All of its isotopes are radioactive; it is extremely rare, with only about 500–600 grams naturally occurring in the Earth's crust at any given time.

In 1926, two groups (one Italian and one American) claimed to have isolated a sample of element 61; both "discoveries" were soon proven to be false.

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".

Because natural promethium is exceedingly scarce, it is typically synthesized by bombarding uranium-235 (enriched uranium) with thermal neutrons to produce promethium-147 as a fission product.

For example, the melting point, the first three ionization energies, and the hydration energy are greater than those of neodymium and lower than those of samarium;[6] similarly, the estimate for the boiling point, ionic (Pm3+) radius, and standard heat of formation of monatomic gas are greater than those of samarium and less than those of neodymium.

[8] This low-temperature alpha form converts into a beta, body-centered cubic (bcc) phase upon heating to 890 °C.

[9] Promethium belongs to the cerium group of lanthanides and is chemically very similar to the neighboring elements.

[11][12] In May 2024, a promethium coordination complex with neutral PyDGA ligands was characterized in aqueous solution.

[13] Treatment of acidic solutions containing Pm3+ ions with ammonia results in a gelatinous light-brown sediment of hydroxide, Pm(OH)3, which is insoluble in water.

The most stable of them is promethium-148m, with a half-life of 43.1 days; this is longer than the half-lives of the ground states of all promethium isotopes, except for promethium-143 to 147.

[4] In 1934, Willard Libby reported that he had found weak beta activity in pure neodymium, which was attributed to a half-life over 1012 years.

[27] In 1965, Olavi Erämetsä separated out traces of 147Pm from a rare earth concentrate purified from apatite, resulting in an upper limit of 10−21 for the abundance of promethium in nature; this may have been produced by the natural nuclear fission of uranium, or by cosmic ray spallation of 146Nd.

[35] With the knowledge of a gap in the periodic table several groups started to search for the predicted element among other rare earths in the natural environment.

After separating a mixture of a few rare earth elements nitrate concentrate from the Brazilian mineral monazite by fractionated crystallization, they yielded a solution containing mostly samarium.

[39][40][41][42][43][44] Also in 1926, a group of scientists from the University of Illinois at Urbana–Champaign, Smith Hopkins and Len Yntema published the discovery of element 61.

[49][50] Promethium was first produced and characterized at Oak Ridge National Laboratory (Clinton Laboratories at that time) in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell by separation and analysis of the fission products of uranium fuel irradiated in the graphite reactor; however, being too busy with military-related research during World War II, they did not announce their discovery until 1947.

[49] It is derived from Prometheus, the Titan in Greek mythology who stole fire from Mount Olympus and brought it down to humans[49] and symbolizes "both the daring and the possible misuse of the mankind intellect".

[17] In 1963, ion-exchange methods were used at ORNL to prepare about ten grams of promethium from nuclear reactor fuel processing wastes.

[60] As early as the 1960s, Oak Ridge National Laboratory could produce 650 grams of promethium per year[61] and was the world's only large-volume synthesis facility.

[62] Gram-scale production of promethium was discontinued in the U.S. in the early 1980s, but will possibly be resumed after 2010 at the High Flux Isotope Reactor.

[65] In atomic batteries, the beta particles emitted by promethium-147 are converted into electric current by sandwiching a small promethium source between two semiconductor plates.

[12][22][49] The first promethium-based battery was assembled in 1964 and generated "a few milliwatts of power from a volume of about 2 cubic inches, including shielding".

[72] In general, gloves, footwear covers, safety glasses, and an outer layer of easily removed protective clothing should be used.

If a promethium leak is found, the area should be identified as hazardous and evacuated, and emergency services must be contacted.