Promethium
Promethium is a synthetic chemical element with the symbol Pm and atomic number 61, classified as a lanthanide in the periodic table and notable for being one of only two elements (along with technetium) that occur naturally on Earth in only trace amounts due to the absence of stable isotopes.[1] As a soft, silvery metal, it tarnishes slowly in air and reacts readily with water, exhibiting typical lanthanide properties such as a melting point of 1042°C and a boiling point of approximately 3000°C, while its electron configuration is [Xe] 4f⁵ 6s².[1] All known isotopes of promethium are radioactive, with promethium-145 having the longest half-life 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.[1] Discovered in 1945 at Oak Ridge National Laboratory by Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell through ion-exchange separation of fission products from uranium, promethium was named after the Titan Prometheus from Greek mythology, who stole fire from the gods, symbolizing humanity's quest for knowledge.[2][3] Although not found in significant natural deposits on Earth, 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.[1][4] Recent advances in promethium chemistry, such as synchrotron X-ray studies of its coordination complexes, are enhancing understanding of lanthanide contraction and potentially unlocking new applications in nuclear technology and materials science.[5]Properties
Physical properties
Promethium is the chemical element with atomic number 61 and electron configuration [Xe] 4f⁵ 6s².[6][7] Its atomic radius is approximately 185 pm, based on empirical measurements for lanthanides.[6] Promethium is a silvery metal that tarnishes in air due to oxidation.[8] It has an extrapolated density of 7.26 g/cm³ at room temperature, a melting point of 1042°C, and a boiling point of 3000°C.[6][7] These values are derived from limited experimental data and analogies with neighboring lanthanides, as promethium's radioactivity complicates direct measurements.[9] The metal exhibits a double hexagonal close-packed crystal structure in its α-phase, with lattice parameters a = 3.65 Å and c = 11.65 Å.[9] Promethium is paramagnetic, consistent with its unpaired f-electrons.[10]| Property | Value | Notes/Source |
|---|---|---|
| Thermal conductivity | 17.9 W/(m·K) | Estimated at room temperature[11] |
| Electrical conductivity | 1.3 × 10⁶ S/m | Derived from resistivity of ~0.75 µΩ·m[12] |
Chemical properties
Promethium, as a member of the lanthanide series, predominantly exhibits the +3 oxidation state, forming the Pm³⁺ ion with an ionic radius of 97.0 pm in six-coordinate environments.[13] 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 water to produce hydrogen gas and promethium(III) hydroxide (Pm(OH)₃), with the reaction accelerating in hot water.[14] It tarnishes rapidly in moist air by oxidizing to form promethium(III) oxide (Pm₂O₃) and dissolves readily in dilute acids, such as hydrochloric or nitric acid, to generate corresponding salts like PmCl₃ or Pm(NO₃)₃, accompanied by hydrogen evolution.[14] Among its key compounds, promethium(III) oxide (Pm₂O₃) is a pale yellow-white solid prepared by calcining promethium oxalate or hydroxide at elevated temperatures around 800–1000°C.[15] Promethium(III) chloride (PmCl₃), a yellow, water-soluble salt, is synthesized by dissolving promethium metal or oxide in hydrochloric acid or by fusing the oxide with ammonium chloride.[15] Similarly, promethium(III) fluoride (PmF₃), a white crystalline compound, is obtained via precipitation from aqueous Pm³⁺ solutions with fluoride ions or by direct reaction of the metal with hydrogen fluoride gas.[15] 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.[16] 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.[16] 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] 4f⁵ 6s² electron configuration, which influences orbital energies and leads to intermediate properties between neodymium (4f⁴) and samarium (4f⁶), including variations in bond lengths and reactivity trends within the series.[15] The half-filled nature approaching the 4f subshell contributes to distinct spectroscopic signatures, though its overall reactivity remains governed by the +3 ionic state.[15]Isotopes
Promethium possesses no stable 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 electron capture leading to neodymium daughters, while heavier isotopes predominantly undergo beta-minus decay to samarium.[17][6] 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.[6][18] Traces of promethium, specifically ^{145}Pm and ^{147}Pm, occur in uranium ores such as pitchblende due to the spontaneous fission of ^{238}U, though in quantities less than one microgram per million tonnes of ore (as of current nuclear data, 2023); these amounts are insufficient for practical isolation and decay rapidly.[15] 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 Number | Half-Life | Decay Mode(s) | Daughter Product(s) |
|---|---|---|---|
| ^{143}Pm | 265 days | Electron capture (100%) | ^{143}Nd |
| ^{144}Pm | 360 days | Electron capture (100%) | ^{144}Nd |
| ^{145}Pm | 17.7 years | Electron capture (nearly 100%); α (2.8 × 10^{-7} %) | ^{145}Nd; ^{141}Pr |
| ^{146}Pm | 5.53 years | Electron capture (66%); β⁻ (34%) | ^{146}Nd; ^{146}Sm |
| ^{147}Pm | 2.623 years | β⁻ (100%) | ^{147}Sm |
| ^{148}Pm | 5.37 days | β⁻ (100%) | ^{148}Sm |
| ^{149}Pm | 2.21 days | β⁻ (100%) | ^{149}Sm |