Promethium

Promethium (named after Prometheus, a Titan from Greek mythology) is a chemical element with the symbol Pm and atomic number 61. In the periodic table, it belongs to the lanthanide group and is thus also classified among the rare earth metals. All isotopes of promethium are radioactive, meaning that every atomic nucleus containing 61 protons is unstable and undergoes decay. Promethium and the lighter technetium (atomic number 43) are the only elements with atomic numbers lower than lead (82) that exhibit this property. The first discovery was published by the Italian scientists Luigi Rolla and Lorenzo Fernandes from Florence. After separating a didymium nitrate concentrate by fractional crystallization from the Brazilian mineral monazite, which consists of about 70% dysprosium and neodymium and about 30% other lanthanides, they obtained a solution mainly containing samarium. This solution produced X-ray spectra that they interpreted as samarium and element 61. They named element 61 “Florentium” in honor of their city. Their results were published in 1926, although they claimed to have conducted the experiments in 1924. In the same year, Smith Hopkins and Len Yntema at the University of Illinois in Urbana-Champaign also announced the discovery of element 61, naming it “Illinium” after their university. Neither of these discoveries could be verified. Several groups claimed to have produced the element, but due to the difficulties in separating promethium from other elements, none could confirm their findings.

Promethium was finally discovered in 1945 at the Oak Ridge National Laboratory (ORNL) in Tennessee, USA, by Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell as a fission product of uranium. Because they were heavily involved in military research during World War II, their discovery was only published in 1947. The name promethium was chosen in reference to the Greek Titan Prometheus, who brought fire to humanity and thereby angered the gods. This was intended as a warning to mankind, which was then beginning the nuclear arms race. The name was suggested by Grace Mary Coryell, Charles Coryell’s wife. In 1963, ion exchange methods at ORNL were used to obtain about 10 grams of promethium from nuclear reactor fuel waste. That same year, Fritz Weigel was able to produce metallic promethium for the first time by heating promethium(III) fluoride (PmF₃) with lithium in a tantalum crucible.

In nature, promethium is mostly found as a product of spontaneous uranium fission and from the alpha decay of the europium isotope ¹⁵¹Eu. Trace amounts are present in pitchblende at concentrations of about (4±1)·10⁻¹⁵ grams of ¹⁴⁷Pm per kilogram. The steady-state occurrence of promethium in the earth’s crust is approximately 560 grams from uranium fission and about 12 grams from alpha decay of ¹⁵¹Eu. Promethium has also been detected in the emission spectrum of the star GY Andromedae, and possibly in HD 101065 (Przybylski’s star) and HD 965.

Since isotope ¹⁴⁷Pm can be artificially produced as a fission product in measurable quantities, its properties can be studied quite well. As a metal, promethium is a typical representative of the lanthanides. The silver-white, relatively soft metal oxidizes fairly quickly in air and reacts slowly with water.

Promethium occurs only in the +3 oxidation state in its compounds ([Xe] 4f⁴). It loses two 6s electrons and one 4f electron. Its solutions are pinkish violet in color. Promethium forms slightly soluble compounds such as fluoride, oxalate, and carbonate. Promethium(III) oxide (Pm₂O₃) exists in three different modifications: a hexagonal A-form (violet-brown), a monoclinic B-form (pink-violet), and a cubic C-form (coral red). Its melting point is 2130 °C (3866 °F). All halides from fluorine to iodine are known in the +3 oxidation state.

Promethium(III) fluoride (PmF₃) is sparingly soluble in water and is obtained from a nitric acid solution of Pm³⁺ ions by adding hydrofluoric acid; the precipitate is pale pink. The melting point of the anhydrous compound is 1338 °C (2440 °F). When hydrated PmF₃·x H₂O is heated, promethium(III) oxyfluoride (PmOF) forms, which is pink-violet in color.

Promethium(III) chloride (PmCl₃) is violet with a melting point of 655 °C (1211 °F). When PmCl₃ is heated in the presence of water, pale pink promethium(III) oxychloride (PmOCl) forms.

Promethium(III) bromide (PmBr₃) is prepared by heating Pm₂O₃ in a dry hydrogen bromide stream. It is red and melts at 660 °C (1220 °F).

Promethium(III) iodide (PmI₃) cannot be produced from Pm₂O₃ by reaction with hydrogen iodide–hydrogen mixtures; instead, promethium(III) oxyiodide (PmOI) forms. The desired product can be obtained by reacting Pm₂O₃ with molten aluminum iodide (AlI₃) at 500 °C (932 °F). It is red and melts at 695 °C (1283 °F).

Promethium(III) hydroxide (Pm(OH)₃) is obtained by introducing ammonia into a nitric acid solution of Pm³⁺ ions. It is violet-pink in color.

There are no hazard classifications under chemical safety regulations because these only cover chemical hazards, which are negligible compared to the dangers arising from radioactivity. However, such regulations apply only when relevant quantities are involved.

Due to its short-lived isotopes and very limited availability, promethium is used technically only in minute quantities. The isotopes ¹⁴⁶Pm and ¹⁴⁷Pm serve as beta radiation sources in luminous watch dials and in cold light sources for signal devices. Additionally, promethium’s beta radiation is utilized for radiometric thickness gauging and level measurement.

In aerospace applications, small radioisotope batteries containing promethium are employed.