A silvery-gray radioactive metal, the first synthetically produced element, having isotopes with masses ranging from 85 to 118 and half-lives up to 4.2 million years. It is principally used as a tracer in a variety of medical applications. It is a remarkable inhibitor of corrosion in steel, but this use is limited because of radioactivity hazards. Atomic number 43; melting point 2,157°C; boiling point 4,265°C; specific gravity 11.50 (calculated); valence 0, 2, 4, 5, 6, 7. See Periodic Table.