scandium

scandium (Sc), chemical element, a rare-earth metal of Group 3 of the periodic table.

Scandium is a silvery white, moderately soft metal. It is fairly stable in air but will slowly change its colour from silvery white to a yellowish appearance because of formation of Sc₂O₃ oxide on the surface. The metal slowly dissolves in diluted acids—except hydrofluoric acid (HF), in which a protective trifluoride layer prevents further reaction. Scandium is paramagnetic from 0 K (−273 °C, or −460 °F) to its melting point (1,541 °C, or 2,806 °F). It becomes superconducting at −273.1 °C (−459.6 °F) at pressures exceeding 186 kilobars.

After Russian chemist Dmitry Ivanovich Mendeleyev in 1871 predicted this element’s existence, tentatively calling it ekaboron, Swedish chemist Lars Fredrik Nilson in 1879 discovered its oxide, scandia, in the rare-earth minerals gadolinite and euxenite, and Swedish chemist Per Teodor Cleve later in 1879 identified scandium as the hypothetical ekaboron. Scandium is found in small proportions, generally less than 0.2 percent, in many of the heavy lanthanide ores and in many tin, uranium, and tungsten ores. Thortveitite (a scandium silicate) is the only mineral containing large amounts of scandium, about 34 percent, but unfortunately this mineral is quite rare and is not an important source of scandium. The cosmic abundance of scandium is relatively high. Although it is only about the 50th most abundant element on Earth (its abundance is similar to that of beryllium), it is about the 23rd most abundant element in the Sun.

In nature, scandium exists in the form of one stable isotope, scandium-45. Among 25 (excluding nuclear isomers) radioactive isotopes with masses ranging from 36 to 61, the most stable is scandium-46 (half-life of 83.79 days), and the least stable is scandium-39 (half-life of less than 300 nanoseconds).

Scandium is separated from the other rare earths by precipitation of the insoluble potassium scandium sulfate or by extraction of scandium thiocyanate by diethyl ether. The metal itself was first prepared in 1938 by the electrolysis of potassium, lithium, and scandium chlorides in a eutectic mixture (i.e., a mixture having the lowest melting point possible with those components). Scandium is now produced mostly as a by-product of uranium extraction from the mineral davidite, which contains about 0.02 percent scandium oxide. Scandium exists in two allotropic (structural) forms. The α-phase is close-packed hexagonal with a = 3.3088 Å and c = 5.2680 Å at room temperature. The β-phase is body-centred cubic with an estimated a = 3.73 Å at 1,337 °C (2,439 °F).

Only a few uses of this unusual transition metal have been developed, mostly due to scandium’s limited availability and high cost. Its low density and high melting point suggest applications as an alloying agent for lightweight metals for military and high-performance applications. The major uses of scandium are as an alloy additive to aluminum-based alloys for sporting goods and in high-intensity metal halide lamps. When alloyed with aluminum and aluminum-based alloys, scandium limits high-temperature grain growth.

The chemistry of scandium bears a closer resemblance to that of the other rare-earth elements of oxidation state +3 than to that of aluminum or titanium. Some of its behaviour, however, is atypical of the rare earths because of its significantly smaller ionic radius (1.66 Å for coordination number 12) as compared with the rare-earth average (1.82 Å for coordination number 12). For this reason, the Sc³⁺ ion is a relatively strong acid and has a much greater tendency to form complex ions.