technetiumn.

A dense, silvery grey, radioactive metallic element, atomic number 43, chemically similar to rhenium, which occurs naturally only in trace amounts but is produced in reactors as a long-lived by-product of the fission of uranium-235. Symbol Tc.Technetium is the lightest element to have no stable isotopes. The longest-lived isotope is technetium-98, with a half-life of 4.2 million years. Its applications include use as a tracer in scintigraphy, and as an emitter of beta particles for instrument calibration purposes.Mendeleev predicted the existence of an element between molybdenum (atomic number 42) and ruthenium (44) that would be chemically similar to manganese. He called it ekamanganese. Subsequently there were a number of reports of the isolation of element 43, but the substances concerned proved to be either a mixture of known elements or an impure sample of a known element. In 1925 W. Noddack et al. reported to have identified element 43 from X-ray diffraction spectra, and called it masurium n., but the result was not generally accepted. The name technetium was given to the element in 1947 when it became the first element to be synthesized, though Perrier and Segrè had identified it in 1937 in radioactive cyclotron parts.