Protactinium
Protactinium is a radioactive chemical element with atomic number 91 and symbol Pa, classified as a dense, silvery-gray actinide metal in the f-block of the periodic table.[1] It has an atomic mass of 231.036 and occurs naturally in trace amounts in uranium ores, rendering it one of the rarest of all naturally occurring elements on Earth.[2][1] The discovery of protactinium unfolded in the early 20th century through the identification of its isotopes. The first isotope, 234Pa, was discovered in 1913 by Kasimir Fajans and Oswald Göhring as a short-lived decay product in the uranium-238 series.[1] In 1917–1918, the more stable isotope 231Pa was independently identified by Otto Hahn and Lise Meitner, as well as by Frederick Soddy and John Cranston, from uranium ore residues.[1] The element was named "protactinium" in 1949 by the International Union of Pure and Applied Chemistry (IUPAC), deriving from its role as the progenitor of actinium in the decay chain.[1] Physically, protactinium is a solid at room temperature with a density of 15.4 g/cm³, a melting point of 1572 °C, and a boiling point of 4000 °C.[2] It exhibits metallic luster but tarnishes rapidly in air due to its reactivity. Chemically, it displays oxidation states of +3, +4, and +5, with +5 being the most stable, and it readily reacts with oxygen, water vapor, and acids to form compounds such as protactinium oxide (Pa₂O₅).[1] The primary isotope, 231Pa, has a half-life of 32,700 years and decays via alpha emission.[1] Due to its extreme rarity—estimated at only a few parts per trillion in the Earth's crust—and intense radioactivity, protactinium has no practical industrial applications and is extracted only in microgram quantities from spent nuclear fuel for basic scientific research in nuclear physics and chemistry.[1]Physical and atomic properties
Appearance and bulk properties
Protactinium metal exhibits a silvery-gray appearance with a bright metallic luster when freshly prepared. It retains this luster for some time upon exposure to air but gradually tarnishes due to the formation of a surface oxide layer.[1] The density of protactinium is 15.37 g/cm³ at 20 °C, positioning it among the denser naturally occurring elements and comparable to neighboring actinides like thorium.[3] Protactinium has a melting point of 1,568 °C and a boiling point of approximately 4,000 °C, reflecting its high thermal stability typical of refractory metals.[1] In its pure form, protactinium is ductile and malleable, allowing it to be shaped under mechanical stress, although the presence of impurities can increase its hardness and brittleness. Its specific heat capacity is 0.12 J/g·K, indicating relatively low thermal responsiveness compared to lighter metals.[4][5] The electrical resistivity of protactinium is 1.7 × 10^{-7} Ω·m, consistent with its metallic character, while its thermal conductivity is 47 W/m·K, enabling moderate heat dissipation.[6] Protactinium displays paramagnetic behavior, with a magnetic susceptibility of χ = 1.7 × 10^{-4} cm³/mol, showing no magnetic ordering transitions at standard temperatures.[7] Due to its radioactivity, protactinium experiences self-heating, which can influence its handling and storage.[1]Atomic structure
Protactinium (Pa) is a chemical element with atomic number 91, positioned in the periodic table as the second member of the actinide series, following thorium (atomic number 90) and preceding uranium (atomic number 92).[3][2] This placement highlights its role in the f-block, where the 5f orbitals contribute to its electronic structure and chemical behavior.[2] The ground-state electron configuration of protactinium is [Rn] 5f² 6d¹ 7s², reflecting the onset of 5f-orbital filling in the actinide series after the core [Rn] configuration and partial occupation of the 6d and 7s valence shells.[2] Relativistic effects, arising from the high nuclear charge, cause significant contraction of the 6s and 6p orbitals, which poorly shield the nucleus and lead to a gradual stabilization and filling of the 5f orbitals across the actinides, influencing protactinium's atomic properties.[8] The empirical atomic radius of protactinium is 163 pm, indicative of its metallic character within the actinide group.[5] For its ions, the effective ionic radius is 104 pm for Pa⁴⁺ and 92 pm for Pa⁵⁺, both in six-fold coordination, demonstrating the expected decrease in size with higher oxidation states due to greater effective nuclear charge.[9] The successive ionization energies are 568 kJ/mol for the first (removal from 7s), 1,180 kJ/mol for the second, and approximately 2,000 kJ/mol for the third, underscoring the increasing difficulty of electron removal from the contracted orbitals.[10] In natural samples, protactinium occurs exclusively as the isotope ²³¹Pa, with an atomic mass of 231.03588 u, which dominates its measured atomic weight of approximately 231.036 u.[3]Chemical properties
Reactivity and oxidation states
Protactinium is a highly reactive metal that tarnishes slowly in air upon exposure to oxygen and water vapor, though the bulk metal retains its silvery luster for some time before corroding. The powdered form is particularly reactive and ignites spontaneously in air at elevated temperatures.[1][11] The metal reacts vigorously with water vapor when heated, producing hydrogen gas and protactinium oxide. It also combines with oxygen to form the pentoxide according to the equation: $4\mathrm{Pa} + 5\mathrm{O_2} \rightarrow 2\mathrm{Pa_2O_5} This reaction underscores protactinium's affinity for oxygen, leading to the formation of the stable Pa(V) oxide.[5] In chemical compounds, protactinium exhibits +5 (Pa(V)) as the most stable oxidation state, with +4 (Pa(IV)) also common and accessible under reducing conditions; lower states such as +3 are unstable and tend to disproportionate or oxidize readily. The prevalence of higher oxidation states arises from the electronic configuration of protactinium, where the 5f¹ electron participates minimally in bonding, favoring actinide-like behavior with strong oxidizing tendencies.[3][1] Due to the large ionic radius of Pa⁵⁺ (approximately 0.94 Å for coordination number 6) combined with its high charge density, protactinium in higher oxidation states readily forms coordination complexes with various ligands, including halides, oxyanions, and chelating agents. These complexes stabilize the metal ion in solution and influence its separation and analytical chemistry. For instance, in acidic media, Pa(V) coordinates water molecules and anionic ligands to mitigate hydrolysis.[12][13] Protactinium metal demonstrates corrosion resistance toward dilute inorganic acids such as nitric, hydrochloric, and sulfuric, where it reacts slowly or forms protective oxide layers, but it dissolves readily in hydrofluoric acid due to the formation of stable anionic fluoride complexes like [PaF₈]³⁻. This selective solubility is exploited in protactinium purification processes.[14]Thermodynamic properties
The thermodynamic properties of protactinium provide key insights into its chemical stability and reactivity, particularly in oxidation states relevant to actinide chemistry. The standard reduction potential for the Pa⁵⁺/Pa⁴⁺ couple is approximately +0.33 V in molten salt environments, indicating the relative ease of reduction from the pentavalent to tetravalent state under such conditions.[15] For the Pa⁴⁺/Pa couple, the standard reduction potential is approximately -1.4 V, reflecting the strong reducing nature required to obtain metallic protactinium from its tetravalent ion.[16] Standard enthalpies and Gibbs free energies of formation are available for select protactinium compounds, aiding in the prediction of reaction feasibility. The standard enthalpy of formation for Pa₂O₅ is -1060 kJ/mol, derived from early calorimetric estimates, underscoring the thermodynamic favorability of protactinium oxide formation.[17] For PaCl₅ in the crystalline state, the Gibbs free energy of formation is -846.8 ± 8.8 kJ/mol, calculated from measured enthalpies of solution and auxiliary thermodynamic data.[18] Bulk thermodynamic data for protactinium metal include a heat of vaporization of 481 kJ/mol, which highlights the significant energy required to transition the metal to the gas phase.[19] The standard molar entropy of protactinium metal at 298 K is approximately 50 J/mol·K, consistent with values for other early actinides and reflecting its metallic bonding.[20] In the Pa-O system, phase diagram studies reveal the stability of various oxides, with Pa₂O₅ adopting layered structures akin to those in Nb₂O₅ and Ta₂O₅. Density functional theory calculations indicate that the ζ-Nb₂O₅-type structure is the most stable form of Pa₂O₅ at ambient conditions, with a formation energy of -27.92 eV per formula unit, corresponding to enhanced stability due to optimized Pa⁵⁺ coordination in distorted octahedral and pyramidal geometries.[21] Lower oxides like PaO₂ exhibit stability up to higher temperatures, but Pa₂O₅ predominates under oxidizing conditions, influencing the overall phase equilibria in oxygen-rich environments.[21]History
Discovery and isolation
The earliest indication of protactinium's existence came in 1900 when William Crookes examined uranium residues and observed intense radioactivity, which he attributed to an unknown constituent but could not fully characterize.[22] These observations highlighted the challenges posed by the element's extreme rarity and high radioactivity, complicating early detection efforts.[23] Protactinium was first identified as a distinct element in 1913 by Kasimir Fajans and Oswald Göhring at the University of Karlsruhe in Germany.[23] Working with uranium-238 decay products, they isolated a short-lived isotope (later known as the metastable protactinium-234m) from thorium-234 using the recoil method, where alpha decay imparts momentum to separate daughter products; they named it "brevium" or "uranium-X₂" due to its brief half-life of about 1.17 minutes.[1][2] This confirmation via radiochemical separation marked a breakthrough, though the element's fleeting nature and intense radiation made further study difficult.[23] In 1917, Frederick Soddy and John Arnold Cranston at the University of Glasgow independently confirmed protactinium as an element by isolating its longer-lived isotope, protactinium-231, from uranium ore residues through repeated chemical precipitations.[1] Simultaneously, Otto Hahn and Lise Meitner in Berlin achieved the same isolation using similar purification techniques on pitchblende waste, establishing protactinium's position as the parent of actinium in the decay chain; they proposed the name "protactinium" to reflect this relationship.[23] These parallel discoveries underscored the element's scarcity, with only trace amounts available from natural sources. Pure protactinium was first isolated in 1927 by Aristid von Grosse, then working with Hahn in Berlin, who obtained about 2 milligrams of protactinium(V) oxide (Pa₂O₅) from 100 tons of uranium residues via fractional crystallization from concentrated mineral acids like hydrochloric and nitric acid.[24] This laborious process, involving hundreds of precipitations to separate protactinium from tantalum and other interferents, yielded the first macroscopic sample despite the element's rarity (estimated at one part per 10 million in uranium ores).[23] In 1934, von Grosse further refined the metal by thermal decomposition of protactinium pentaiodide (PaI₅), producing elemental protactinium for the first time.[25][2]Naming and early research
The element was initially identified in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring, who named the short-lived isotope ^{234m}Pa brevium due to its brief half-life of approximately 1.17 minutes.[2] In 1917–1918, Otto Hahn and Lise Meitner discovered the more stable isotope ^{231}Pa and proposed the name protoactinium, derived from the Greek word protos (meaning "first" or "before") combined with actinium, highlighting its position as the immediate precursor to actinium in the uranium-235 decay series.[2] This nomenclature emphasized the element's radioactive genealogy, distinguishing it from Fajans's earlier term.[2] In 1949, the International Union of Pure and Applied Chemistry (IUPAC) officially adopted the shortened name protactinium, resolving competing designations like brevium and confirming Hahn and Meitner as co-discoverers alongside Fajans and Göhring.[1] This standardization facilitated ongoing research, as the element's scarcity—estimated at about one part per trillion in Earth's crust—had previously hindered systematic study.[2] Early investigations into protactinium's properties began in earnest during the late 1920s and 1930s, led by Aristid von Grosse, who isolated approximately 2 mg of protactinium(V) oxide (Pa_2O_5) in 1927 from uranium residues, marking the first macroscopic preparation.[1] In 1934, von Grosse achieved the isolation of metallic protactinium on a milligram scale using two methods: thermal decomposition of protactinium(V) iodide (PaI_5) in a vacuum at 1200–1400 °C, and reduction of purified Pa_2O_5 with calcium in a vacuum furnace.[1][2] These efforts not only confirmed protactinium's chemical analogy to tantalum but also enabled initial determinations of its atomic weight through synthesis of compounds like K_2PaF_7.[26] Throughout the 1920s to 1940s, researchers focused on protactinium's solution chemistry to develop separation techniques from uranium ores, exploiting its tendency to hydrolyze and form colloids.[26] Von Grosse and others observed that Pa(V) exhibits low solubility in non-complexing acids like HClO_4, HCl, and HNO_3, precipitating readily above pH 5, while stable solutions required highly acidic conditions at tracer levels.[26] Precipitation methods, such as co-precipitation with manganese dioxide or hydrous oxides, proved effective for isolating protactinium from uranium matrices, informing early purification strategies amid growing interest in actinide chemistry during nuclear research initiatives.[26]Occurrence and production
Natural occurrence
Protactinium is one of the rarest naturally occurring elements, with an estimated abundance in Earth's crust of a few parts per trillion by mass, equivalent to approximately 5 × 10^{-10} % or about 5 parts per trillion.[1] This scarcity arises because protactinium has no stable isotopes, and all natural protactinium consists solely of the radioisotope ^{231}Pa, which has a half-life of 32,760 years and forms as an intermediate in the ^{235}U decay chain.[27] In equilibrium with uranium in the crust, ^{231}Pa accumulates to trace levels, underscoring its geochemical rarity and dependence on parent nuclides for its presence.[1] The element is primarily associated with uranium- and thorium-bearing minerals, where it occurs at low concentrations due to decay chain production. In uranium ores such as pitchblende (uraninite), protactinium levels range from 0.1 to 1 ppm, reflecting the equilibrium ratio with ^{235}U content.[14] It is also present in accessory amounts in monazite and thorianite, phosphate and oxide minerals rich in thorium and uranium, respectively, from which ^{231}Pa can be preconcentrated during processing. These mineral associations highlight protactinium's role as a geochemical tracer in actinide-rich deposits, though its total crustal inventory remains minuscule.[1] Beyond Earth, protactinium participates in cosmic nucleosynthesis, where isotopes are synthesized via the rapid neutron-capture (r-) process in core-collapse supernovae, contributing to the production of heavy actinides.[28] Trace quantities of protactinium have been identified in lunar regolith samples returned by Apollo missions, such as those from Apollo 14, confirming its presence in extraterrestrial materials at levels consistent with solar system uranium decay.[29] In aqueous environments like seawater, dissolved protactinium concentrations are extremely low, on the order of 10^{-15} g/L, derived from the ongoing decay of uranium isotopes in ocean waters.[30][31]Synthesis and extraction methods
Protactinium-231, the principal isotope, is primarily obtained through extraction from residues generated during uranium ore processing, where it accumulates as a decay product of uranium-235. These residues, often from minerals like pitchblende, are dissolved in nitric acid-hydrofluoric acid mixtures to solubilize the protactinium, followed by separation techniques such as ion exchange chromatography using anion-exchange resins to isolate Pa(IV) from uranium and other impurities. Solvent extraction methods employing tributyl phosphate (TBP) in hydrochloric acid media have also been utilized to selectively extract protactinium from such aqueous solutions, enabling its separation from thorium and uranium based on differences in distribution coefficients.[32][33][34] The pure protactinium metal is synthesized by reducing protactinium tetrafluoride (PaF₄) with barium or calcium vapor at temperatures around 1,400°C under an inert argon atmosphere to prevent oxidation. This metallothermic reduction process yields metallic protactinium as a distillate or bead, which can then be purified further by vacuum distillation. The reaction is typically conducted in a tantalum or molybdenum crucible, with the reducing agent vaporized to facilitate complete conversion of PaF₄ to the metal.[17][7] In nuclear reactors, protactinium-233 is produced as an intermediate in the thorium fuel cycle through neutron capture by thorium-232, leading to thorium-233, which beta-decays to protactinium-233 with a half-life of 27 days; this protactinium subsequently decays to uranium-233, supporting the breeding of fissile material. This reactor-based production allows for the isolation of protactinium-233 from irradiated thorium targets via chemical separation, though it is often allowed to decay in situ for uranium-233 generation.[35] A notable large-scale production occurred in 1961 by the United Kingdom Atomic Energy Authority, which purified approximately 125 g of protactinium by a 12-stage process from 60 tons of radioactive waste, followed by adsorption onto silica columns and elution for further refinement. Overall, extraction from natural sources has yielded only about 125 g of protactinium historically, with achievable purities reaching up to 99.5%.[1][32]Isotopes
Principal isotopes
Protactinium has no stable isotopes, with all 30 known radioisotopes being radioactive and spanning mass numbers from ²¹⁰Pa to ²³⁹Pa.[36] These isotopes primarily decay via alpha (α) emission or beta-minus (β⁻) decay, reflecting the element's position in the actinide series.[37] The principal isotopes are those with the longest half-lives or significant natural or synthetic relevance, including ²³¹Pa, ²³³Pa, and ²³⁴Pa (with its isomer). The most stable isotope, ²³¹Pa, has a half-life of 32,760 years and undergoes α decay to ²²⁷Ac.[37] It occurs naturally as part of the ²³⁵U decay chain (actinium series), constituting nearly all terrestrial protactinium.[3] Another key isotope, ²³³Pa, has a half-life of 26.97 days and decays via β⁻ emission to ²³³U.[37] It is produced synthetically in thorium-based nuclear reactors through neutron capture on ²³²Th, followed by β⁻ decay of ²³³Th.[2] The isotope ²³⁴Pa exists in two forms: the ground state with a half-life of 6.70 hours and the metastable isomer ²³⁴mPa with a half-life of 1.17 minutes.[37] Both primarily undergo β⁻ decay to ²³⁴U, with the isomer first decaying via isomeric transition (IT) to the ground state; ²³⁴Pa occurs naturally in trace amounts in the ²³⁸U decay chain (uranium series).[3] Among neutron-deficient isotopes, ²¹⁰Pa represents a recent discovery in 2025, with a half-life of approximately 6 ms and α decay.[36] It was synthesized via multinucleon transfer reactions and marks the current limit of known protactinium isotopes on the proton-rich side.[36]| Isotope | Half-life | Decay mode(s) | Principal decay product | Occurrence/Production |
|---|---|---|---|---|
| ²³¹Pa | 32,760 years | α | ²²⁷Ac | Natural (²³⁵U chain) |
| ²³³Pa | 26.97 days | β⁻ | ²³³U | Synthetic (thorium reactors) |
| ²³⁴Pa (ground) | 6.70 hours | β⁻, EC | ²³⁴U, ²³⁴Th | Natural (²³⁸U chain) |
| ²³⁴mPa | 1.17 minutes | IT, β⁻ | ²³⁴Pa, ²³⁴U | Natural (²³⁸U chain) |
| ²¹⁰Pa | ~6 ms | α | (not specified) | Synthetic (2025 discovery) |