/mol Chondritic (CI) abundance: 5.81 ppm Silicate Earth abundance: 16.4 ppm Crustal abundance: 21.9 Seawater abundance: 2–20 pmol/L Core abundance: $0 Properties Scandium is the lightest group 3 (IIIB) element and is the lightest of the transition metals. Its atomic number (proton number) is 21, has only one long-lived isotope, and has an atomic mass of 44.95591 u. Scandium's electronic configuration is [Ar]4s 2 3d 1 and only occurs in the trivalent state (Sc 3+) in nature. Thus, unlike most other period 4 transition metals, the geochemical behavior of scandium is not affected by redox conditions and shows lithophile behavior. The effective ionic radii in six-and eightfold coordination are 74.5 and 87 pm, respectively (Shannon, 1976), and its Pauling electronegativ-ity is 1.36. Pure scandium metal has a melting point of 1541 C at 1 atm. The International Union of Pure and Applied Chemistry considers Sc to be a rare earth element (REE) (Damhus et al., 2005); however, the ionic radius and electronic configuration of Sc are sufficiently different from yttrium and the lanthanides that it is generally excluded from discussions of the REE. History and Use Scandium was " discovered " by Lars Fredrik Nilson in 1879 by separation from rare earth mixtures from euxenite and gadolinite and is named for Scandinavia. Global production of Sc is small ($10 tonnes per year) as a by-product from mining of ores of titanium, rare earths, apatite, and uranium. Bauxites are another potential source of Sc, as it gets concentrated, along with other elements, in the " red mud " residue that results from Al processing (Deady et al., 2016). The principal uses of Sc are in Sc-Al alloys and in solid oxide fuel cells. Minor amounts of Sc are also used in a variety of other applications including electronics, lasers, and lighting. Natural Abundances The abundance of Sc in the solar system, as estimated from chondritic meteorites, is ca. 5.81 ppm (McDonough and Sun, 1995). The refractory behavior of this element in the solar nebula as well as its lithophile behavior led to the relative concentration of this element in the silicate Earth, with an estimated abundance of about 16 ppm, which is also th