Promethium
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.
There are two possible sources for natural promethium: rare decays of natural europium-151 (producing promethium-147), and uranium (various isotopes). Practical applications exist only for chemical compounds of promethium-147, which are used in luminous paint, atomic batteries and thickness measurement devices, even though promethium-145 is the most stable promethium isotope. Since natural promethium is exceedingly scarce, the element is typically synthesized by bombarding uranium-235 (enriched uranium) with thermal neutrons to produce promethium-147.
Occurrence
In 1934, Willard Libby found weak beta activity in pure neodymium, which was attributed to a half-life over 1012 years. Almost 20 years later, it was claimed that the element occurs in natural neodymium in equilibrium in quantities below 10−20 grams of promethium per one gram of neodymium. However, these observations were disproved by newer investigations, because for all seven naturally occurring neodymium isotopes, any single beta decays (which can produce promethium nuclides) are forbidden by energy conservation. In particular, careful measurements of atomic masses show that the mass difference 150Nd-150Pm is negative (−87 keV), which absolutely prevents the single beta decay of 150Nd to 150Pm.
Both isotopes of natural europium have larger mass excesses than sums of those of their potential alpha daughters plus that of an alpha particle; therefore, they (stable in practice) may alpha decay. Research at Laboratori Nazionali del Gran Sasso showed that europium-151 experimentally decays to promethium-147 with the half-life of 5×1018 years. It has been shown that europium is "responsible" for about 12 grams of promethium in the Earth's crust. Alpha decays for europium-153 have not been found yet, and its theoretically calculated half-life is so high (due to low energy of decay) that this process will probably never be observed.
Finally, promethium can be formed in nature as a product of spontaneous fission of uranium-238. Only trace amounts can be found in naturally occurring ores: a sample of pitchblende has been found to contain promethium at a concentration of four parts per quintillion (1018) by mass. Uranium is thus "responsible" for 560 g promethium in Earth's crust.
Promethium has also been identified in the spectrum of the star HR 465 in Andromeda; it also has been found in HD 101065 (Przybylski's star) and HD 965. Because of the short half-life of promethium isotopes, they should be formed near the surface of those stars.
| Symbol | Pm | |
| Atomic Number | 61 | |
| Atomic Weight | 144.9127 | |
| Oxidation States | +3 | |
| Electronegativity, Sanderson | 0.94 | |
| State at RT | Solid, Metal | |
| Melting Point, K | 1441 | |
| Boiling Point, K | 3000 |
Harmful effects:
Promethium is harmful due to its radioactivity.
Characteristics:
- Promethium is the only rare earth radioactive metal.
- In the dark, its salts luminesce with a pale blue or green glow due to its high radioactivity.
- Isotope 147Pm, the most useful isotope, decays with energetic gamma rays.
Uses of Promethium
- Promethium is useful as a beta source for thickness gauges.
- Promethium is also used in atomic batteries for spacecraft and guided missiles.
