technetium(redirected from 99m)
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technetium (tĕknēˈshēəm) [Gr. technetos=artificial], artificially produced radioactive chemical element; symbol Tc; at. no. 43; mass no. of most stable isotope 98; m.p. 2,200℃; b.p. 4,877℃; sp. gr. 11.5 (calculated); valence +4, +6, or +7. Technetium is a radioactive silver-gray metal. In some of its chemical properties it resembles rhenium, the element below it in Group 7 of the periodic table. It tarnishes slowly when exposed to moist air. Although it is not attacked by hydrochloric acid, it dissolves in concentrated sulfuric or nitric acid and in aqua regia. The pure metal may be prepared by chemical reduction of certain of its compounds with hydrogen gas.
The most stable isotope, technetium-98, has a half-life of 6.6 million years; most of the other 30 known isotopes are much less stable. Technetium-95m is a gamma-ray emitter with a half-life of 61 days that is used in radioactive tracer studies. The most useful isotope of technetium, however, is technetium-99m, which is used in many medical radioactive isotope tests because of its short half-life (6.01 hours), the energy of the gamma radiation it emits, and its ability to bind chemically to many biologically active molecules. Potassium technetate, KTcO4, has found some use in alloys with iron and steel; the addition of a small amount renders the alloy highly resistant to corrosion. This use is limited by the radioactivity of the element.
Technetium was once very rare and expensive but is now obtained in quantity from nuclear reactor fission products. Although the spectra of some stars show that they contain technetium, the naturally occurring element has not been found on earth. It is called technetium because it was the first element to be prepared synthetically. Its existence was predicted from the periodic table. Discovery of the element in nature was erroneously claimed in 1925 by the German chemists I. W. and W. K. Noddack, who called it masurium. The element was discovered in 1937 by C. Perrier and E. G. Segrè of Italy in a sample of molybdenum that was bombarded with deuterons in a cyclotron at the Univ. of California at Berkeley and sent to them by E. O. Lawrence.
Tc, a radioactive chemical element of group VII of Mendeleev’s periodic system. Atomic number, 43; atomic weight, 98.9062; a malleable, ductile metal.
D. I. Mendeleev predicted the existence of an element with atomic number 43. Technetium was prepared artificially in 1937 by the Italian scientists E. Segré and C. Perrier during the deuteron bombardment of molybdenum nuclei. The name comes from the Greek technetos, meaning “artificial.”
Technetium has no stable isotopes. Only two of the approximately 20 radioisotopes are of practical importance: 99Tc and 99mTc, having half-lives of 2.12 × 105 years and 6.04 hours, respectively. The element occurs naturally in negligible quantities—10–10 g per ton of pitchblende.
Physical and chemical properties. Metallic technetium in powder form is gray, resembling Re, Mo, and Pt; the consolidated metal (ingots from molten metal, foil, wire) is silvery gray. In the crystalline state, technetium has a hexagonal close-packed structure, with a = 2.735 angstroms (Å) and c = 4.391 Å; in layers less than 150 Å thick, the lattice is face-centered cubic (a= 3.68 ± 0.0005 Å). Technetium (hexagonal lattice) has a density of 11.487 g/cm3, a melting point of 2200° ± 50°C, a boiling point of 4700°C, and an electrical resistivity of 69 × 10–6 ohm-cm (100°C). The element’s transition temperature to the superconducting state is 8.24°K. Technetium is paramagnetic, with a magnetic susceptibility of 2.7 × 10–4 at 25°C. The configuration of the outer electron subshells of the Tc atom is 4d55s2. The atomic radius is 1.358 Å, and the ionic radius of Tc7+ is 0.56 Å.
With regard to chemical properties, Tc resembles Mn and particularly Re; in compounds it exhibits oxidation states from –1 to +7. The most stable and thoroughly investigated Tc compounds are those in which the oxidation state is +7. Technetium and its compounds react with oxygen to yield the oxides Tc2O7and TcO2 and with chlorine and fluorine to yield the halides TcX6, TcX5, and TcX4, where X is the halogen. The formation of oxyhalides, such as TcO3X, is also possible. Technetium and its compounds react with sulfur to form the sulfides Tc2S7 and TcS2. The element forms pertechnic acid (HTcO4) and salts, or per-technetates, of the acid with the formula MTcO4, where M is a metal. In addition, technetium forms carbonyl, complex, and organometallic compounds.
In the electromotive series, technetium is positioned to the right of hydrogen; it does not react with hydrochloric acid of any concentration, but it readily dissolves in nitric and sulfuric acids, aqua regia, hydrogen peroxide, and bromine water.
Preparation. By-products of the atomic industry are the primary source of technetium. The 99Tc yield upon fission of 235U is approximately 6 percent. Technetium in the form of pertechne-tates, oxides, and sulfides is obtained from mixtures of fission products using extraction with organic solvents, ion-exchange methods, and precipitation of weakly soluble derivatives. The metal is obtained either by the reduction of NH4TcO4, TcO2, and Tc2S7 with hydrogen at 600°–1000°C or by electrolysis.
Use. Technetium holds great promise for use in technology; it can be used as a catalyst and as a high-temperature and superconducting material. Technetium compounds are effective corrosion inhibitors. The isotope 99mTc is used in medicine as a source of gamma radiation. Since technetium is hazardous because of radiation, all work with the element requires special airtight equipment.
REFERENCESKotegov, K. V., O. N. Pavlov, and V. P. Shvedov. Tekhnetsii. Moscow, 1965.
“Poluchenie 99Tc v vide metalla i ego soedinenii iz otkhodov atomnoi promyshlennosti.” In Proizvodstvo izotopov. Moscow, 1973.
A. F. KUZINA