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 degrees Celsius; b.p. 4,877 degrees Celsius; 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 VIIb 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. 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. The most stable isotope, technetium-98, has a half-life of 4.2 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 sometimes used in radioactive tracer studies. 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.

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