Rare earths, the final frontier, a 151 year mission to explore for a new group of elements. Promethium, the last of the rare-earth elements to be discovered, was the most elusive. From 1794 to 1945, it took the best scientists in the world countless hours of research and liquid refreshments to uncover this assemblage of 17 elements, so chemically similar, yet curiously diverse.


  • Luminous promethium range-marks are used in the targeting sights of shoulder-fired missiles.
  • Promethium is applied on watch hands and dials to glow in the dark.
  • Promethium is used as a starter switch in energy-efficient compact fluorescent lamps (CFL).
  • The radiation from promethium is used as a thickness gauge for thin plastics, sheet metal, rubber, textiles, and paper.

Interesting Facts

  • Promethium is used in nuclear powered batteries that have a useful life of five years.
  • It is used in Electric blanket thermostats.


A trio of American scientists at Oak Ridge National Laboratory (ORNL), Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell discovered promethium in 1945 (Weeks and Leicester, 1968, p. 835). The element was separated and analyzed from the fission products of uranium fuel irradiated in a graphite reactor. The discovery team did not announce their findings until 1947 because of critical research that was underway at ORNL during World War II. The name Prometheum was suggested by Mary Coryell, wife of one of the discoverers, after the mythological Greek Titan, Prometheus, who stole fire from the gods on Mount Olympus and brought it down to Earth (Marinsky, Glendenin, and Coryell, 1947). The International Union of Pure and Applied Chemistry accepted the discovery, however, they changed the spelling to Promethium to conform to the other elements (Weeks and Leicester, 1968, p. 835).


Promethium is a silver-white metal that tarnishes slowly in air to a pink tint and burns readily in air at 150 °C. No stable isotopes of the element exist and 38 radioisotopes have been identified. The original isotope discovered was Promethium-147 which has a half-life of 2.62 years, and is a uranium fission product from nuclear reactors via the beta decay of neodymium-147. Promethium-145 has the longest half-life at 17.7 years. The metal has a double hexagonal close-packed structure, a density of 7.22 g/cm3, a melting point of 1168 °C, and an estimated boiling point of 2460 °C (Wheelwright, 1969). Promethium oxide, or promethia, is a pale pink color and occurs as a sesquioxide with the formula Pm2O3. Dependant on temperature, the oxide occurs in three crystal systems. At ambient temperatures, promethium oxide occurs in cubic form with a specific gravity of 6.85 g/cm3, above 750-800 °C it occurs in monoclinic form with a specific gravity of 7.48 g/cm3, and above 1740 °C it occurs in hexagonal form with a specific gravity of 7.62 g/cm3 (Chikalla, McNeilly, and Roberts, 1972).

Preparation of Metal

The first promethium metal was prepared in 1963 by the reduction of PmF3 with Li vapor (Weigel, 1963). Promethium-147 metal is prepared using calciothermic reduction of the trihalide, typically anhydrous PmCl3. The reduction uses high-purity calcium metal as a reductant to reduce the PmCl3 in a magnesium oxide crucible. Additional purification to remove the calcium and magnesium impurities requires a vacuum distillation step to yield a high-purity promethium metal (Wheelwright, 1969).


Naturally Occurring: Trace amounts of promethium have been found in naturally occurring minerals. A sample of the mineral pitchblende has been found to contain promethium at a concentration of four parts per quintillion (1018) by mass (Attrep and Kuroda, 1968). Promethium occurs naturally in the Earth's crust at miniscule concentrations. By calculating the content of promethium at secular equilibrium in the Earth's crust, about 560 grams would naturally occur as a result of the spontaneous fission of uranium-238 and about 12 grams would naturally occur due to the alpha decay of europium-151 (Belli and others, 2007).

Manmade: All commercially used promethium is created in nuclear reactors. Traditional production of promethium-147 is from processing uranium fission products with high-activity and a yield of 2.25%. Although high-levels of radioactive waste are generated during the processing, this process is the primary method of production for commercial quantities. An alternate method of producing promethium-147 uses a High Flux Isotope Reactor (HFIR). In this process, enriched neodymium-146 is irradiated in the HFIR to create neodymium-147 in the peripheral target positions which decay to promethium-147. The yield from the HFIR process is lower than that from uranium fission but will provide promethium-147 in useful quantities of 500 to 1000 curies for research and development (Knapp, Mirzadeh, and Boll, 2007).


Promethium does not occur in sufficient quantities in the Earth's crust to be economically mined.

Selected promethium minerals

No promethium minerals have been discovered.

Selected minerals containing promethium

Uraninite, variety pitchblende1 UO2.

1 Extremely small trace amounts of promethium have been identified in the uranium mineral pitchblende.


Attrep, Moses, Jr.; and P. K. Kuroda, 1968, Promethium in pitchblende: Journal of Inorganic and Nuclear Chemistry v. 30, no. 3, p. 699-703.

Beaudry and Bernard J. and Karl A. Gschneidner, Jr., 1978, Preparation and Basic Properties of the Rare Earth Metals: chapter 2 in Handbook of the Physics and Chemistry of Rare Earths-Volume 1:Metals, (Gschneidner, Jr. and Eyring, editors), North-Holland, New York, p. 173-232.

Belli, P., R. Bernabei, F. Cappella, R. Cerulli, C.J. Dai, F.A. Danevich, A. d'Angelo, A. Incicchitti, V.V. Kobychev, S.S. Nagorny, S. Nisi, F. Nozzoli, D. Prosperi, V.I. Tretyak, S.S. Yurchenko , 2007, Search for a decay of natural europium: Nuclear Physics, v. A, no. 789, p. 15-29.

Chikalla, T.D., C E. McNeilly, and E.P. Roberts, 1972, Polymorphic Modifications of Pm2O3: Journal of the American Ceramic Society, v. 55, no. 8, p. 428.

Knapp, F.F. (Russ), Saed Mirzadeh, and Rose Boll, 2007, ORNL Nuclear medicine program develops efficient method for separation of promethium-147 from reactor-produced neodymium-147: Isotope News, U.S. Department of Energy, Spring/Summer 2007, v. 1, issue 1, p. 5-7.

Marinsky, Jacob A., Lawrence E. Glendenin, and Charles D. Coryell, 1947, The chemical identification of radioisotopes of neodymium and element 61: Journal of the American Chemical Society, v. 69, p. 2781-2785.

Weeks, Mary E., and Henry M. Leicester, 1968, Discovery of the Elements (7th ed.): Easton, Pennsylvania, Journal of Chemical Education, 896 p.

Weigel, Fritz., 1963, Darstellung von metallischen promethium: Angewandte Chemie, May 21, v. 75, issue 10, p. 451.

Wheelwright, Earl J., 1969, Preparation of promethium-147 metal and determination of the density and melting points: Journal of Physical Chemistry, September, v. 78, no. 9, p. 2867-2871.

Electrons per shell:
2, 8, 18, 23, 8, 2
Atomic number,
Protons, Electrons:
Number of Neutrons:
Atomic Mass:
145.0 amu
Melting Point:
Boiling Point:
Density @ 293 K:
6.475 g/cm3
Crystal structure: