Promethium, polonium, astatine and radon are radioactive, and therefore present radioactive hazards. Thulium is the least abundant lanthanide on earth except for the radioactive promethium. Elements heavier than lead, and the elements technetium and promethium, exist only as radionuclides. Promethium briefly replaced radium as a radiation source. Thus it is the only promethium isotope with an experimentally observed alpha decay. The artificial chemical element promethium is named after Prometheus. The second-lightest exclusively radioactive element, promethium, has an atomic number of 61. Promethium may be favored over tritium for nuclear safety reasons. Only the highly unstable and radioactive promethium "rare earth" is quite scarce. No dangers from promethium aside from the radioactivity are known. Because of its instability, chemical studies of promethium are incomplete. In 1963, promethium(III) fluoride was used to make promethium metal. A promethium atom has 61 electrons, arranged in the configuration [Xe]4f6s. While saving Kord from an industrial accident, Calhoun is covered with metals laced with promethium. Promethium shows only one stable oxidation state of +3.