scandium

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scandium

(skăn`dēəm), metallic chemical element; symbol Sc; at. no. 21; at. wt. 44.95591; m.p. 1,541°C;; b.p. 2,831°C;; sp. gr. 2.99 at 20°C;; valence +3. Scandium is a soft silver-white metal. It is a member of Group 3 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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; because of its chemical and physical properties, its scarcity, and the difficulty in extracting the metal, it is sometimes regarded as one of the rare-earth metalsrare-earth metals,
in chemistry, group of metals including those of the lanthanide series and actinide series and usually yttrium, sometimes scandium and thorium, and rarely zirconium. Promethium, which is not found in nature, is not usually considered a rare-earth metal.
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. At ordinary temperatures it crystallizes in a hexagonal close-packed structure. It tarnishes slightly when exposed to air. It reacts with many acids. It forms an oxide and a number of colorless salts. Its compounds are found widely distributed in minute amounts in nature. It is a major component of the rare Norwegian mineral thortveitite. It is found in many of the rare-earth minerals and in certain tungsten and uranium ores. Scandium is found in relatively greater abundance in the sun and certain stars than on earth. The metal has little commercial importance. In 1970 pure scandium cost several thousand dollars per pound. Scandium oxide (scandia) finds use as a catalyst and in making crucibles and other ceramic parts. Scandium sulfate in very dilute aqueous solution is used in agriculture as a seed treatment to improve the germination of corn, peas, wheat, and other plants. Scandium was discovered by L. F. Nilson in 1879 by spectroscopic analysis of euxenite and gadolinite. It was later shown by P. T. Cleve to correspond to the ekaboron predicted in 1871 by MendeleevMendeleev, Dmitri Ivanovich
, 1834–1907, Russian chemist. He is famous for his formulation (1869) of the periodic law and the invention of the periodic table, a classification of the elements; with Lothar Meyer, who had independently reached similar conclusions, he was
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 from his periodic law.

Scandium

 

Sc, a chemical element in Group III of Mendeleev’s periodic system. Atomic number, 21; atomic weight, 44.9559. Scandium is a light metal with a characteristic yellowish cast appearing upon contact with air. One natural stable isotope, 45Sc, is known. Of the ten artificial radioisotopes, 46Sc, with a half-life of 84 days, is the most important. Scandium was predicted by D. I. Mendeleev in 1870 and isolated by L. F. Nil-son in 1879 from the minerals gadolinite and euxenite. These minerals are found in Scandinavia (Latin Scandia), from which the element takes its name.

Distribution in nature. The average content of scandium in the earth’s crust (clarke) is 2.2 × 10-3 percent by weight. The content of scandium in rocks varies. There is 5 × 10-4 percent in ultrabasic rocks, 2.4 × 10-3 percent in basic rocks, 2.5 × 10-4 percent in intermediate rocks, 3 × 10-4 percent in granites and syenites, and from 1 × 10-4 to 1.3 × 10-4 percent in sedimentary rock. Scandium is concentrated in the earth’s crust as a result of magmatic and hydrothermal processes and of hypergen-esis (surficial alteration). The two known scandium minerals, thortveitite and sterrettite, are rare. In general, scandium is a typical dispersed element, with only a slight capacity for migration. The content of scandium in seawater is 4 × 10-5 gram per liter.

Physical and chemical properties. The crystal structure of scandium has α and β modifications. At ordinary temperatures, α-scandium is hexagonal close-packed (a = 3.3080 angstroms [Å] and c = 5.2653 Å) and stable; at temperatures above 1350°C, the crystal structure is that of β-scandium and is body-centered cubic. The density of α-scandium is 3.020 g/cm3 at 25°C; the atomic radius is 1.64 A and the ionic radius is 0.75 A. The element has a melting point of 1539° ±5°C and a boiling point of 2700°C; it evaporates above 1600°C. At 25°C, the specific heat capacity is 25.158 kilojoules/kg.°K (6.01 kilocalories/g.°C) and electrical resistivity ranges from 54 × 10-6 to 70.7 × 10-6 ohm-cm. Scandium is weakly paramagnetic and its atomic magnetic susceptibility is 236 × 10-6 at 20°C. Scandium is the first transition element with one 3d electron, and its configuration of outer electrons is 3d14s2.

Scandium is a soft metal, which, in the pure state, is easily worked by forging, rolling, and stamping.

Scandium is similar in chemical behavior to the other transition elements in the +3 oxidation state (for example, Ti3+, Fe3+, and Mn3+), to elements of the Al and Be subgroups, and to elements of the yttrium subgroup. Scandium, together with the yttrium subgroup, is sometimes considered a rare-earth element. In the presence of air, scandium is covered by a protective oxide film having a thickness up to 600 Å; marked oxidation commences at 250°C. Upon reaction with hydrogen at 450°C, scandium forms the hydride ScH2; it reacts with nitrogen at 600°–800°C to form the nitride ScN and with halogens at 400°–600°C to form compounds of the ScCl3 type. Scandium also reacts with boron and silicon above 1000°C. The metal dissolves easily in hydrochloric, nitric, and sulfuric acids. Here, the rate of dissolution drops sharply with decreasing acid concentration and reaches zero with 0.001 N solutions. The scandium salts of hydrochloric, sulfuric, nitric, thiocyanic, and acetic acids dissolve well in water, while those of phosphoric, oxalic, and hydrofluoric acids are poorly soluble. Scandium acetylacetonate and the fluorine derivatives have moderate volatility. Dilute solutions of NaOH (10 percent) and a mixture of concentrated HNO3 and HF (1:1) have practically no effect on scandium. In water, compounds of scandium undergo marked hydrolysis with the formation of basic salts. Sc3+ ions tend toward polymerization and the formation of the ions depends on the nature of the anion and the pH of the medium; examples are Scandium. The basic salts of scandium are easily converted in solution into an amorphous hydroxide.

Production and use. Scandium is extracted primarily as a byproduct in oxide form in the hydrometallurgical and pyro-metallurgical treatment of bauxites and of tungsten, tin, titanium, and uranium ores. The oxides are chlorinated or fluorinated at elevated temperatures. Hard metallic scandium (yield ∼99.5 percent) is then obtained by the reduction of scandium chloride or scandium fluoride. The reduction is carried out by heating the compound with metallic calcium and is followed by a distillation process in a high vacuum (133.3 × 10-6 newton/m2[10-6 mm Hg]) at 1600°–1700°C.

Scandium is used on only a limited scale. Scandium oxide is used in the production of ferrites for memory elements of computers. Radioactive 46Sc is used in neutron activation analysis and in medicine. Alloys of scandium, which have low densities and high melting points, are promising as structural materials in rocket and aircraft construction. Several compounds of scandium are suitable for use in the preparation of luminophors and oxide cathodes, in glass and ceramic production, and in the chemical industry, where the compounds serve as catalysts.

REFERENCES

Borisenko, L. F. Skandii. Moscow, 1961.
Favorskaia, L. V. Khimicheskaia tekhnologiia skandiia. Alma-Ata, 1969.
Kogan, B. I., and V. A. Nazvanova, Skandii. Moscow, 1961.
Spravochnik po redkim metallam. Moscow, 1965. (Translated from English.)
Vickery, R. C. The Chemistry of Yttrium and Scandium. Oxford, 1960.

L. N. KOMISSAROVA

scandium

[′skan·dē·əm]
(chemistry)
A transition element, symbol Sc, atomic number 21; melts at 1200°C; found associated with rare-earth elements.

scandium

a rare light silvery-white metallic element occurring in minute quantities in numerous minerals. Symbol: Sc; atomic no.: 21; atomic wt.: 44.955910; valency: 3; relative density: 2.989; melting pt.: 1541°C; boiling pt.: 2836°C
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