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Promethium, 61Pm
Promethium
Pronunciation/prəˈmiːθiəm/ ⓘ ​(prə-MEE-thee-əm)
Appearancemetallic
Mass number[145]
Promethium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
–
↑
Pm
↓
Np
neodymium ← promethium → samarium
Atomic number (Z)61
Groupf-block groups (no number)
Periodperiod 6
Block  f-block
Electron configuration[Xe] 4f5 6s2
Electrons per shell2, 8, 18, 23, 8, 2
Physical properties
Phase at STPsolid
Melting point1315 K ​(1042 °C, ​1908 °F)
Boiling point3273 K ​(3000 °C, ​5432 °F)
Density (at 20° C)α-145Pm: 7.149 g/cm3
α-147Pm: 7.247 g/cm3 [1]
Heat of fusion7.13 kJ/mol
Heat of vaporization289 kJ/mol
Atomic properties
Oxidation statescommon: +3
+2?
ElectronegativityPauling scale: 1.13 (?)
Ionization energies
  • 1st: 540 kJ/mol
  • 2nd: 1050 kJ/mol
  • 3rd: 2150 kJ/mol
Atomic radiusempirical: 183 pm
Covalent radius199 pm
Color lines in a spectral range
Spectral lines of promethium
Other properties
Natural occurrencefrom decay
Crystal structure ​double hexagonal close-packed (dhcp) (hP4)
Lattice constants
Double hexagonal close packed crystal structure for promethium
a = 0.36393 nm
c = 1.1739 nm (at 20 °C)[1]
Thermal expansion9.0×10−6/K (at r.t.)[2][a]
Thermal conductivity17.9 W/(m⋅K)
Electrical resistivityest. 0.75 µΩ⋅m (at r.t.)
Magnetic orderingparamagnetic[3]
Young's modulusα form: est. 46 GPa
Shear modulusα form: est. 18 GPa
Bulk modulusα form: est. 33 GPa
Poisson ratioα form: est. 0.28
CAS Number7440-12-2
History
Namingderived from Prometheus, the Titan in Greek mythology
DiscoveryJacob A. Marinsky, Lawrence E. Glendenin, Charles D. Coryell (1945)
First isolationF. Weigel (1963)
Named byGrace Mary Coryell (1945)
Isotopes of promethium
Main isotopes[4] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
143Pm synth 265 d ε 143Nd
144Pm synth 363 d ε 144Nd
145Pm synth 17.7 y ε 145Nd
α 141Pr
146Pm synth 5.53 y ε 146Nd
β− 146Sm
147Pm trace 2.6234 y β− 147Sm
 Category: Promethium
| references

Promethium is a chemical element; it has symbol Pm and atomic number 61. 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. Promethium is one of the only two radioactive elements that are both preceded and followed in the periodic table by elements with stable forms, the other being technetium. Chemically, promethium is a lanthanide. Promethium shows only one stable oxidation state of +3.

In 1902 Bohuslav Brauner suggested that there was a then-unknown 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 that the element with atomic number 61 was missing. 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. 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 much 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". A sample of the metal was made only in 1963.

The two sources of natural promethium are rare alpha decays of natural europium-151 (producing promethium-147) and spontaneous fission of uranium (various isotopes). Promethium-145 is the most stable promethium isotope, but the only isotope with practical applications is promethium-147, chemical compounds of which are used in luminous paint, atomic batteries and thickness-measurement devices. 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.

Properties

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Physical properties

[edit]

A promethium atom has 61 electrons, arranged in the configuration [Xe] 4f5 6s2. The seven 4f and 6s electrons are valence electrons.[5] In forming compounds, the atom loses its two outermost electrons and one 4f-electron, which belongs to an open subshell. The element's atomic radius is the second largest among all the lanthanides but is only slightly greater than those of the neighboring elements.[5] It is the most notable exception to the general trend of the contraction of lanthanide atoms with the increase of their atomic numbers (lanthanide contraction[6]). 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 first three ionization energies, and the hydration energy are greater than those of neodymium and lower than those of samarium;[5] 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.[5]

Promethium has a double hexagonal close packed (dhcp) structure and a hardness of 63 kg/mm2.[7] This low-temperature alpha form converts into a beta, body-centered cubic (bcc) phase upon heating to 890 °C.[8]

Chemical properties and compounds

[edit]
Promethium nitrate
Solution containing Pm3+ ions

Promethium belongs to the cerium group of lanthanides and is chemically very similar to the neighboring elements.[9] Because of its instability, chemical studies of promethium are incomplete. Even though a few compounds have been synthesized, they are not fully studied; in general, they tend to be pink or red in color.[10][11] In May 2024, a promethium coordination complex with neutral PyDGA ligands was characterized in aqueous solution.[12] 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.[13] When dissolved in hydrochloric acid, a water-soluble yellow salt, PmCl3, is produced;[13] 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 to Nd(NO3)3.[13] The electron configuration for Pm3+ is [Xe] 4f4, and the color of the ion is pink. The ground state term symbol is 5I4.[14] 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)3·8H2O is 2.86 g/cm3.[15] The oxalate, Pm2(C2O4)3·10H2O, has the lowest solubility of all lanthanide oxalates.[16]

Unlike the nitrate, the oxide is similar to the corresponding samarium salt and not the neodymium salt. As-synthesized, e.g. by heating the oxalate, it is a white or lavender-colored powder with disordered structure.[13] This powder crystallizes in a cubic lattice upon heating to 600 °C. Further annealing at 800 °C and then at 1750 °C irreversibly transforms it to monoclinic and hexagonal phases, respectively, and the last two phases can be interconverted by adjusting the annealing time and temperature.[17]

Formula symmetry space group No Pearson symbol a (pm) b (pm) c (pm) Z density,
g/cm3
α-Pm dhcp[7][8] P63/mmc 194 hP4 365 365 1165 4 7.26
β-Pm bcc[8] Fm3m 225 cF4 410 410 410 4 6.99
Pm2O3 cubic[17] Ia3 206 cI80 1099 1099 1099 16 6.77
Pm2O3 monoclinic[17] C2/m 12 mS30 1422 365 891 6 7.40
Pm2O3 hexagonal[17] P3m1 164 hP5 380.2 380.2 595.4 1 7.53

Promethium forms only one stable oxidation state, +3, in the form of ions; this is in line with other lanthanides. Promethium can also form the +2 oxidation state.[18] Thermodynamic properties of Pm2+ suggests that the dihalides are stable, similar to NdCl2 and SmCl2.[19]

Promethium halides[20]
Formula color coordination
number 		symmetry space group No Pearson symbol m.p. (°C)
PmF3 Purple-pink 11 hexagonal P3c1 165 hP24 1338
PmCl3 Lavender 9 hexagonal P63/mc 176 hP8 655
PmBr3 Red 8 orthorhombic Cmcm 63 oS16 624
α-PmI3 Red 8 orthorhombic Cmcm 63 oS16 α→β
β-PmI3 Red 6 rhombohedral R3 148 hR24 695

Isotopes

[edit]
Main article: Isotopes of promethium

Promethium is the only lanthanide and one of only two elements among the first 82 with no stable or long-lived (primordial) 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.[4] The primary decay products are neodymium and samarium isotopes (promethium-146 decays to both, the lighter isotopes generally to neodymium via positron decay and electron capture, and the heavier isotopes to samarium via beta decay). Promethium nuclear isomers may decay to other promethium isotopes and one isotope (145Pm) has a very rare alpha decay mode to stable praseodymium-141.[4]

The most stable isotope of the element is promethium-145, which has a specific activity of 139 Ci/g (5.1 TBq/g) and a half-life of 17.7 years via electron capture.[4] Because it has 84 neutrons (two more than 82, which is a magic number corresponding to a stable neutron configuration), it may emit an alpha particle (which has 2 neutrons) to form praseodymium-141 with 82 neutrons. Thus it is the only promethium isotope with an experimentally observed alpha decay.[21] Its partial half-life for alpha decay is about 6.3×109 years, and the relative probability for a 145Pm nucleus to decay in this way is 2.8×10−7 %. Several other promethium isotopes such as 144Pm, 146Pm, and 147Pm also have a positive energy release for alpha decay; their alpha decays are predicted to occur but have not been observed. In total, 41 isotopes of promethium are known, ranging from 126Pm to 166Pm.[4][22]

The element also has 18 nuclear isomers, with mass numbers of 133 to 142, 144, 148, 149, 152, and 154 (some mass numbers have more than one isomer). The most stable of them is promethium-148m, with a half-life of 41.3 days; this is longer than the half-lives of its ground state, and all promethium isotopes except for 143-147.[4]

Occurrence

[edit]
Uraninite, a uranium ore and the host for most of Earth's promethium

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.[23] 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.[23] However, these observations were disproved by newer investigations, because for all seven naturally occurring neodymium isotopes, any single beta decays (which can produce promethium isotopes) are forbidden by energy conservation.[24] In particular, careful measurements of atomic masses show that the mass difference between 150Nd and 150Pm is negative (−87 keV), which absolutely prevents the single beta decay of 150Nd to 150Pm.[25]

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 neutron capture of 146Nd.[26]

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 to promethium.[27] Research at Laboratori Nazionali del Gran Sasso showed that europium-151 decays to promethium-147 with the half-life of 5×1018 years;[27] later measurements gave the half-life as (4.62 ± 0.95(stat.) ± 0.68(syst.)) × 1018 years.[28] It has been shown that europium is "responsible" for about 12 grams of promethium in the Earth's crust.[27] 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 not be observed in the near future.[29]

Promethium can also be formed in nature as a product of spontaneous fission of uranium-238.[23] 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 (4×10−18) by mass.[30] Uranium is thus "responsible" for 560 g of promethium in Earth's crust.[27]

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.[31] Because of the short half-life of promethium isotopes, they should be formed near the surface of those stars.[32]

History

[edit]

Searches for element 61

[edit]

In 1902, Czech chemist Bohuslav Brauner found out that the differences in properties between neodymium and samarium were the largest between any two consecutive lanthanides in the sequence then known; as a conclusion, he suggested there was an element with intermediate properties between them.[33] This prediction was supported in 1914 by Henry Moseley who, having discovered that atomic number was an experimentally measurable property of elements, found that a few atomic numbers had no known corresponding elements: the gaps were 43, 61, 72, 75, 85, and 87.[34] 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.[35][36][37]

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.[38][39][40][41][42][43] 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.[44][45][46] 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).[35]

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.[35][47] From 1938, a nuclear experiment was conducted by H. B. Law et al. at the Ohio State University. Nuclides were produced in 1941 which 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 was not largely recognized.[48][49]

Discovery and synthesis of promethium metal

[edit]

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.[50][51] 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 the discoverers.[48] It is derived from Prometheus, the Titan in Greek mythology who stole fire from Mount Olympus and brought it down to humans[48] and symbolizes "both the daring and the possible misuse of the mankind intellect".[52] The spelling was then changed to "promethium", as this was in accordance with most other metals.[48]

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 crucible contained lithium metal (10 times excess compared to promethium).[10][16] After creating a vacuum, the chemicals were mixed to produce promethium metal:

PmF3 + 3 Li → Pm + 3 LiF

The promethium sample produced was used to measure a few of the metal's properties, such as its melting point.[16]

In 1963, ion-exchange methods were used at ORNL to prepare about ten grams of promethium from nuclear reactor fuel processing wastes.[32][53][54]

Promethium can be either recovered from the byproducts of uranium fission or produced by bombarding 146Nd with neutrons, turning it into 147Nd, which decays into 147Pm through beta decay with a half-life of 11 days.[55]

Production

[edit]

The production methods for different isotopes vary, and only those for promethium-147 are given because it is the only isotope with industrial applications. Promethium-147 is produced in large quantities (compared to other isotopes) by bombarding uranium-235 with thermal neutrons. The output is relatively high, at 2.6% of the total product.[56] Another way to produce promethium-147 is via neodymium-147, which decays to promethium-147 with a short half-life. Neodymium-147 can be obtained either by bombarding enriched neodymium-146 with thermal neutrons[57] or by bombarding a uranium carbide target with energetic protons in a particle accelerator.[58] Another method is to bombard uranium-238 with fast neutrons to cause fast fission, which, among multiple reaction products, creates promethium-147.[59]

As early as the 1960s, Oak Ridge National Laboratory could produce 650 grams of promethium per year[60] and was the world's only large-volume synthesis facility.[61] 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. [needs update] In 2010, Russia was the only country producing promethium-147 on a relatively large scale.[57]

Applications

[edit]
Promethium(III) chloride being used as a light source for signals in a heat button

Only promethium-147 has uses outside laboratories.[48] It is obtained as the oxide or chloride,[62] in milligram quantities.[48] This isotope has a relatively long half-life and its radiation has a relatively small penetration depth in matter.[62]

Some signal lights use a luminous paint containing a phosphor that absorbs the beta radiation emitted by promethium-147 and emits light.[32][48] This isotope does not cause aging of the phosphor, as alpha emitters do,[62] and therefore the light emission is stable for a few years.[62] Originally, radium-226 was used for the purpose, but it was later replaced by promethium-147 and tritium (hydrogen-3).[63] Promethium may be favored over tritium for nuclear safety.[64]

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. These batteries have a useful lifetime of about five years.[11][32][48] 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".[65]

Promethium is also used to measure the thickness of materials by measuring the amount of radiation from a promethium source that passes through the sample.[32][10][66] It has possible future uses in portable X-ray sources, and as auxiliary heat or power sources for space probes and satellites[67] (although the alpha emitter plutonium-238 has become standard for most space-exploration-related uses).[68]

Promethium-147 is also used, albeit in very small quantities (less than 330nCi), in some Philips CFL (Compact Fluorescent Lamp) glow switches in the PLC 22W/28W 15mm CFL range.[69]

Precautions

[edit]

Promethium has no biological role. Promethium-147 can emit gamma rays, which are dangerous for all lifeforms, during its beta decay.[70] Interactions with tiny quantities of promethium-147 are not hazardous if certain precautions are observed.[71] In general, gloves, footwear covers, safety glasses, and an outer layer of easily removed protective clothing should be used.[72]

It is not known what human organs are affected by interaction with promethium; a possible candidate is the bone tissues.[72] Sealed promethium-147 is not dangerous. However, if the packaging is damaged, then promethium becomes dangerous to the environment and humans. If radioactive contamination is found, the contaminated area should be washed with water and soap, but, even though promethium mainly affects the skin, the skin should not be abraded. If a promethium leak is found, the area should be identified as hazardous and evacuated, and emergency services must be contacted. No dangers from promethium aside from the radioactivity are known.[72]

Notes

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References

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Bibliography

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Promethium

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from Grokipedia
Promethium is a synthetic with the symbol Pm and 61, classified as a in the periodic table and notable for being one of only two elements (along with ) that occur naturally on in only trace amounts due to the absence of stable isotopes. As a soft, silvery metal, it tarnishes slowly in air and reacts readily with water, exhibiting typical lanthanide properties such as a of 1042°C and a of approximately 3000°C, while its is [Xe] 4f⁵ 6s². All known isotopes of promethium are radioactive, with promethium-145 having the longest of 17.7 years and promethium-147 being the most commonly used due to its 2.62-year half-life and pure beta emission, making it suitable for specialized applications. Discovered in 1945 at by Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell through ion-exchange separation of fission products from , promethium was named after the Titan from , who stole fire from the gods, symbolizing humanity's quest for knowledge. Although not found in significant natural deposits on , promethium has been detected in trace amounts in stars and is produced artificially via nuclear reactors for its primary uses, including as a beta source in thickness gauges for measuring thin films, in nuclear batteries powering devices like pacemakers and space probes, and in luminous paints for self-sustaining light sources. Recent advances in promethium chemistry, such as studies of its coordination complexes, are enhancing understanding of and potentially unlocking new applications in and .

Properties

Physical properties

Promethium is the with and [Xe] 4f⁵ 6s². Its is approximately 185 pm, based on empirical measurements for lanthanides. Promethium is a silvery metal that tarnishes in air due to oxidation. It has an extrapolated of 7.26 g/cm³ at , a of 1042°C, and a of 3000°C. These values are derived from limited experimental data and analogies with neighboring lanthanides, as promethium's radioactivity complicates direct measurements. The metal exhibits a double hexagonal close-packed in its α-phase, with lattice parameters a = 3.65 and c = 11.65 . Promethium is paramagnetic, consistent with its unpaired f-electrons.
PropertyValueNotes/Source
Thermal conductivity17.9 W/(m·K)Estimated at
Electrical conductivity1.3 × 10⁶ S/mDerived from resistivity of ~0.75 µΩ·m
Due to its radioactivity, all physical properties of promethium are studied using short-lived isotopes like ¹⁴⁷Pm, with data often extrapolated from bulk samples containing impurities.

Chemical properties

Promethium, as a member of the series, predominantly exhibits the +3 , forming the with an ionic radius of 97.0 pm in six-coordinate environments. This trivalent state arises from the loss of the 6s² electrons and one 4f electron, resulting in a [Xe] 4f⁴ configuration for the ion, which is characteristic of lanthanide chemistry. Higher oxidation states are unstable and rarely observed due to the poor shielding of the 4f electrons. The element is highly electropositive, reflecting its position among the reactive lanthanides, and thus displays vigorous reactivity with common substances. Promethium reacts slowly with cold to produce gas and promethium(III) (Pm(OH)₃), with the reaction accelerating in hot . It tarnishes rapidly in moist air by oxidizing to form promethium(III) oxide (Pm₂O₃) and dissolves readily in dilute acids, such as hydrochloric or , to generate corresponding salts like PmCl₃ or Pm(NO₃)₃, accompanied by evolution. Among its key compounds, promethium(III) oxide (Pm₂O₃) is a pale -white solid prepared by calcining promethium or at elevated temperatures around 800–1000°C. Promethium(III) (PmCl₃), a , water-soluble salt, is synthesized by dissolving promethium metal or in or by fusing the oxide with . Similarly, promethium(III) (PmF₃), a white crystalline compound, is obtained via from aqueous Pm³⁺ solutions with ions or by direct reaction of the metal with gas. In coordination chemistry, Pm³⁺ ions typically form complexes with coordination numbers ranging from 6 to 9, favoring high coordination due to the large ionic radius and electrostatic bonding preferences of lanthanides. A landmark advancement came in 2024 with the characterization of the first stable promethium coordination complex in aqueous solution, a 1:3 homoleptic species formed with a tridentate diglycolamide ligand, where nine oxygen atoms coordinate the metal center, enabling detailed study via X-ray absorption spectroscopy. This complex highlights promethium's ability to form chelates despite its radioactivity. Promethium's chemical behavior is largely analogous to adjacent lanthanides but exhibits subtle differences attributable to its neutral [Xe] , which influences orbital energies and leads to intermediate properties between (4f⁴) and (4f⁶), including variations in bond lengths and reactivity trends within the series. The half-filled nature approaching the 4f subshell contributes to distinct spectroscopic signatures, though its overall reactivity remains governed by the +3 ionic state.

Isotopes

Promethium possesses no isotopes, with all known isotopes being radioactive. As of recent evaluations, 38 isotopes have been characterized, with mass numbers ranging from 128 to 166. The primary decay modes for lighter isotopes (below mass 146) are leading to daughters, while heavier isotopes predominantly undergo beta-minus decay to . The most stable isotope is ^{145}Pm, with a half-life of 17.7 years, decaying primarily by electron capture (nearly 100%) to stable ^{145}Nd, with an extremely rare alpha decay branch (2.8 × 10^{-7} %) to ^{141}Pr. The next longest-lived is ^{147}Pm, with a half-life of 2.623 years, undergoing pure beta-minus decay to stable ^{147}Sm. Another notable isotope, ^{146}Pm, has a half-life of 5.53 years and decays via both electron capture (66%) to ^{146}Nd and beta-minus decay (34%) to stable ^{146}Sm. These isotopes are the only ones with half-lives exceeding one year, making them relevant for potential applications despite their radioactivity. Traces of promethium, specifically ^{145}Pm and ^{147}Pm, occur in uranium ores such as pitchblende due to the of ^{238}U, though in quantities less than one per million tonnes of (as of current nuclear data, 2023); these amounts are insufficient for practical isolation and decay rapidly. The following table summarizes selected promethium isotopes, focusing on those with relatively longer half-lives, including their decay modes and daughter products (abundances are negligible and not naturally occurring):
Mass NumberHalf-LifeDecay Mode(s)Daughter Product(s)
^{143}Pm265 daysElectron capture (100%)^{143}Nd
^{144}Pm360 daysElectron capture (100%)^{144}Nd
^{145}Pm17.7 yearsElectron capture (nearly 100%); α (2.8 × 10^{-7} %)^{145}Nd; ^{141}Pr
^{146}Pm5.53 yearsElectron capture (66%); β⁻ (34%)^{146}Nd; ^{146}Sm
^{147}Pm2.623 yearsβ⁻ (100%)^{147}Sm
^{148}Pm5.37 daysβ⁻ (100%)^{148}Sm
^{149}Pm2.21 daysβ⁻ (100%)^{149}Sm
Data derived from nuclear databases; shorter-lived isotopes (half-lives <1 day) are omitted for brevity. Regarding nuclear properties, ^{147}Pm has thermal neutron capture cross sections of 84 ± 10 barns to ^{148}Pm (ground state) and 72.4 ± 3.0 barns to ^{148m}Pm, influencing its behavior in nuclear reactors where it acts as a neutron absorber.

Occurrence and production

Natural occurrence

Promethium is one of the rarest elements in the Earth's crust, with an estimated abundance of less than 10−1510^{-15} g/kg, making it orders of magnitude scarcer than other lanthanides. This trace presence arises almost exclusively from the spontaneous fission of , which produces short-lived isotopes such as 145^{145}Pm (half-life 17.7 years) and 147^{147}Pm (half-life 2.62 years) with cumulative fission yields of approximately 2% for 147^{147}Pm and similar for other short-lived isotopes. A minor contribution comes from the alpha decay of naturally occurring 151^{151}Eu. These isotopes contribute to a total natural inventory of approximately 500–600 grams distributed across the entire crust at any given time. Promethium has been detected in minute traces within uranium-bearing ores, such as pitchblende, where it forms as a byproduct of spontaneous fission processes. However, no concentrated deposits or minerals containing promethium exist, due to its rapid radioactive decay and the infinitesimal production rates. In cosmic environments, promethium is synthesized via the rapid neutron-capture process () in core-collapse supernovae and neutron star mergers, events that generate neutron-rich heavy nuclei including lanthanides. Its cosmic abundance remains negligible, as the unstable isotopes decay quickly, but trace detections have been reported in the spectra of peculiar stars like HD 25354 through analysis of promethium II lines. Unlike other rare earth elements, which feature stable isotopes and form economically viable ores like monazite or bastnäsite, promethium fills a unique gap in the lanthanide series with virtually no stable accumulation, rendering its natural occurrence insignificant for geological or practical purposes.

Synthetic production

Promethium is primarily produced synthetically through neutron bombardment of enriched neodymium-146 targets in high-flux nuclear reactors, such as the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). The key nuclear reaction involves the capture of a thermal neutron by 146Nd^{146}\mathrm{Nd}, forming 147Nd^{147}\mathrm{Nd}, which undergoes beta-minus decay to yield the most commonly produced isotope, 147Pm^{147}\mathrm{Pm}: 146Nd(n,γ)147Nd→β−→147Pm^{146}\mathrm{Nd}(n,\gamma)^{147}\mathrm{Nd} \rightarrow \beta^- \rightarrow ^{147}\mathrm{Pm}. This process achieves yields of up to 2.75 mCi per milligram of target material after approximately 24 days of irradiation at neutron fluxes around 2.1×10152.1 \times 10^{15} n/cm²/s, though longer irradiations do not significantly boost output due to the high neutron capture cross-section of 147Pm^{147}\mathrm{Pm}. Impurities, such as 147Nd^{147}\mathrm{Nd} (up to 3.13%) and minor isotopes like 148mPm^{148\mathrm{m}}\mathrm{Pm} (0.64%), arise from competing reactions but can be minimized using highly enriched targets (>99% 146Nd^{146}\mathrm{Nd}). An alternative production route exploits promethium as a fission byproduct in nuclear reactors fueled by or plutonium-239. The cumulative fission yield for 147Pm^{147}\mathrm{Pm} from thermal neutron-induced fission of 235U^{235}\mathrm{U} is approximately 2.23%, meaning about 2.23 atoms of 147Pm^{147}\mathrm{Pm} are produced per 100 fissions. This method historically supplied larger quantities before shifts in reprocessing, with promethium extracted from alongside other fission products. Separation techniques include solvent extraction using organic extractants like di(2-ethylhexyl)orthophosphoric acid (HDEHP) and ion-exchange , which achieve decontamination factors exceeding 10410^4 from and other lanthanides, yielding purities over 99% with recovery rates up to 90%. To obtain metallic promethium, purified 147Pm^{147}\mathrm{Pm} compounds, typically the fluoride PmF3\mathrm{PmF_3}, are reduced using or metals at elevated temperatures. Reduction with vapor in a crucible proceeds via 2PmF3+3Li→2Pm+3LiF2\mathrm{PmF_3} + 3\mathrm{Li} \rightarrow 2\mathrm{Pm} + 3\mathrm{LiF}, while calcium reduction follows 2PmF3+3Ca→2Pm+3CaF22\mathrm{PmF_3} + 3\mathrm{Ca} \rightarrow 2\mathrm{Pm} + 3\mathrm{CaF_2}; both reactions occur under at temperatures around or above the metal's of 1042°C to ensure complete conversion and minimize oxidation. These methods have produced milligram-scale samples of promethium metal with densities near 7.26 g/cm³, though the material's radioactivity limits handling to glove boxes or hot cells. Recent advances at ORNL in 2025 have improved separation efficiency by characterizing promethium coordination complexes in aqueous solution for the first time, using ligands like diglycolamides to form stable Pm³⁺-oxygen bonds. X-ray absorption spectroscopy revealed bond lengths and electronic structures that explain promethium's separation behavior relative to neighboring lanthanides, enabling higher-purity isolation (>99.9%) from mixed fission or irradiation products via targeted chelation and chromatography. These developments, supported by quantum chemistry simulations, enhance yield and scalability for research applications. Worldwide annual production of promethium remains limited to tens of grams, predominantly as 147Pm^{147}\mathrm{Pm}, with the U.S. Department of Energy's Program at ORNL serving as the primary supplier alongside limited contributions from Russian reactors.

History

Early searches for element 61

The existence of element 61 was anticipated in the late as part of the series in Dmitri Mendeleev's periodic table, where gaps appeared between (atomic number 60) and (62) due to the progressive filling of 4f orbitals leading to the —a phenomenon causing atomic radii to decrease across the series, making chemical separation challenging for unstable elements like this one. In 1902, Czech chemist Bohuslav Brauner refined Mendeleev's table by extending the row after , explicitly predicting an undiscovered element at position 61 with properties intermediate between and , based on atomic weight discrepancies in rare earth minerals. This prediction gained empirical support in 1914 when Henry Moseley's experiments confirmed a missing at 61 among the lanthanides, solidifying the theoretical gap. Searches for element 61 intensified from the early 1900s through the 1940s, primarily involving fractional crystallization and spectroscopic analysis of rare earth concentrates from minerals like , but these efforts were hampered by the element's absence in nature owing to its lack of stable isotopes—all known isotopes have half-lives shorter than 18 years, preventing geological accumulation. Pioneering work began around 1912 with American chemist Charles James at the , who amassed large quantities of rare earths for separation studies, followed by B. Smith Hopkins at the starting in 1923, both using laborious techniques without success. Several premature claims emerged during this period, later debunked as misidentifications of impurities. In 1924, Italian chemists Luigi Rolla and Rita Brunetti at the reported isolating a rare earth fraction with spectral lines suggesting a new element, proposing the name florentium after their city; however, reanalysis in the 1930s revealed the lines belonged to known lanthanides like , and Rolla retracted the claim in 1941 following scrutiny by . Similarly, in 1926, B. Smith Hopkins, along with Len Yntema and J.A. Harris at the , announced the discovery of illinium from monazite residues, based on faint emissions and naming it after their state; subsequent independent verifications, including by Austrian chemist Friedrich Adolf Paneth in 1927, showed the sample was contaminated with and other impurities, invalidating the claim. Another unconfirmed report came in 1938 from Laurence Quill's team at , who produced short-lived isotopes via bombardment and suggested the name cyclonium, but lacked chemical characterization. These pre-1945 searches culminated during the , where the need to analyze uranium fission products from atomic bomb development provided the breakthrough. In 1945, Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell at successfully identified element 61 using ion-exchange chromatography on fission products from , revealing its beta-emitting isotopes and confirming its position in the series—thus completing the rare earth row after decades of elusive pursuit.

Discovery and initial synthesis

Promethium, element 61, was first identified in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell at the in . Working under wartime secrecy during the , the team isolated the element from the fission products of irradiated in the Graphite Reactor. They employed ion-exchange chromatography to separate rare earth fission products and identified promethium through its characteristic properties, specifically the decay chain involving isotopes such as promethium-147, which emits beta particles with energies around 0.225 MeV. This breakthrough filled the last gap in the lanthanide series, as prior attempts to find a stable isotope had failed due to its radioactivity. The discovery was not publicly announced until December 1947, when Marinsky, Glendenin, and Coryell published their findings in the Journal of the American Chemical Society, detailing the chemical separation and radioactive identification of the new element. Independent confirmation came in 1951 through absorption and emission spectroscopy by William F. Meggers, Bourdon F. Scribner, and William R. Bozman at the National Bureau of Standards, who analyzed a sample of promethium chloride and identified spectral lines consistent with its position between neodymium and samarium in the periodic table, including prominent absorption bands at 494.5, 548.5, 568.0, 685.5, and 735.5 nm. Early isolations yielded only microgram quantities, limited by the low fission yield of promethium isotopes (about 0.25% for Pm-147 from U-235 fission) and the challenges of handling highly radioactive materials. In 1949, the International Union of Pure and Applied Chemistry (IUPAC) officially approved the name "promethium" (symbol Pm) for element 61, honoring the Greek Titan who stole fire from the gods to benefit humanity—a nod to the element's nuclear origins and potential. This replaced temporary designations like "cl" proposed in some early reports. The first synthesis of pure promethium metal occurred in 1963, when Fritz Weigel at the University of Munich reduced promethium(III) fluoride (PmF₃) with vapor in a crucible at 1100-1200°C, producing approximately 50 mg of the metal with reported purity over 99%. This marked the initial production of weighable amounts of the element, enabling further studies of its physical properties despite its rapid self-irradiation damage due to .

Recent research advances

In the early 2000s, research on promethium shifted toward advanced spectroscopic techniques to overcome its scarcity and radioactivity, enabling the first detailed studies of its coordination chemistry. A pivotal advancement occurred in 2024 through collaborative efforts at (ORNL) and (BNL), where scientists synthesized and characterized the first stable promethium in using the bispyrrolidine diglycolamide (PyDGA) . This [Pm(PyDGA)3]3+ complex allowed for the determination of Pm–O bond lengths at 2.476(16) Å via synchrotron (XAS) at BNL's National Synchrotron Light Source II (NSLS-II). These measurements confirmed the effect, with promethium exhibiting an accelerated shortening of ionic radii compared to neighboring elements, completing the structural trend across the lanthanide series from to . Building on this, DOE-funded research in March 2025 at ORNL further characterized a promethium-147 coordination complex, providing insights into its electronic structure and bonding behavior distinct from other lanthanides. By stabilizing the complex with an organic ligand in solution, researchers used XAS to reveal subtle electronic differences that influence , advancing techniques for separating promethium from nuclear fission byproducts—building on the 2024 discovery of the first Pm complex. This work, supported by the DOE Office of Science and Isotope Program, highlighted how promethium's properties can inform improved purification methods for rare earth elements. In April and May 2025, ORNL studies uncovered hidden electronic properties of promethium through , demonstrating deviations in its orbital interactions that aid in modeling the behavior of rare earth elements and their analogs. These findings, derived from stabilized promethium-147 complexes, revealed how promethium's position in the periodic table influences overall bonding trends, with potential applications in simulating chemistry for nuclear waste processing. For instance, the observed trends support the development of ligands that selectively extract and from , enhancing efficiency in waste remediation. Despite these progress, challenges persist due to promethium's short isotope half-lives—such as 2.62 years for 147Pm—and its synthetic , limiting experiments to microgram-scale samples produced via nuclear reactors. These constraints necessitate specialized facilities like hot cells at ORNL and sources, restricting sample volumes to approximately 20 μg in solution for accurate measurements. Ongoing research continues to address these limitations through computational modeling and targeted production to expand promethium's study for broader geochemical and nuclear applications.

Applications

Established uses

Promethium, particularly the isotope ¹⁴⁷Pm, has found limited but established applications due to its beta-emitting and . These uses primarily emerged during the mid-20th century, leveraging its radioactivity for low-power, long-duration sources and measurement tools. One key application is in atomic batteries, specifically betavoltaic cells that convert energy into electricity. ¹⁴⁷Pm-powered betavoltaic batteries were developed for cardiac pacemakers in the , providing reliable, long-term power without frequent replacements; the Betacel model 400, for instance, used ¹⁴⁷Pm coupled with semiconductors to deliver stable output for medical implants. Similar devices powered remote sensors and guided missiles, capitalizing on the isotope's 2.62-year for sustained low-level energy in harsh environments. Production of ¹⁴⁷Pm for these batteries peaked during the era at facilities like the , where gram-scale quantities were extracted from fission products for military and space-related needs. In luminous paints, ¹⁴⁷Pm is mixed with to create self-luminous materials that emit light without external excitation, ideal for low-light visibility. These paints were applied to watch dials, instrument panels, and military equipment like gauges during the mid-20th century, offering persistent glow from beta-induced ; however, they were largely phased out by the in favor of tritium-based alternatives due to handling concerns. Historical use extended to space applications, including illumination of instruments in Apollo lunar modules. Promethium also serves as a beta source in industrial thickness gauges, where ¹⁴⁷Pm emissions measure material density and thickness in manufacturing processes. Beta particles from the isotope are attenuated by passing through thin films, , plastics, or metal sheets, allowing non-destructive, precise gauging of thicknesses down to micrometers; this technique has been standard in for and converting industries since the 1950s. In research settings, ¹⁴⁷Pm acts as a tracer for chemical analysis, tracking reaction pathways and material flows in laboratory studies due to its detectability via .

Potential and emerging applications

Promethium's potential in nuclear batteries centers on developing improved long-life versions for deep-space probes, leveraging isotopes like ¹⁴⁵Pm with its 17.7-year half-life to provide sustained power over extended missions. In 2025, researchers at advanced promethium-147 production by extracting it from byproducts, potentially increasing availability for nuclear battery applications. A 2024 study on coalescent energy transducers highlights promethium-147's role in micronuclear batteries, achieving higher efficiency through beta-emitting phosphors integrated with photovoltaic cells, which could extend operational lifespans for probes in harsh environments. Advances in promethium research, including the first promethium complex observed in solution using diglycolamide ligands, have improved separation techniques. These separation breakthroughs correlate with lanthanide contraction trends, enabling more precise isolation that could streamline industrial recycling of rare earths. Additionally, quantum chemical modeling of promethium's f-block electronic structure, using absorption and computational simulations, provides accurate data. In and tracers, promethium's beta emission offers promise for targeted , particularly with like promethium-149, which deliver high-energy electrons to destroy tumor cells while minimizing damage to surrounding tissues. Studies on promethium-149 DOTA-bombesin analogs demonstrate selective binding to cancer receptors, enabling precise delivery with low gamma emission for safer administration. A 2022 review of rare earth radionuclides underscores promethium's potential in theranostics, combining and , though clinical translation lags due to isotope availability. Despite these prospects, promethium's applications face significant challenges from its extreme , with annual production limited to tens of grams, and inherent , which complicates handling, increases costs, and limits scalability for widespread use. Its beta poses risks, including tissue damage, necessitating stringent shielding and specialized facilities that hinder commercial viability. Ongoing aims to mitigate these barriers through improved synthesis, but current production constraints from byproducts remain a primary obstacle.

Safety and precautions

Health effects

Promethium poses health risks primarily through its , with the most common isotope, (¹⁴⁷Pm), emitting beta particles that can cause internal tissue damage upon or . These beta particles have a maximum energy of 0.225 MeV and an average energy of 0.062 MeV, enabling them to penetrate the outer layers of but not deeper tissues, resulting in potential superficial burns from external exposure to high-activity sources; however, the greater danger arises from internal exposure, where particles deposit energy locally in organs. As a , promethium exhibits chemical akin to other rare earth elements, with low gastrointestinal absorption (approximately 0.007% in adult rats) but potential accumulation in bones and the liver following uptake, which may lead to or other organ damage over time. Promethium-147 is classified as highly radiotoxic, with an annual limit of intake () for occupational exposure via ingestion of 1.5 × 10⁸ Bq (4 × 10³ µCi), corresponding to a committed effective dose equivalent of 0.05 Sv, and critical organ effects on the lower wall; vary by class (D: 2.6 × 10⁸ Bq, W: 7.4 × 10⁷ Bq, Y: 7.4 × 10⁷ Bq). No specific (LD₅₀) data exist for promethium, but extrapolations from lanthanide analogs suggest acute oral in the range of several grams per kilogram body weight in . Chronic exposure to promethium increases cancer risk due to the ionizing effects of its beta , particularly in the and , though promethium has no known essential biological role in humans. Epidemiological data on promethium specifically are limited owing to its rarity and synthetic nature, with insights derived from studies on analogs showing associations with and hepatic dysfunction in occupationally exposed populations.

Handling and environmental considerations

Promethium, primarily in the form of the promethium-147, requires careful handling due to its beta and chemical reactivity. Operations involving promethium are typically conducted within gloveboxes to contain radioactive aerosols and prevent , as demonstrated in purification processes at facilities like (ORNL). For , beta emissions from promethium-147 can be effectively shielded using as little as 0.2 mm of plastic material, while alpha shielding is unnecessary for this but may apply if trace contaminants are present. Storage must occur in inert atmospheres, such as argon-filled gloveboxes with oxygen levels below 500 ppm, to inhibit oxidation and formation of promethium oxide (Pm₂O₃), which could compromise sample integrity. Regulatory oversight for promethium falls under international and national frameworks for radioactive materials, rather than special nuclear materials classification, given its status as a non-fissile fission product. The (IAEA) governs its safe transport through standards outlined in SSR-6, requiring that maintains under normal and accident conditions. In the United States, the (NRC) regulates possession, use, and disposal via 10 CFR Part 20, with transport compliant to rules aligned with IAEA, often under 2915 for low specific activity radioactive material in excepted packages. These protocols emphasize labeling, monitoring with body and ring badges, and limits on airborne concentrations to 4 × 10⁻¹¹ μCi/mL for promethium-147 in uncontrolled areas. Environmental impacts from promethium are limited by its negligible natural occurrence and low global production volumes, estimated at less than 1 gram annually from processes. As a component of , promethium can bioaccumulate in aquatic organisms, with studies showing uptake by freshwater , potentially entering food chains if released from waste streams. However, its relatively short of 2.62 years for promethium-147 restricts long-term environmental persistence, as the decay product samarium-147 poses reduced radiological risks over time. Disposal of promethium-bearing follows high-level nuclear protocols, primarily through , where fission products are incorporated into matrices for immobilization and long-term storage in geological repositories. opportunities arise during reprocessing, where promethium-147 can be separated via ion-exchange or solvent extraction for reuse in applications like thickness gauges, thereby diverting it from streams. As of 2025, advances in coordination chemistry and synchrotron-based characterization have enabled more efficient separation techniques, such as improved complexation in aqueous solutions, reducing overall volumes by enhancing recovery yields from production byproducts.

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