Technetium is a chemical element with symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive; none are stable. Nearly all technetium is produced synthetically, and only minute amounts are found in the Earth's crust. Naturally occurring technetium is a spontaneous fission product in uranium ore or the product of neutron capture in molybdenum ores. The chemical properties of this silvery gray, crystalline transition metal are intermediate between rhenium and manganese. Many of technetium's properties were predicted by Dmitri Mendeleev before the element was discovered. Mendeleev noted a gap in his periodic table and gave the undiscovered element the provisional name ekamanganese (Em). In 1937, technetium (specifically the technetium-97 isotope) became the first predominantly artificial element to be produced, hence its name (meaning "artificial", + -ium). One short-lived gamma ray-emitting nuclear isomer of technetium—technetium-99m—is used in nuclear medicine for a wide variety of diagnostic tests. The ground state of this nuclide, technetium-99, is used as a gamma-ray-free source of beta particles. Long-lived technetium isotopes produced commercially are by-products of the fission of uranium-235 in nuclear reactors and are extracted from nuclear fuel rods. Because no isotope of technetium has a half-life longer than 4.2 million years (technetium-98), the 1952 detection of technetium in red giants, which are billions of years old, helped to prove that stars can produce heavier elements.