Technetium is a chemical element in the periodic table that has the symbol Tc and atomic number 43. The chemical properties of this silvery gray, radioactive, crystalline transition metal are intermediate between rhenium and manganese and it is rarely found in nature. Its short-lived isotope Tc-99m is used in nuclear medicine to diagnose certain cancers, Tc-99 is used as a gamma ray-free source of beta rays, and its pertechnate ion could find use as a corrosion preventer for steel (this possible use is hindered by technetium's radioactivity).
Dmitri Mendeleev predicted many of the properties of element 43, which he called ekamanganese, well before its actual discovery (see Mendeleev's predicted elements). In 1937 its isotope Tc-97 became the first element to be artificially produced, hence its name (from the Greek technètos, meaning "artificial"). Most technetium produced on Earth is a by-product of fission of uranium-235 in nuclear reactors and is extracted from nuclear fuel rods. No isotope of technetium has a half life longer than 4.2 million years (Tc-98), so its detection in red giants in 1952 helped bolster the theory that stars can produce heavier elements.