• Element 61 Promethium Pm, 1945
• Chemist Jacob Akiba Marinsky, 1918
• Chemist Lawrence Elgin Glendenin, 1918
• Chemist Charles DuBois Coryell, 1912

Promethium, originally prometheum, is a chemical element with the symbol Pm and atomic number 61. It is notable for being the only exclusively radioactive element besides technetium that is followed by chemical elements with stable isotopes. Chemically, promethium is a lanthanide, which forms salts when combined with other elements; the compounds, however, are understudied due to extreme radioactivity of promethium. Promethium shows only one stable oxidation state of +3, however, a few +2 compounds may be capable of existence.

In 1902, Bohuslav Brauner suggested there was an element with intermediate properties between the known neodymium and samarium; this was confirmed in 1914 by Henry Moseley who, having measured atomic numbers of all then known elements, 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 that was 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 not largely recognized. Promethium was first produced and characterized at Oak Ridge National Laboratory in 1945 by separation and analysis of the fission products of uranium fuel irradiated in the graphite reactor. The discoverers proposed the name "prometheum" (the spelling was changed shortly), derived from Prometheus, the Titan, in Greek mythology, who stole fire from Mount Olympus and brought it down to mankind, to symbolize "both the daring and the possible misuse of the mankind intellect." However, a sample of the metal was made only in 1963.

There are three possible sources for promethium: rare decays of natural (primordial) neodymium (producing promethium - 150), europium (promethium - 147), and uranium (various isotopes). The most stable isotope, promethium - 145, can alpha decay with a very small probability, making the alpha half-life long enough for theoretical possibility of primordial promethium; however, this has not been experimentally confirmed. The only isotope used in industry is promethium - 147, which is used (as ¹⁴⁷Pm₂O₃), in radioionizators, light producers, and atomic batteries, with the latter used in guided missiles. Since there is not enough natural promethium to be at least a slightly significant impurity, it is typically man made by bombarding uranium - 235 (enriched uranium) with thermal neutrons (for promethium - 147).

A promethium atom has 61 electrons, arranged in order [Xe]4f⁵6s². When entering a chemical reaction, the atom loses its two outermost electrons and one of the 4f - electron, which belongs to an open subshell. Promethium atomic radius is third largest among all lanthanides but is only slightly greater than those of the neighboring elements. It is the only exception from the general trend of the contraction of the atoms with increase of atomic radius (caused by the lanthanide contraction) that is not caused by the filled (or half-filled) 4f - subshell.

Many properties of promethium rely on its position among lanthanides and are intermediate between those of neodymium and samarium. For example, the melting point, the three first ionization energies, and the hydration energy are greater than those of neodymium and under those of samarium; 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 under those of neodymium. Most properties, however, never were measured due to the intense radioactivity.

Promethium belongs to the cerium group of lanthanides and is very similar the neighboring elements chemically. Because of its instability, chemical studies of promethium are incomplete. Even though a few compounds have been synthesized, they are still understudied; in general, they tend to have pink or red color. Treatment of acidic solutions containing Pm³⁺ ions with ammonia results in gelatinous light brown sediment of hydroxide, Pm(OH)₃, only 10⁻²⁹ grams of which dissolve in water. When dissolved in hydrochloric acid, it turns into a water soluble yellow salt, PmCl₃; similarly, when dissolved in nitric acid, a nitrate results, Pm(NO₃)₃. The latter is also well soluble; when dried, it forms pink crystals, similar with Nd(NO₃)₃. Unlike nitrate, the oxide is similar with the corresponding samarium salt and not the neodymium salt; white powder, it changes its structure only at very high temperatures. The sulfate is slightly soluble, like the other cerium group sulfates. Cell parameters have been calculated for its octahydrate; they lead to conclusion that the density of Pm₂(SO₄)₃•8 H₂O is 2.86 g•cm⁻³. The oxalate, Pm₂(C₂O₄)₃•10 H₂O, has the lowest solubility of all lanthanide oxalates.

Promethium forms only one stable oxidation state, +3, in form of ions; this is in line with other lanthanides. According to its position in the periodic table, the element cannot be expected to form stable +4 or +2 oxidation states; treating chemical compounds containing Pm³⁺ ions with strong oxidizers or reducers showed that the ion does not show significant ability to oxidize or reduce. Calculations show, however, that a few promethium (II) compounds are capable of existence, such as promethium (II) chloride and promethium (II,III) fluoride. Since all cerium group elements (the lighter rare earth elements) form stable diiodides, this may also be true for promethium.

Promethium is the only lanthanide and one of two elements among the first 82 that has no stable (or even long lived) isotopes; this is a result of a rarely occurring effect of the liquid drop model and stabilities of neighbor element isotopes; it is also the least stable element of the first 84. The primary decay products are neodymium and samarium isotopes (promethium - 146 decays to both, the lighter generally to neodymium, and heavier to samarium). Promethium nuclear isomers may decay to other promethium isotopes and one isotope can decay to praseodymium.

The most stable isotope of the element is promethium - 145, which has a half-life of 17.7 years. It is the most interesting isotope from the scientific point of view: Because it has 84 neutrons (two more than 82, which is a magic number and corresponds to a stable neutron configuration), it may emit an alpha particle (which has 2 neutrons) to form praseodymium - 141 with 82 neutrons. For this reason, it is the only promethium isotope that can alpha decay. Its alpha half-life is about 6,3×10⁹ years; this may be a rationale for searches of primordial promethium - 145 in nature.

The element also has 18 nuclear isomers with mass numbers of 133 — 142, 144, 148, 149, 152, and 154 (not every mass number corresponds to only one isomer). The most stable of them is promethium - 148m, with half-life of 43.1 day; this is longer than half-lives of the ground states of all promethium isotopes, except only for promethium - 143 to 147 (note that promethium - 148m has a longer half-life than the ground state, promethium - 148).

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 confirmed that the element occurs in natural neodymium in equilibrium in quantities below 10⁻²⁰ grams of promethium per one gram of neodymium. Earlier failures to find this promethium are probably due to that the time needed for fractional division of rare earth elements is too long and promethium - 150 (half-life 2.7 h) completely decays.

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. Since the energy of an alpha decay of europium - 151 was greater than that of europium - 153, it was first studied. Researches 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.

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.

All promethium isotopes have their half-lives under 20 years. Therefore, the promethium created directly when all original atoms of heavier elements were created must have decayed. However, unlike other elements that are commonly thought not to have stable isotopes, promethium may be an exception. The most stable isotope of the element, promethium - 145, decays with the average half-life of 17.7 years. The primary decay mode of the isotope is beta decay, but it also alpha decays with the probability of 2.8×10⁻⁹; therefore, if all ignore all promethium - 145 which beta decays, the promethium - 145 that ever decayed or will ever decay via alpha decay has the half-life (also called "alpha half-life") of 6.3×10⁹ years. This is longer than the age of the Earth's crust, which is 4.5×10⁹ years, and is shorter than (but comparable) with the age of the universe, which is about 13.75×10⁹ years. Since its creation, the original alpha decaying part of promethium - 145 only underwent through two complete half-lives, which means there is still some promethium - 145. Therefore, it possibly occurs not only as the result of decay of other elements, but also primordially. However, this has not been confirmed experimentally, as no primordial promethium has been found in nature yet.

In 1902, Bohuslav Brauner found out that the difference between neodymium and samarium is the largest of all neighboring lanthanides pairs; as a conclusion, he suggested there was an element with intermediate properties between them. This prediction was supported in 1914 by Henry Moseley who, having discovered that atomic number was an experimentally measurable property of elements, found a few atomic numbers had no element to correspond: the gaps were 43, 61, 72, 75, 85, and 87. 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.

The first claim of a discovery was published by Luigi Rolla and Lorenzo Fernandes of Florence, Italy. 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. This solution gave x-ray spectra attributed to samarium and element 61. In honor of their city, they named element 61 "florentium." The results were published in 1926, but the scientists claimed that the experiments were done in 1924. 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. They named it "illinium," after the university. Both of these reported discoveries were shown to be erroneous because the spectrum line that "corresponded" to element 61 was identical to that of didymium; the lines thought to belong to element 61 turned out to belong to a few impurities (barium, chromium, and platinum).

In 1934, Josef Mattauch finally formulated the isobar rule. One of the indirect consequences of this rule was that element 61 was unable to form stable isotopes. In 1938, a nuclear experiment was conducted by H.B. Law et al. at Ohio State University. The nuclides produced certainly were not radioisotopes of neodymium or samarium, and the name "cyclonium" was proposed, but there was a lack of chemical proof that element 61 was produced and the discovery not largely recognized.

Promethium was first produced and characterized at Oak Ridge National Laboratory (Clinton Laboratories at the point) 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. The original proposed name was "clintonium," after the laboratory where the work was conducted; however, the name "prometheum" was suggested by Grace Mary Coryell, the wife of one of discoverers. It is derived from Prometheus, the Titan, in Greek mythology, who stole fire from Mount Olympus and brought it down to mankind and symbolizes "both the daring and the possible misuse of the mankind intellect." The spelling was then changed to "promethium," as this was in closer in accordance with other metals.

In 1963, promethium (III) fluoride was used to make promethium metal. Provisionally purified from impurities of samarium, neodymium, and americium, it was put into a tantalum crucible which was located in another tantalum crucible; the outer one contained lithium metal (10 times excess compared to promethium). After creating vacuum, the chemical were mixed to give promethium:

PmF₃ + 3 Li → Pm + 3 LiF

The created promethium sample was used to experimentally measure a few properties, such as the melting point.

Promethium - 147 (the production methods vary for different isotopes, so the one for promethium - 147 is given since it is the only one to have any industrial uses) is produced in large (compared to other isotopes) quantities by bombarding uranium - 235 with thermal neutrons. The outcome is relatively high and is 2.6%. Yet another way of production of promethium - 147 is producing neodymium - 147, which decays into the desired nuclide shortly. It may be obtained by either bombarding enriched neodymium - 146 with thermal neutrons or by bombarding uranium carbide target with energetic protons in particle accelerator. The last method is bombarding uranium - 238 with fast neutrons to cause fast fission, which among multiple reaction products, creates promethium - 147.

Even as long ago as 1960s, Oak Ridge National Laboratory was able to produce 650 grams per year. At the time, the United States was the only country to produce it in significant quantities. However, the large scale production has stopped in the U.S., and now the only country that produces promethium - 147 on relatively large is scales is Russia. However, the U.S. is planning to build a reactor to come back into the promethium - 147 production, since the interest for the nuclide increased in 2000s (as of 2010).

Promethium is most commonly used for research purposes; however, one promethium isotope is used outside laboratories. It is promethium - 147, most commonly used as the oxide, obtained in milligram quantities. The isotope does not emit gamma rays, small average run in matter, and relatively long half-life.

• Radioionizators based on promethium - 147 eliminate electrostatic charges by giving off microampere currents.
• As a light source for signals, using phosphor to absorb the beta radiation and produce light. The nuclide does not cause aging of the phosphor, like alpha emitters do. Despite its small size, it provides stable work for a few years. Originally, radium - 226 was used for the purpose, but it was later replaced with promethium - 147 and tritium (hydrogen - 3). Promethium may be favored to tritium for safety reasons.
• In an atomic battery in which cells convert the beta emissions into electric current to be used in guided missiles, watches or radios. They have a useful lifetime of about five years.
• Promethium has possible future uses in portable X-ray sources, and as auxiliary heat or power sources for space probes and satellites (although the alpha emitter plutonium - 238 has become standard for most space exploration related uses).
• Promethium is used to measure the thickness of materials. Detectors can measure the thickness of a piece of metal by measuring the amount of radiation from a promethium sample above the metal that passes through the metal. The detectors can automatically stop the production of these metal pieces when too much or too little radiation passes through the metal, indicating that the thickness of the metal is not correct.

The element, like other lanthanides, has no biological role. Promethium - 147 can emit X-rays during its beta decay, which are dangerous for all lifeforms. Minute interactions with promethium - 147 are not hazardous if certain precautions are observed. In general, gloves, footwear covers, safety glasses, and an outer layer or easily removed protective clothing should be used.

It is not known what human organs are affected after interaction with promethium; a possible candidate is the bone tissues. Sealed promethium - 147 is not dangerous. However, if the packaging is deformed, then promethium becomes dangerous to environment and humans. If the radioactive contamination was found, the contamination place should be washed with water and soap, but, even though promethium affects the skin mostly, it should not be ripped off. If a promethium leak was found, the area should be identified as hazardous and left and emergency services must be contacted.

No dangers aside from the radioactivity have been shown.

Jacob Akiba Marinsky (1918 – September 1, 2005) was a chemist who was the co-discoverer of the element promethium.

Marinsky was born in Buffalo, New York, and attended the University at Buffalo, entering at age 16 and receiving a bachelor's degree in chemistry in 1939.

During World War II he was employed as a chemist for the Manhattan Project, working at Clinton Laboratories (now Oak Ridge National Laboratory) from 1944 to 1946. In 1945, together with Lawrence E. Glendenin and Charles D. Coryell, he isolated the previously undocumented rare earth element 61. Marinsky and Glendenin produced promethium both by extraction from fission products and by bombarding neodymium with neutrons. They isolated it using ion - exchange chromatography. Publication of the finding was delayed until later due to the war. Marinsky and Glendenin announced the discovery at a meeting of the American Chemical Society in September 1947. Upon the suggestion of Coryell's wife, the team named the new element for the mythical Prometheus, who stole fire from the gods and was punished for the act by Zeus. They had also considered naming it "clintonium" for the facility where it was isolated.

Marinsky was among the Manhattan Project scientists who in 1945 signed a petition against dropping an atomic bomb on Japan.

He resumed his education after the war, obtaining a Ph.D. in Nuclear and Inorganic Chemistry from the Massachusetts Institute of Technology in 1949. He worked in industrial research before joining the faculty of the University at Buffalo in 1957. His research was concerned with nuclear inorganic chemistry, physicochemical studies of ion exchange, and polyelectrolyte and electrolyte systems. In the late 1960s when the university required faculty to sign an oath of loyalty to the United States, Marinsky refused, calling it a violation of civil liberties, a position that caused some other faculty members to lose their jobs. He retired in 1988, becoming a professor emeritus.

In the early 1960s Marinsky was a Fulbright Research Scholar at the Weizmann Institute of Science in Israel. In 1990, he received the Clifford Furnas Memorial Award of the University at Buffalo, awarded to graduates whose scientific accomplishments brought prestige to the university.

Marinsky died September 1, 2005, from multiple myeloma. He was buried in Pine Hill Cemetery in Buffalo. He was married to the former Ruth Slick, who survived him, for 63 years. They were the parents of four daughters.

Glendenin was born in Bay City, Michigan, on November 8, 1918. He attended the University of Chicago, graduating in 1941.

He worked as a chemist at the Clinton Laboratories (now Oak Ridge National Laboratory) during the World War II Manhattan Project, engaged in separating, identifying and characterizing the radioactive elements produced by nuclear fission. In 1945, he, together with Jacob A. Marinsky and Charles D. Coryell, isolated the previously undocumented rare earth element 61. Marinsky and Glendenin produced promethium both by extraction from fission products and by bombarding neodymium with neutrons. They isolated it using ion - exchange chromatography. Publication of the finding was delayed until later due to the war. In September 1947, Marinsky and Glendenin announced the discovery at a meeting of the American Chemical Society. Upon the suggestion of Coryell's wife, the team named the new element for the titan Prometheus, who stole fire from the gods and was punished for the act by Zeus. They had also considered naming it "clintonium" for the facility where it was isolated.

In 1945, Glendenin and 154 other Manhattan Project scientists signed the Szilárd petition. The petition urged President Harry S. Truman not to use the first atomic bomb "without restriction," urging him instead to "describe and demonstrate" its power and give Japan "the opportunity to consider the consequences of further refusal to surrender."

In 1949, Glendenin earned his Ph.D from the Massachusetts Institute of Technology. That same year he joined Argonne National Laboratory, where he remained until his retirement in 1985.

He published extensively on the properties of fission products. He served as Scientific Secretary on the U.S. delegation to the Atoms for Peace Conference and received the American Chemical Society's Glenn T. Seaborg Award for Nuclear Chemistry in 1974.

Glendenin was married for 63 years to Ethel Glendenin (née Long), who survived him at his death in November 2008. The couple were the parents of two daughters and two sons.

Charles DuBois Coryell (February 21, 1912 – January 7, 1971) was an American chemist who was one of the discoverers of the element promethium.

Coryell earned a Ph.D at California Institute of Technology in 1935 under Arthur A. Noyes. During the late 1930s he engaged in research on the structure of hemoglobin in association with Linus Pauling. He also taught at UCLA before 1942. In 1942 he took a position in the Manhattan Project, for which he was Chief of the Fission Products Section, both at the University of Chicago (1942 – 1946) and at Clinton Laboratories (now Oak Ridge National Laboratory) in Oak Ridge, Tennessee (1943 – 1946). His group had responsibility for characterizing radioactive isotopes created by the fission of uranium and for developing of a process for chemical separation of plutonium.

In 1945 he was a member of the Clinton Laboratories team, with Jacob Marinsky and Lawrence E. Glendenin, that isolated the previously undocumented rare earth element 61. Marinsky and Glendenin produced promethium both by extraction from fission products and by bombarding neodymium with neutrons. They isolated it using ion - exchange chromatography. Publication of the finding was delayed until later due to the war. Marinsky and Glendenin announced the discovery at a meeting of the American Chemical Society in September 1947. Upon the suggestion of Coryell's wife, the team named the new element for the mythical Prometheus, who stole fire from the gods and was punished for the act by Zeus. They had also considered naming it "clintonium" for the facility where it was isolated.

Coryell was among the Manhattan Project scientists who in 1945 signed the Szilárd petition urging President Harry S. Truman not to use the first atomic bomb "without restriction," urging him instead to "describe and demonstrate" its power and give Japan "the opportunity to consider the consequences of further refusal to surrender."

With Dr. Nathan Sugarman, Coryell was co-editor of Radiochemical Studies: The Fission Projects, a volume of 336 research papers from the Manhattan Project.

After World War II he joined the Massachusetts Institute of Technology (MIT) in 1945 as a faculty member in inorganic and radiochemistry. At MIT he conducted research in fission fine - structure and beta decay theory until his death in 1971.

In 1954 he received the Louis Lipsky Fellowship at the Weizmann Institute of Science in Rehovot, Israel. In 1960 he received the American Chemical Society's Glenn T. Seaborg Award for Nuclear Chemistry.

The Charles D. Coryell Award of the Division of Nuclear Chemistry and Technology of the American Chemical Society, which is awarded annually to undergraduate students doing research projects in nuclear related areas, is named in his honor.