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tellurium(tĕlo͝or`ēəm) [Lat.,=earth], semimetallic chemical element; symbol Te; at. no. 52; at. wt. 127.60; m.p. 450°C;; b.p. 990°C;; sp. gr. 6.24 at 20°C;; valence −2, +4, or +6. Tellurium is a lustrous, brittle, crystalline, silver-white metalloid. A powdery brown form of the element is also known. Tellurium forms many compounds corresponding to those of sulfur and selenium, the elements above it in Group 16 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
..... Click the link for more information. . The dioxide, TeO2, is formed when the element is burned in air. Tellurium forms two weak acids and a number of halogen compounds. With hydrogen and with some metals it forms tellurides. Tellurium and its compounds are probably poisonous. Tellurium is occasionally found uncombined in nature but is more often found combined with metals, as in the minerals calaverite (gold telluride) and sylvanite (silver-gold telluride). Tellurium is recovered as a byproduct of the electrolytic refining of blister copper. It is used as an additive to steel and is often alloyed with aluminum, copper, lead, or tin. It is used in vulcanizing rubber, as a coloring agent in glass and ceramics, and in catalysts for petroleum cracking. Tellurium is a semiconductor material and is slightly photosensitive. It is used with bismuth in thermoelectric devices. Tellurium was discovered in 1782 by Franz Muller von Reichenstein. It was named by M. H. Klaproth, who isolated it in 1798.
Te, a chemical element in the major subgroup of group VI of Mendeleev’s periodic system. Atomic number, 52; atomic weight, 127.60. A rare dispersed element. Tellurium occurs in nature in the form of eight stable isotopes, with mass numbers of 120, 122–26, 128, and 130; the most widespread are 128Te (31.79 percent) and 130Te (34.48 percent). Of the artificially produced radioisotopes, 127Te (half-life, 105 days) and 129Te (half-life, 33.5 days) are widely used as tracers. Tellurium was discovered by F. Müller von Reichenstein in 1782. The German scientist M. H. Klaproth confirmed the discovery and named the element tellurium after the Latin tellus, meaning “earth.” The first systematic studies on the chemistry of tellurium were carried out in the 1830’s by J. J. Berzelius.
Occurrence. Tellurium is one of the rarest elements; its average content in the earth’s crust (clarke) is ~1 × 10–7 percent by weight. Tellurium is dispersed in magma and in the biosphere; it precipitates with S, Ag, Au, Pb, and other elements from certain subterranean thermal springs. Hydrothermal deposits of Au and nonferrous metals enriched with tellurium are known; approximately 40 minerals of this element, the more important including altaite, tellurobismuthite, and other natural tellurides, are associated with these deposits. Admixtures of tellurium in pyrite and other sulfides are typical. Tellurium is extracted from complex ores.
Physical and chemical properties. Tellurium is silvery white in color and has a metallic luster; it is brittle and becomes ductile with heating. The element crystallizes in the hexagonal system: a = 4.4570 angstroms (Å), and c = 5.9290 A. It has a density of 6.25 g/cm3 at 20°C, a melting point of 450°C, a boiling point of 990° ± 1.0°C, and a specific heat capacity at 20°C of 0.204 kilojoule/kg.K (0.047 calorie/g.°C). Its thermal conductivity at 20°C is 5.999 watts/m.°K (0.014 calorie/cm.sec.°C), and its coefficient of linear thermal expansion is 1.68 × 10–5 (20°C). Tellurium is diamagnetic, with a specific magnetic susceptibility of 0.31 × 10–6 at 18°C. It has a Brinell hardness of 184.3 meganewtons/m2 (18.43 kilograms-force/mm2), an atomic radius of 1.7 Å, and ionic radii of 2.22 Å (Te2), 0.89 Å (Te4+), and 0.56 Å (Te6+).
Tellurium is a semiconductor; its energy gap is equal to 0.34 electron volts. Under ordinary conditions and at temperatures up to its melting point, pure tellurium exhibits p-type conductivity. A reduction in temperature in the interval between – 100°C and – 80°C brings about a transition to n-type conductivity. The temperature of the transition is dependent on the purity of the metal: the purer the metal, the lower the temperature.
The configuration of the outer subshells of the tellurium atom is 5s25p4. In compounds, the element exhibits oxidation states of –2, +4, +6, and, more rarely, +2. Tellurium is the chemical analogue of sulfur and selenium, but its metallic properties are more pronounced. It reacts with oxygen to form the oxide TeO, dioxide TeO2, and trioxide TeO3. TeO exists in the gaseous phase at temperatures above 1000°C. TeO2 is obtained by burning Te in air; it has amphoteric properties and is sparingly soluble in water but freely soluble in acidic and alkaline solutions. TeO 3is unstable and can be obtained only upon decomposition of telluric acid. When heated, tellurium reacts with hydrogen to form hydrogen telluride (H2Te), a colorless toxic gas with a pungent, unpleasant odor.
Tellurium reacts readily with halogens; the characteristic halides for tellurium are of the type TeX2 and TeX4, where X is Cl or Br. The compounds TeF4 and TeF6 have also been obtained. All these halides are highly volatile and are hydrolyzed with water. Tellurium reacts directly with nonmetals (S, P), as well as with metals; at room temperature, it reacts with concentrated nitric and sulfuric acids, in the latter case forming TeSO3, which is oxidized upon heating to TeOSO4. Relatively weak Te acids are known, including hydrotelluric acid (solution of H2Te in water), tellurous acid (H2TeO3), and telluric acid (H6TeOa); their salts (tellurides, tellurites, and tellurates, respectively) are either sparingly soluble or completely insoluble in water, with the exception of the salts of alkali metals and ammonium. Certain organic tellurium derivatives are known, for example, RTeH and dialkyl tellurides (R2Te), which are low-boiling liquids with an unpleasant odor.
Preparation. Tellurium is extracted as a by-product during the processing of sulfide ores from intermediate products of copper and lead-zinc production, as well as from certain gold ores. The main source of tellurium is the slime from copper electrolysis, which contains from 0.5 to 2 percent Te, as well as Ag, Au, Se, Cu, and other elements. Cu and Se are first removed from the slime, and the residue, which contains precious metals, Te, Pb, Sb, and other components, is remelted in order to obtain an alloy of gold and silver. During this process, tellurium, in the form of Na2TeO3, enters a soda slag, where the Te content can reach 20–35 percent. The slag is then pulverized and leached with water. From this solution, tellurium is deposited on a cathode through electrolysis. The tellurium concentrate thus obtained is processed with an alkali in the presence of aluminum powder, thereby dissolving tellurium in telluride form. The solution is separated from the insoluble residue, which has a high concentration of heavy metal impurities; air is then passed through the liquid. At this point, tellurium (99 percent pure) is precipitated in its elementary state. Tellurium of higher purity is obtained by repeating the process involving the tellurides. The purest tellurium is produced by combining methods of chemical purification, distillation, and zone melting.
Use. Tellurium is used in semiconductor technology; it is also used as an alloying additive in alloys of lead, cast iron, and steel in order to improve fabricating characteristics and mechanical properties. The compounds Bi2Te3 and Sb2Te3 are used in thermoelectric generators, and CdTe is used in solar batteries and as a semiconductor laser material. Tellurium is also used for the chilling of cast iron, the vulcanization of latex mixes, and the production of brown and red glasses and enamels.
T. N. GREIVER
In the organism. Tellurium is always present in plant and animal tissue. For plants growing on soil rich in tellurium, the Te concentration can reach 2 × 10–4–2.5 × 10–3 percent; in terrestrial animals, the concentration is approximately 2 × 10–6 percent. The daily human intake of tellurium with food and water is approximately 0.6 mg; the element is eliminated from the body primarily in the urine (more than 80 percent), as well as in the feces. Tellurium is moderately toxic for plants and highly toxic for mammals, causing, for example, cessation of growth, loss of hair, and paralysis.
Industrial tellurium intoxication can occur during melting and other manufacturing processes. Symptoms include chills, headaches, asthenia, rapid pulse, loss of appetite, metallic taste in the mouth, garlic breath, nausea, dark coloring of tongue, irritation of the respiratory tract, perspiration, and loss of hair. Preventive measures include adherence to the requirements of occupational hygiene, use of coverings to protect the skin, and periodic medical examinations.
REFERENCESKudriavtsev, A. A. Khimiia i tekhnologiia selena i tellura, 2nd ed. Moscow, 1968.
Osnovy metallurgii, vol. 4, ch. 8. Moscow, 1967.
Filiand, M. A., and E. I. Semenova. Svoistva redkikh elementov, 2nd ed. Moscow, 1964.
Buketov, E. A., and V. P. Malyshev. lzvlechenie selena i tellura iz medeelektrolitnykh shlamov. Alma-Ata, 1969.
Bowen, H. I. M. Trace Elements in Biochemistry. London-New York, 1966.