TECHNETIUM

(Tc), chemical element, synthetic radioactive metal of Group VIIb of the periodic table, the first element to be artificially produced. The isotope technetium-97 (2,600,000-year half-life) was discovered (1937) by the Italian mineralogist Carlo Perrier and the Italian-born American physicist Emilio Segr in a sample of molybdenum that had been bombarded by deuterons in the Berkeley (California) cyclotron. This isotope is the longest-lived member of a set from technetium-92 to technetium-107 that has since been produced. The most important isotope, because it is the only one available on a large scale, is technetium-99 (212,000-year half-life); it is produced in kilogram quantities as a fission product in nuclear reactors. Technetium metal looks like platinum but is usually obtained as a gray powder. It crystallizes in the hexagonal close-packed structure and is a superconductor below 11.2 K. Except for technetium-99, technetium-97, and technetium-98 (1,500,000-year half-life), technetium isotopes are short-lived. Technetium occurs in the Earth's crust as minute traces from the spontaneous fission of uranium; the relatively short half-lives preclude the existence of any primordial technetium on Earth. The American astronomer Paul W. Merrill's discovery in 1952 that technetium-99 is present in S-type stars was a valuable piece of evidence concerning stellar evolution and nucleosynthesis. Technetium, chemically similar to rhenium (atomic number 75), exists in oxidation states of +7, +6, and +4 in compounds such as KTcO4, potassium technetate(VII), TcCl6, technetium(VI) chloride, and TcS2, technetium(IV) sulfide, respectively. atomic number 43 commonest isotope (99) melting point 2,172 C (3,942 F) boiling point 4,877 C (8,811 F) specific gravity 11.5 (20 C) valence 4,6,7 electronic config. 2-8-18-14-1 or (Kr)4d65s1