Promethium, originally prometheum, is a Chemical element with the symbol Pm and Atomic number 61. All its isotopes are radioactive; it is one of only two elements with Z < 84 which have no stable isotopes; the other is Technetium. Chemically, promethium is a Lanthanide, which forms salts when combined with other elements. Promethium shows only one stable oxidation state of +3; however, a few +2 compounds may exist.
In 1902, Bohuslav Brauner suggested there was an element with properties intermediate between those of the known elements Neodymium (60) and Samarium (62); this was confirmed in 1914 by Henry Moseley who, having measured the atomic numbers of all the elements then known, found there was no element with atomic number 61. In 1926, an Italian and an American group claimed to have isolated a sample of element 61; both "discoveries" were soon proven to be false. In 1938, during a nuclear experiment conducted at Ohio State University, a few radioactive nuclides were produced that certainly were not radioisotopes of neodymium or samarium, but there was a lack of chemical proof that element 61 was produced, and the discovery was not generally recognized. Promethium was first produced and characterized at Oak Ridge National Laboratory in 1945 by the separation and analysis of the fission products of uranium fuel irradiated in a graphite reactor. The discoverers proposed the name "prometheum" (the spelling was subsequently changed), derived from Prometheus, the Titan in Greek mythology who stole fire from Mount Olympus and brought it down to humans, to symbolize "both the daring and the possible misuse of mankind's intellect." However, a sample of the metal was made only in 1963.
Practical applications exist only for chemical compounds of promethium-147, which are used in luminous paint, atomic batteries and thickness measurement devices.
There are two possible sources for natural promethium. Europium-151 is an effectively stable nuclide whose mole fraction in solar-system Eu is around 0.48. Average crustal abundance of Eu is around 2 ppm, so it's moderately rare. Although stable for all practical purposes, 151Eu does decay to 147Pm, with a half-life around 4E18 yr. Its specific activity is near 1E-19 (decays/mole)/yr, It contributes only to production of 147Pm. If it were the only source of 147Pm, it would come into equilibrium with 151Eu when [147Pm]/[151Eu] = 7E-19,
The other mechanism which produces Pm is fission of actinide elements. In reactors and laboratories, induced fission of 235U or 239Pu generates Pm among its fission products. Elsewhere, in places like highly evolved stars (2nd ascent red giants) and terrestrial planets, neutrons come from spontaneous decay, and almost all spontaneous decay is that of 238U. Neutron capture by stable Nd isotopes up to 146Nd, which leads to 147Nd, which beta decays to 147Pm. Neutron capture by 148Nd (0.057 of all Nd captures) leads to 149Nd, then 149Pm. Neutron capture by 150Nd lead to 151Pm. Of these possibilities, 147Pm will form in by far the largest amounts both because of its long (2.62 yr) half-life and the high concentration of 146Nd (0.172 per unit).
Promethium is synthesized in stars by the slow capture of neutrons (s process). Two stars [GY Andromedae and Przybylski's star (HD 101065)] show Pm lines in their spectra. Its presence there is a puzzle. Pm forms deep in a star, while its spectrum originates at the surface. Since the longest-lived isotope of Pm has a half-life of 17.7 years, it should not survive to reach the surface. There have been a number of explanations given for the presence of Pm lines, some involving aliens. One suggestion is that Pm is produced at the surface by spontaneous fission decay of upper actinides or superheavy elements. The fact that only two stars, and not the brightest ones of their type, show Pm lines support this idea by providing a way for fission-decaying elements to be deposited on these stars. Neutron star mergers are very rare (about 2 per million years per galaxy), they eject large quantities of upper actinides and superheavy elements, and the diffuse remnant becomes completely invisible within a few decades. It is possible that GY Andromedae and Przybylski's star have recently hit kilonova (neutron star merger) remnants.
Since production of Pm on earth is constant on time scales less than a century or so. Pm 147 is the most abundant isotope of the element, and the only one used commercially, so the atomic mass of Pm can be called that of Pm 147, except for close work or special cases. That number does not have to change with time. Rather than atomic mass number it is appropriate to report atomic mass for Pm.