Protactinium
Protactinium | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Pronunciation | /ˌproʊtækˈtɪniəm/ | |||||||||||||||||||||||||||||||||||||||||||||||||
Appearance | bright, silvery metallic luster | |||||||||||||||||||||||||||||||||||||||||||||||||
Standard atomic weight Ar°(Pa) | ||||||||||||||||||||||||||||||||||||||||||||||||||
Protactinium in the periodic table | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Kasimir Fajans and Oswald Helmuth Göhring (1913) | ||||||||||||||||||||||||||||||||||||||||||||||||||
Named by | Otto Hahn and Lise Meitner (1917–8) | |||||||||||||||||||||||||||||||||||||||||||||||||
Isotopes of protactinium | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Protactinium is a
The element was first identified in 1913 by
The longest-lived and most abundant (nearly 100%) naturally occurring isotope of protactinium, protactinium-231, has a half-life of 32,760 years and is a decay product of uranium-235. Much smaller trace amounts of the short-lived protactinium-234 and its nuclear isomer protactinium-234m occur in the decay chain of uranium-238. Protactinium-233 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 thorium-232. It is an undesired intermediate product in thorium-based nuclear reactors, and is therefore removed from the active zone of the reactor during the breeding process. Ocean science utilizes 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 sediments up to 175,000 years old, and in modeling of various geological processes.[10]
History
In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium.[11] The actinide series was unknown at the time, so Mendeleev positioned uranium below tungsten in group VI, and thorium below zirconium in group IV, leaving the space below tantalum in group V empty. Until the general acceptance of the actinide concept in the late 1940s, periodic tables were published with this structure.[12] For a long time, chemists searched for eka-tantalum[note 1] as an element with similar chemical properties to tantalum, making a discovery of protactinium nearly impossible. Tantalum's heavier analogue was later found to be the transuranic element dubnium – although dubnium is more chemically similar to protactinium, not tantalum.[13]
In 1900,
91Pa
(uranium X2).[17][18][19][20][21][22] 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.[7][21][23] 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 Greek: πρῶτος prôtos, meaning "first, before").[24] The IUPAC confirmed this naming in 1949.[25][26] The discovery of protactinium completed one of the last gaps in early versions of the periodic table, and brought fame to the involved scientists.[27]
- 2 PaI5 → 2 Pa + 5 I2
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.[25][31] For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies.[11] The Oak Ridge National Laboratory in the US provided protactinium at a cost of about US$280/gram.[32]
Isotopes
Twenty-nine
The primary
Nuclear fission
The longest-lived and most abundant isotope, 231Pa, can fission from fast neutrons exceeding ~1 MeV.[34] 233Pa, the other isotope of protactinium produced in nuclear reactors, also has a fission threshold of 1 MeV.[35]
Occurrence
Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes – 231Pa and 234Pa, with the isotope 234Pa occurring in two different energy states. Nearly all natural protactinium is protactinium-231. It is an alpha emitter and is formed by the decay of uranium-235, whereas the beta radiating protactinium-234 is produced as a result of uranium-238 decay. Nearly all uranium-238 (99.8%) decays first to the shorter-lived 234mPa isomer.[36]
Protactinium occurs in uraninite (pitchblende) at concentrations of about 0.3-3 parts 231Pa per million parts (ppm) of ore.[11] Whereas the usual content is closer to 0.3 ppm[37] (e.g. in Jáchymov, Czech Republic[38]), some ores from the Democratic Republic of the Congo have about 3 ppm.[25] 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.[36][39]
In nuclear reactors
Two major protactinium isotopes, 231Pa and 233Pa, are produced from thorium in
Protactinium-233 is formed upon neutron capture by 232Th. It either further decays to uranium-233, or captures another neutron and converts into the non-fissile uranium-234.
Preparation
Before the advent of nuclear reactors, protactinium was separated for scientific experiments from uranium ores. Since reactors have become more common, it is mostly produced as an intermediate product of nuclear fission in thorium high-temperature reactors as an intermediate in the production of the fissile uranium-233:
The isotope 231Pa can be prepared by irradiating thorium-230 with slow neutrons, converting it to the beta-decaying thorium-231; or, by irradiating thorium-232 with fast neutrons, generating thorium-231 and 2 neutrons.
Protactinium metal can be prepared by reduction of its fluoride with calcium,[42] lithium, or barium at a temperature of 1300–1400 °C.[43][44]
Properties
Protactinium is an actinide positioned in the periodic table to the left of uranium and to the right of thorium, and many of its physical properties are intermediate between its neighboring actinides. Protactinium is denser and more rigid than thorium, but is lighter than uranium; its melting point is lower than that of thorium, but higher than that of uranium. The thermal expansion, electrical, and thermal conductivities of these three elements are comparable and are typical of post-transition metals. The estimated shear modulus of protactinium is similar to that of titanium.[45] Protactinium is a metal with silvery-gray luster that is preserved for some time in air.[25][31] Protactinium easily reacts with oxygen, water vapor, and acids, but not with alkalis.[11]
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.[42][46] The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9×10−6/°C.[42]
Protactinium is paramagnetic and no magnetic transitions are known for it at any temperature.[47] It becomes superconductive at temperatures below 1.4 K.[11][43] Protactinium tetrachloride is paramagnetic at room temperature, but becomes ferromagnetic when cooled to 182 K.[48]
Protactinium exists in two major oxidation states: +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 [Rn]5f26d17s2, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f0 configuration. Both +4 and +5 states easily form hydroxides in water, with the predominant ions being Pa(OH)3+, Pa(OH)2+2, Pa(OH)+3, and Pa(OH)4, all of which are colorless.[49] Other known protactinium ions include PaCl2+2, PaSO2+4, PaF3+, PaF2+2, PaF−6, PaF2−7, and PaF3−8.[50][51]
Chemical compounds
Formula | color | symmetry | space group | No | Pearson symbol | a (pm) | b (pm) | c (pm) | Z | density (g/cm3) |
---|---|---|---|---|---|---|---|---|---|---|
Pa | silvery-gray | tetragonal[3] | I4/mmm | 139 | tI2 | 392.5 | 392.5 | 323.8 | 2 | 15.37 |
PaO | rocksalt[44] | Fm3m | 225 | cF8 | 496.1 | 4 | 13.44 | |||
PaO2 | black | fcc[44] | Fm3m | 225 | cF12 | 550.5 | 4 | 10.47 | ||
Pa2O5 | white | Fm3m[44] | 225 | cF16 | 547.6 | 547.6 | 547.6 | 4 | 10.96 | |
Pa2O5 | white | orthorhombic[44] | 692 | 402 | 418 | |||||
PaH3 | black | cubic[44] | Pm3n | 223 | cP32 | 664.8 | 664.8 | 664.8 | 8 | 10.58 |
PaF4 | brown-red | monoclinic[44] | C2/c | 15 | mS60 | 2 | ||||
PaCl4 | green-yellow | tetragonal[52] | I41/amd | 141 | tI20 | 837.7 | 837.7 | 748.1 | 4 | 4.72 |
PaBr4 | brown | tetragonal[53][54] | I41/amd | 141 | tI20 | 882.4 | 882.4 | 795.7 | ||
PaCl5 | yellow | monoclinic[55] | C2/c | 15 | mS24 | 797 | 1135 | 836 | 4 | 3.74 |
PaBr5 | red | monoclinic[54][56] | P21/c | 14 | mP24 | 838.5 | 1120.5 | 1214.6 | 4 | 4.98 |
PaOBr3 | monoclinic[54] | C2 | 1691.1 | 387.1 | 933.4 | |||||
Pa(PO3)4 | orthorhombic[57] | 696.9 | 895.9 | 1500.9 | ||||||
Pa2P2O7 | cubic[57] | Pa3 | 865 | 865 | 865 | |||||
Pa(C8H8)2 | golden-yellow | monoclinic[58] | 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 formula units per unit cell; fcc stands for the face-centered cubic symmetry. Density was not measured directly but calculated from the lattice parameters.
Oxides and oxygen-containing salts
Protactinium oxides are known for the metal oxidation states +2, +4, and +5. The most stable is the white pentoxide
Protactinium forms mixed binary oxides with various metals. With alkali metals A, the crystals have a chemical formula APaO3 and
Protactinium oxides are basic; they easily convert to hydroxides and can form various salts, such as sulfates, phosphates, nitrates, etc. The nitrate is usually white but can be brown due to radiolytic decomposition. Heating the nitrate in air at 400 °C converts it to the white protactinium pentoxide.[64] 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.[57]
Halides
Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and coordinated by 7 other ions. The coordination is the same in protactinium(V) chloride, but the color is yellow. The coordination changes to octahedral in the brown protactinium(V) bromide, but is unknown for protactinium(V) iodide. The protactinium coordination in all its tetrahalides is 8, but the arrangement is square antiprismatic in protactinium(IV) fluoride and dodecahedral in the chloride and bromide. Brown-colored protactinium(III) iodide has been reported, where protactinium ions are 8-coordinated in a bicapped trigonal prismatic arrangement.[65]
Protactinium(V) fluoride and protactinium(V) chloride have a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The 7-coordination of protactinium originates from the five halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water.[66] The pentachloride 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.[44]
Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachloride at temperatures of 200–300 °C.[44] The by-products (such as PaOCl3) are removed by fractional sublimation.[55] 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.[44]
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.[44] 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.[54][56]
Protactinium iodides can be produced by reacting protactinium metal with elemental iodine at 600 °C, and by reacting Pa2O5 with AlO3 at 600 °C.[44] Protactinium(III) iodide can be obtained by heating protactinium(V) iodide in vacuum.[66] 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.[50]
More complex protactinium fluorides are also known, such as Pa2F9[66] 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).[51]
Other inorganic compounds
Oxyhalides and oxysulfides of protactinium are known. PaOBr3 has a monoclinic structure composed of double-chain units where protactinium has coordination 7 and is arranged into pentagonal bipyramids. The chains are interconnected through oxygen and bromine atoms, and each oxygen atom is related to three protactinium atoms.[54] PaOS is a light-yellow, non-volatile solid with a cubic crystal lattice isostructural to that of other actinide oxysulfides. It is obtained by reacting protactinium(V) chloride with a mixture of hydrogen sulfide and carbon disulfide at 900 °C.[44]
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.[44] Protactinium forms borohydrides, which include Pa(BH4)4. It has an unusual polymeric structure with helical chains, where the protactinium atom has coordination number of 12 and is surrounded by six BH4− ions.[67]
Organometallic compounds
Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C5H5)4) with four cyclopentadienyl rings, which can be synthesized by reacting protactinium(IV) chloride with molten Be(C5H5)2. One ring can be substituted with a halide atom.[68] Another organometallic complex is the golden-yellow bis(π-cyclooctatetraene) protactinium, or protactinocene (Pa(C8H8)2), which is analogous in structure to uranocene. There, the metal atom is sandwiched between two cyclooctatetraene ligands. Similar to uranocene, it can be prepared by reacting protactinium tetrachloride with dipotassium cyclooctatetraenide (K2C8H8) in tetrahydrofuran.[58]
Applications
Although protactinium is situated in the periodic table between uranium and thorium, both of which have numerous applications, there are currently no uses for protactinium outside scientific research owing to its scarcity, high radioactivity, and high toxicity.[36]
Protactinium-231 arises naturally from the decay of natural uranium-235, and artificually 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 nuclear weapons; the physicist Walter Seifritz once estimated the associated critical mass as 750±180 kg.[69] However, the possibility of criticality of 231Pa has since been ruled out.[34][70]
With the advent of highly sensitive
Precautions
Protactinium is both toxic and highly radioactive; thus, it is handled exclusively in a sealed
Isotope | 231Pa | 227Ac | 227Th | 223Ra | 219Rn | 215Po | 211Pb | 211Bi | 207Tl |
---|---|---|---|---|---|---|---|---|---|
SA (Ci/g) | 0.048 | 73 | 3.1×104 | 5.2×104 | 1.3×1010 | 3×1013 | 2.5×107 | 4.2×108 | 1.9×108 |
Decay | α | α, β | α | α | α | α | β | α, β | β |
Half-life | 33 ka | 22 a | 19 days | 11 days | 4 s | 1.8 ms | 36 min | 2.1 min | 4.8 min |
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.[36][64] The maximum safe dose of Pa in the human body is 0.03 μCi (1.1 kBq), which corresponds to 0.5 micrograms of 231Pa.[73] The maximum allowed concentrations of 231Pa in the air in Germany is 3×10−4 Bq/m3.[64]
See also
- Ada Hitchins, who helped Soddy in discovering the element protactinium
Notes
References
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Bibliography
- ISBN 978-0080379418.
External links
- Protactinium at The Periodic Table of Videos(University of Nottingham)