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Protactinium is a chemical element; it has chemical symbol Pa and atomic number 91. It is a dense, radioactive, silvery-gray actinide metal which readily reacts with oxygen, water vapor, and inorganic . It forms various chemical compounds, in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity, and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel.
The element was first identified in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring and named "brevium" because of the short half-life of the specific isotope studied, Nuclear isomer. A more stable isotope of protactinium, 231Pa, was discovered in 1917/18 by Lise Meitner in collaboration with Otto Hahn, and they named the element protactinium. In 1949, the IUPAC chose the name "protactinium" and confirmed Hahn and Meitner as its discoverers. The new name meant "(nuclear) precursor of actinium," suggesting that actinium is a product of radioactive decay of protactinium. John Arnold Cranston (working with Frederick Soddy and Ada Hitchins) is also credited with discovering the most stable isotope in 1915, but he delayed his announcement due to being called for service in the First World War. John Arnold Cranston . University of Glasgow
The longest-lived and most abundant (nearly 100%) naturally occurring isotope of protactinium, 231Pa, has a half-life of 32,760 years and occurs in the decay chain of uranium-235. Much smaller trace amounts of the short-lived 234Pa and its nuclear isomer 234mPa occur in the decay chain of uranium-238. 233Pa occurs as a result of the decay of thorium-233 as part of the chain of events necessary to produce uranium-233 by neutron irradiation of 232Th. It is an undesired intermediate product in thorium-based , and is therefore removed from the active zone of the reactor during the breeding process. Ocean science uses the element to understand the ancient ocean's geography: analysis of the relative concentrations of various uranium, thorium, and protactinium isotopes in water and minerals is used in radiometric dating of up to 175,000 years old, and in modeling of various geological processes.
Protactinium is the only element with no primordial isotopes that has a standard atomic weight, due to the existence of primordial uranium-235 that can decay into it.
In 1900, William Crookes isolated protactinium as an intensely radioactive material from uranium; however, he could not characterize it as a new chemical element and thus named it uranium X (UX). Crookes dissolved uranium nitrate in diethyl ether, and the residual aqueous phase contained most of the and . His method was used into the 1950s to isolate and from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope 234mPa during their studies of the decay chains of uranium-238: → → → . They named the new element "brevium" (from the Latin word brevis, meaning brief or short) because of the short half-life of 1.16 minutes for (uranium X2).Greenwood, p. 1250Greenwood, p. 1254Eric Scerri, A tale of seven elements, (Oxford University Press 2013) , p.67–74 In 1917–18, two groups of scientists, Lise Meitner in collaboration with Otto Hahn of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered another isotope, 231Pa, having a much longer half-life of 32,760 years. Soddy, F., Cranston, J.F. (1918) The parent of actinium. Proceedings of the Royal Society A – Mathematical, Physical and Engineering Sciences 94: 384-403. Meitner changed the name "brevium" to protactinium as the new element was part of the decay chain of uranium-235 as the parent of actinium (from the prôtos, meaning "first, before"). The IUPAC confirmed this naming in 1949.Greenwood, p. 1251 The discovery of protactinium completed one of the last gaps in early versions of the periodic table, and brought fame to the involved scientists.Shea, William R. (1983) Otto Hahn and the rise of nuclear physics, Springer, p. 213, .
Aristid von Grosse produced 2 milligrams of Pa2O5 in 1927, and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa2O5. He used two different procedures: in the first, protactinium oxide was irradiated by 35 keV electrons in vacuum. In the other, called the van Arkel–de Boer process, the oxide was chemically converted to a halide (chloride, bromide or iodide) and then reduced in a vacuum with an electrically heated metallic filament:
In 1961, the United Kingdom Atomic Energy Authority (UKAEA) produced 127 grams of 99.9% pure protactinium-231 by processing 60 tonnes of waste material in a 12-stage process, at a cost of about US$500,000. For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies. The Oak Ridge National Laboratory in the US provided protactinium at a cost of about US$280/gram.
The primary decay mode for the most stable isotope 231Pa and lighter isotopes (210Pa to 227Pa) is alpha decay, producing isotopes of actinium. The primary decay mode for 228Pa to 230Pa is electron capture or beta plus decay, producing isotopes of thorium, while the primary decay mode for the heavier isotopes (232Pa to 239Pa) is beta decay, producing isotopes of uranium.
Protactinium occurs in uraninite (pitchblende) at concentrations of about 0.3–3 parts 231Pa per million parts (ppm) of ore. Whereas the usual content is closer to 0.3 ppm (e.g. in Jáchymov, Czech Republic), some ores from the Democratic Republic of the Congo have about 3 ppm. Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, corresponding to a radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even when compared to water present in the same sample of soil. Much higher ratios of 2,000 and above are measured in loam soils and clays, such as bentonite.Cornelis, Rita (2005) Handbook of elemental speciation II: species in the environment, food, medicine & occupational health, Vol. 2, John Wiley and Sons, pp. 520–521, .
Protactinium-233 is formed upon neutron capture by 232Th. It either further decays to 233U, or captures another neutron and converts into the non-fissile 234U. 233Pa has a relatively long half-life of 27 days and high cross section for neutron capture (the so-called "neutron poison"). Thus, instead of rapidly decaying to the useful 233U, a significant fraction of 233Pa converts to non-fissile isotopes and consumes neutrons, degrading neutron economy. To limit the loss of neutrons, 233Pa is extracted from the active zone of thorium molten salt reactors during their operation, so that it can only decay into 233U. Extraction of 233Pa is achieved using columns of molten bismuth with lithium dissolved in it. In short, lithium selectively reduces protactinium salts to protactinium metal, which is then extracted from the molten-salt cycle, while the molten bismuth is merely a carrier, selected due to its low melting point of 271 °C, low vapor pressure, good solubility for lithium and actinides, and Miscibility with molten .Groult, Henri (2005) Fluorinated materials for energy conversion, Elsevier, pp. 562–565, .
The isotope 231Pa can be prepared by irradiating 230Th with slow neutrons, converting it to the beta-decaying 231Th; or, by irradiating 232Th with fast neutrons, generating (as one product) 231Th and 2 neutrons.
Protactinium metal has been prepared by reduction of its fluoride with calcium, lithium, or barium at a temperature of 1300–1400 °C.
At room temperature, protactinium crystallizes in the body-centered tetragonal structure, which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa. The structure changes to face-centered cubic ( fcc) upon cooling from high temperature, at about 1200 °C.Young, David A. (1991) Phase diagrams of the elements, University of California Press, p. 222, . The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9/°C.
Protactinium is paramagnetism and no magnetic transitions are known for it at any temperature.Buschow, K. H. J. (2005) Concise encyclopedia of magnetic and superconducting materials, Elsevier, pp. 129–130, . It becomes superconductive at temperatures below 1.4 K. Protactinium tetrachloride is paramagnetic at room temperature, but becomes ferromagnetism when cooled to 182 K.
Protactinium exists in two major : +4 and +5, both in solids and solutions; and the +3 and +2 states, which have been observed in some solids. As the electron configuration of the neutral atom is Rn5f26d17s2, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f0 configuration. Both +4 and +5 states easily form in water, with the predominant ions being Pa(OH)3+, , , and Pa(OH)4, all of which are colorless.Greenwood, p. 1265 Other known protactinium ions include , , , , , , , , , and . (2026). 9780841232556, American Chemical Society. ISBN 9780841232556
| Pa | silvery-gray | tetragonal | I4/mmm | 139 | tI2 | 392.5 | 392.5 | 323.8 | 2 | 15.37 |
| PaO | rocksalt | Fmm | 225 | cF8 | 496.1 | 4 | 13.44 | |||
| PaO2 | black | fcc | Fmm | 225 | cF12 | 550.5 | 4 | 10.47 | ||
| Pa2O5 | white | Fmm | 225 | cF16 | 547.6 | 547.6 | 547.6 | 4 | 10.96 | |
| Pa2O5 | white | orthorhombic | 692 | 402 | 418 | |||||
| PaH3 | black | cubic | Pmn | 223 | cP32 | 664.8 | 664.8 | 664.8 | 8 | 10.58 |
| PaF4 | brown-red | monoclinic | C2/c | 15 | mS60 | 2 | ||||
| PaCl4 | green-yellow | tetragonal | I41/amd | 141 | tI20 | 837.7 | 837.7 | 748.1 | 4 | 4.72 |
| PaBr4 | brown | tetragonal | I41/amd | 141 | tI20 | 882.4 | 882.4 | 795.7 | ||
| PaCl5 | yellow | monoclinic | C2/c | 15 | mS24 | 797 | 1135 | 836 | 4 | 3.74 |
| PaBr5 | red | monoclinic | P21/c | 14 | mP24 | 838.5 | 1120.5 | 1214.6 | 4 | 4.98 |
| PaOBr3 | monoclinic | C2 | 1691.1 | 387.1 | 933.4 | |||||
| Pa(PO3)4 | orthorhombic | 696.9 | 895.9 | 1500.9 | ||||||
| Pa2P2O7 | cubic | Pa3 | 865 | 865 | 865 | |||||
| Pa(C8H8)2 | golden-yellow | monoclinic | 709 | 875 | 1062 |
Here, a, b, and c are lattice constants in picometers, No is the space group number, and Z is the number of per unit cell; fcc stands for the face-centered cubic symmetry. Density was not measured directly but calculated from the lattice parameters.
Protactinium forms mixed binary oxides with various metals. With alkali metals A, the crystals have a chemical formula APaO3 and perovskite structure; A3PaO4 and distorted rock-salt structure; or A7PaO6, where oxygen atoms form a hexagonal close-packed lattice. In all of these materials, the protactinium ions are octahedrally coordinated.Greenwood, p. 1269 The pentoxide Pa2O5 combines with rare-earth metal oxides R2O3 to form various nonstoichiometric mixed-oxides, also of perovskite structure.
Protactinium oxides are Basic oxide; they easily convert to hydroxides and can form various salts, such as , , , etc. The nitrate is usually white but can be brown due to radiolysis decomposition. Heating the nitrate in air at 400 °C converts it to the white protactinium pentoxide. The polytrioxophosphate Pa(PO3)4 can be produced by reacting the difluoride sulfate PaF2SO4 with phosphoric acid (H3PO4) under an inert atmosphere. Heating the product to about 900 °C eliminates the reaction by-products, which include hydrofluoric acid, sulfur trioxide, and phosphoric anhydride. Heating it to higher temperatures in an inert atmosphere decomposes Pa(PO3)4 into the diphosphate PaP2O7, which is analogous to diphosphates of other actinides. In the diphosphate, the PO3 groups form pyramids of C2v symmetry. Heating PaP2O7 in air to 1400 °C decomposes it into the pentoxides of phosphorus and protactinium.
Protactinium(V) chloride has a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all chlorine atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The 7-coordination of protactinium originates from the five chlorine atoms and two bonds to protactinium atoms belonging to the nearby chains. It easily hydrolyzes in water. It melts at 300 °C and sublimates at even lower temperatures.
Protactinium(V) fluoride can be prepared by reacting protactinium oxide with either bromine pentafluoride or bromine trifluoride at about 600 °C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and hydrogen fluoride at 600 °C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction.
Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachloride at temperatures of 200–300 °C. The by-products (such as PaOCl3) are removed by fractional sublimation. Reduction of protactinium(V) chloride with hydrogen at about 800 °C yields protactinium(IV) chloride – a yellow-green solid that sublimes in vacuum at 400 °C. It can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400 °C.
Protactinium bromides are produced by the action of aluminium bromide, hydrogen bromide, carbon tetrabromide, or a mixture of hydrogen bromide and thionyl bromide on protactinium oxide. They can alternatively be produced by reacting protactinium pentachloride with hydrogen bromide or thionyl bromide. Protactinium(V) bromide has two similar monoclinic forms: one is obtained by sublimation at 400–410 °C, and another by sublimation at a slightly lower temperature of 390–400 °C.
Protactinium iodides can be produced by reacting protactinium metal with elemental iodine at 600 °C, and by reacting Pa2O5 with AlI3 at elevated temperatures. Protactinium(III) iodide can be obtained by heating protactinium(V) iodide in vacuum. As with oxides, protactinium forms mixed halides with alkali metals. The most remarkable among these is Na3PaF8, where the protactinium ion is symmetrically surrounded by 8 F− ions, forming a nearly perfect cube.Greenwood, p. 1275
More complex protactinium fluorides are also known, such as Pa2F9Greenwood, p. 1271 and ternary fluorides of the types MPaF6 (M = Li, Na, K, Rb, Cs or NH4), M2PaF7 (M = K, Rb, Cs or NH4), and M3PaF8 (M = Li, Na, Rb, Cs), all of which are white crystalline solids. The MPaF6 formula can be represented as a combination of MF and PaF5. These compounds can be obtained by evaporating a hydrofluoric acid solution containing both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it becomes orthorhombic for larger cations K+, Rb+, Cs+ or NH4+. A similar variation was observed for the M2PaF7 fluorides: namely, the crystal symmetry was dependent on the cation and differed for Cs2PaF7 and M2PaF7 (M = K, Rb or NH4).
In hydrides and nitrides, protactinium has a low oxidation state of about +3. The hydride is obtained by direct action of hydrogen on the metal at 250 °C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is thermally stable to 800 °C in vacuum. Protactinium carbide (PaC) is formed by the reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400 °C. Protactinium forms , which include Pa(BH4)4. It has an unusual polymeric structure with helical chains, where the protactinium atom has coordination number of 14 and is surrounded by six BH4− ions.Greenwood, p. 1277
231Pa arises naturally from the decay of natural 235U, and artificially in nuclear reactors by the reaction 232Th + n → 231Th + 2n and the subsequent beta decay of 231Th. It was once thought to be able to support a nuclear chain reaction, which could in principle be used to build ; the physicist once estimated the associated critical mass as .Seifritz, Walter (1984) Nukleare Sprengkörper – Bedrohung oder Energieversorgung für die Menschheit, Thiemig-Verlag, . However, the possibility of criticality of 231Pa has since been ruled out.
With the advent of highly sensitive mass spectrometers, an application of 231Pa as a tracer in geology and paleoceanography has become possible. In this application, the ratio of 231Pa to 230Th is used for radiometric dating of sediments which are up to 175,000 years old, and in modeling of the formation of minerals. In particular, its evaluation in oceanic sediments helped to reconstruct the movements of North Atlantic water bodies during the last melting of Ice age . Some of the protactinium-related dating variations rely on analysis of the relative concentrations of several long-living members of the uranium decay chain – uranium, protactinium, and thorium, for example. These elements have 6, 5, and 4 valence electrons, thus favoring +6, +5, and +4 oxidation states respectively, and display different physical and chemical properties. Thorium and protactinium, but not uranium compounds, are poorly soluble in aqueous solutions and precipitate into sediments; the precipitation rate is faster for thorium than for protactinium. The concentration analysis for both protactinium-231 (half-life 32,760 years) and 230Th (half-life 75,380 years) improves measurement accuracy compared to when only one isotope is measured; this double-isotope method is also weakly sensitive to inhomogeneities in the spatial distribution of the isotopes and to variations in their precipitation rate.Articles "Protactinium" and "Protactinium-231 – thorium-230 dating" in Encyclopædia Britannica, 15th edition, 1995, p. 737
As protactinium is present in small amounts in most natural products and materials, it is ingested with food or water and inhaled with air. Only about 0.05% of ingested protactinium is absorbed into the blood and the remainder is excreted. From the blood, about 40% of the protactinium deposits in the bones, about 15% goes to the liver, 2% to the kidneys, and the rest leaves the body. The biological half-life of protactinium is about 50 years in the bones, whereas its biological half-life in other organs has a fast and slow component. For example, 70% of the protactinium in the liver has a biological half-life of 10 days, and the remaining 30% for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In each affected organ, protactinium promotes cancer via its radioactivity. The maximum amount of Pa allowed in the human body is , which corresponds to 0.5 micrograms of 231Pa. The maximum allowed concentrations of 231Pa in the air in Germany is .
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