Protactinium

Protactinium (formerly protoactinium) is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. 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.

Protactinium was first identified in 1913 by Kasimir Fajans and Oswald Helmuth Göhring and named brevium because of the short half-life of the specific isotope studied, i.e. protactinium-234. A more stable isotope of protactinium, 231Pa, was discovered in 1917/18 by Otto Hahn and Lise Meitner, and they chose the name proto-actinium, but the IUPAC finally named it "protactinium" in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant "(nuclear) precursor[5] of actinium" and reflected 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 delayed his announcement due to being called up for service in the First World War.[6]

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 results from the decay of thorium-233 as part of the chain of events used 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. Analysis of the relative concentrations of various uranium, thorium and protactinium isotopes in water and minerals is used in radiometric dating of sediments which are up to 175,000 years old and in modeling of various geological processes.

Protactinium,  91Pa
Protactinium
Protactinium
Pronunciation/ˌproʊtækˈtɪniəm/ (PROH-tak-TIN-ee-əm)
Appearancebright, silvery metallic luster
Standard atomic weight Ar, std(Pa)231.03588(1)[1]
Protactinium 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
Pr

Pa

(Uqp)
thoriumprotactiniumuranium
Atomic number (Z)91
Groupgroup n/a
Periodperiod 7
Blockf-block
Element category  actinide
Electron configuration[Rn] 5f2 6d1 7s2
Electrons per shell
2, 8, 18, 32, 20, 9, 2
Physical properties
Phase at STPsolid
Melting point1841 K ​(1568 °C, ​2854 °F)
Boiling point4300 K ​(4027 °C, ​7280 °F) (?)
Density (near r.t.)15.37 g/cm3
Heat of fusion12.34 kJ/mol
Heat of vaporization481 kJ/mol
Atomic properties
Oxidation states+2, +3, +4, +5 (a weakly basic oxide)
ElectronegativityPauling scale: 1.5
Ionization energies
  • 1st: 568 kJ/mol
Atomic radiusempirical: 163 pm
Covalent radius200 pm
Color lines in a spectral range
Spectral lines of protactinium
Other properties
Natural occurrencefrom decay
Crystal structurebody-centered tetragonal[2]
Body-centered tetragonal crystal structure for protactinium
Thermal expansion~9.9 µm/(m·K)[3] (at r.t.)
Thermal conductivity47 W/(m·K)
Electrical resistivity177 nΩ·m (at 0 °C)
Magnetic orderingparamagnetic[4]
CAS Number7440-13-3
History
PredictionDmitri Mendeleev (1869)
Discovery and first isolationKasimir Fajans and Oswald Helmuth Göhring (1913)
Named byOtto Hahn and Lise Meitner (1917–8)
Main isotopes of protactinium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
229Pa syn 1.5 d ε 229Th
230Pa syn 17.4 d ε 230Th
231Pa 100% 3.276×104 y α 227Ac
232Pa syn 1.31 d β 232U
233Pa trace 26.967 d β 233U
234Pa trace 6.75 h β 234U
234mPa trace 1.17 min β 234U

History

Mendelejevs periodiska system 1871
Dmitri Mendeleev's 1871 periodic table with a gap for protactinium on the bottom row of the chart, between thorium and uranium

In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium.[7] The actinide element group was unknown at the time. Therefore, uranium was positioned below tungsten in group VI, and thorium below zirconium in group IV, leaving the space below tantalum in group V empty and, until the 1950s, periodic tables were published with this structure.[8] For a long time chemists searched for eka-tantalum 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 – which, however, does not react like tantalum, but like protactinium.[9]

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).[7][10][11] Crookes dissolved uranium nitrate in ether, and the residual aqueous phase contains most of the 234
90
Th
and 234
91
Pa
. His method was still used in the 1950s to isolate 234
90
Th
and 234
91
Pa
from uranium compounds.[12] Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope 234Pa during their studies of the decay chains of uranium-238: 238
92
U
234
90
Th
234
91
Pa
234
92
U
. They named the new element brevium (from the Latin word, brevis, meaning brief or short) because of its short half-life, 6.7 hours for 234
91
Pa
.[13][14][15][16][17] In 1917/18, two groups of scientists, Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered another isotope of protactinium, 231Pa, having a much longer half-life of about 32,000 years.[17] Thus the name brevium was changed to protoactinium as the new element was part of the decay chain of uranium-235 as the parent of actinium (from Greek: πρῶτος prôtos "first, before"). For ease of pronunciation, the name was shortened to protactinium by the IUPAC in 1949.[18][19] The discovery of protactinium completed one of the last gaps in the early versions of the periodic table, proposed by Mendeleev in 1869, and it brought to fame the involved scientists.[20]

Aristid von Grosse produced 2 milligrams of Pa2O5 in 1927,[21] and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa2O5.[22] He used two different procedures: in the first one, protactinium oxide was irradiated by 35 keV electrons in vacuum. In another method, 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:[18][23]

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 500,000 USD.[18][24] For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies.[7] Oak Ridge National Laboratory in the US provided protactinium at a cost of about 280 USD/gram.[25]

Isotopes

Twenty-nine radioisotopes of protactinium have been discovered, the most stable being 231Pa with a half-life of 32,760 years, 233Pa with a half-life of 27 days, and 230Pa with a half-life of 17.4 days. All of the remaining isotopes have half-lives shorter than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. Protactinium also has two nuclear isomers, 217mPa (half-life 1.2 milliseconds) and 234mPa (half-life 1.17 minutes).[26]

The primary decay mode for isotopes of protactinium lighter than (and including) the most stable isotope 231Pa (i.e., 212Pa to 231Pa) is alpha decay and the primary mode for the heavier isotopes (i.e., 232Pa to 240Pa) is beta decay. The primary decay products of isotopes of protactinium lighter than (and including) 231Pa are actinium isotopes and the primary decay products for the heavier isotopes of protactinium are uranium isotopes.[26]

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.[27]

Protactinium occurs in uraninite (pitchblende) at concentrations of about 0.3-3 parts 231Pa per million parts (ppm) of ore.[7] Whereas the usual content is closer to 0.3 ppm[28] (e.g. in Jáchymov, Czech Republic[29]), some ores from the Democratic Republic of the Congo have about 3 ppm.[18] Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, that corresponds to the radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even the 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.[27][30]

In nuclear reactors

Two major protactinium isotopes, 231Pa and 233Pa, are produced from thorium in nuclear reactors; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular, 232Th via (n,2n) reactions produces 231Th which quickly (half-life 25.5 hours) decays to 231Pa. The last isotope, while not a transuranic waste, has a long half-life of 32,760 years and is a major contributor to the long term radiotoxicity of spent nuclear fuel.[31]

Protactinium-233 is formed upon neutron capture by 232Th. It further either decays to uranium-233 or captures another neutron and converts into the non-fissile uranium-234.[32] 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 the reactor efficiency. To avoid this, 233Pa is extracted from the active zone of thorium molten salt reactors, during their operation, so that it only decays to 233U. This is achieved using several meters tall columns of molten bismuth with lithium dissolved in it. In a simplified scenario, lithium selectively reduces protactinium salts to protactinium metal which is then extracted from the molten-salt cycle, and bismuth is merely a carrier. It is chosen because of its low melting point (271 °C), low vapor pressure, good solubility for lithium and actinides, and immiscibility with molten halides.[31]

Preparation

Uraninite-39029
Protactinium occurs in uraninite ores.

Before the advent of nuclear reactors, protactinium was separated for scientific experiments from uranium ores. Nowadays, it is mostly produced as an intermediate product of nuclear fission in thorium high-temperature reactors:

(The times are half-lives.)

Protactinium metal can be prepared by reduction of its fluoride with calcium fluoride,[33] lithium or barium at a temperature of 1300–1400 °C.[34][35]

Physical and chemical properties

Protactinium is an actinide which is 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 those two actinides. So, protactinium is more dense and rigid than thorium but is lighter than uranium, and its melting point is lower than that of thorium and 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.[36] Protactinium is a metal with silvery-gray luster that is preserved for some time in air.[18][24] Protactinium easily reacts with oxygen, water vapor and acids, but not with alkalis.[7]

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.[33][37] The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9×106/°C.[33]

Protactinium is paramagnetic and no magnetic transitions are known for it at any temperature.[38] It becomes superconductive at temperatures below 1.4 K.[7][34] Protactinium tetrachloride is paramagnetic at room temperature but turns ferromagnetic upon cooling to 182 K.[39]

Protactinium exists in two major oxidation states, +4 and +5, both in solids and solutions, and the +3 and +2 states were observed in some solid phases. 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 colorless.[40] Other known protactinium ions include PaCl2+
2
, PaSO2+
4
, PaF3+, PaF2+
2
, PaF
6
, PaF2−
7
and PaF3−
8
.[41][42]

Chemical compounds

Formula color symmetry space group No Pearson symbol a (pm) b (pm) c (pm) Z density (g/cm3)
Pa silvery-gray tetragonal[2] I4/mmm 139 tI2 392.5 392.5 323.8 2 15.37
PaO rocksalt[35] Fm3m 225 cF8 496.1 4 13.44
PaO2 black fcc[35] Fm3m 225 cF12 550.5 4 10.47
Pa2O5 white Fm3m[35] 225 cF16 547.6 547.6 547.6 4 10.96
Pa2O5 white orthorhombic[35] 692 402 418
PaH3 black cubic[35] Pm3n 223 cP32 664.8 664.8 664.8 8 10.58
PaF4 brown-red monoclinic[35] C2/c 15 mS60 2
PaCl4 green-yellow tetragonal[43] I41/amd 141 tI20 837.7 837.7 748.1 4 4.72
PaBr4 brown tetragonal[44][45] I41/amd 141 tI20 882.4 882.4 795.7
PaCl5 yellow monoclinic[46] C2/c 15 mS24 797 1135 836 4 3.74
PaBr5 red monoclinic[45][47] P21/c 14 mP24 838.5 1120.5 1214.6 4 4.98
PaOBr3 monoclinic[45] C2 1691.1 387.1 933.4
Pa(PO3)4 orthorhombic[48] 696.9 895.9 1500.9
Pa2P2O7 cubic[48] Pa3 865 865 865
Pa(C8H8)2 golden-yellow monoclinic[49] 709 875 1062

Here a, b and c are lattice constants in picometers, No is 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 white pentoxide Pa2O5, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500 °C.[50] Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO2.25. Another phase of this oxide with orthorhombic symmetry has also been reported.[35][51] The black dioxide PaO2 is obtained from the pentoxide by reducing it at 1550 °C with hydrogen. It is not readily soluble in either dilute or concentrated nitric, hydrochloric or sulfuric acids, but easily dissolves in hydrofluoric acid.[35] The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100 °C.[51] The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.[35]

Protactinium forms mixed binary oxides with various metals. With alkali metals A, the crystals have a chemical formula APaO3 and perovskite structure, or A3PaO4 and distorted rock-salt structure, or A7PaO6 where oxygen atoms form a hexagonal close-packed lattice. In all these materials, protactinium ions are octahedrally coordinated.[52][53] The pentoxide Pa2O5 combines with rare-earth metal oxides R2O3 to form various nonstoichiometric mixed-oxides, also of perovskite structure.[54]

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.[55] The polytrioxophosphate Pa(PO3)4 can be produced by reacting difluoride sulfate PaF2SO4 with phosphoric acid (H3PO4) under inert gas atmosphere. Heating the product to about 900 °C eliminates the reaction by-products such as hydrofluoric acid, sulfur trioxide and phosphoric anhydride. Heating 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.[48]

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 and 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.[56]

PaCl5
Coordination of protactinium (solid circles) and halogen atoms (open circles) in protactinium(V) fluoride or chloride.

Protactinium(V) fluoride and protactinium(V) chloride have a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all the halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The coordination 7 of protactinium originates from the 5 halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water.[57] 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.[35]

Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachloride at temperature of 200–300 °C.[35] The by-products (such as PaOCl3) are removed by fractional sublimation.[46] Reduction of protactinium(V) chloride with hydrogen at about 800 °C yields protactinium(IV) chloride – a yellow-green solid which sublimes in vacuum at 400 °C; it can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400 °C.[35]

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. An alternative reaction is between protactinium pentachloride and hydrogen bromide or thionyl bromide.[35] Protactinium(V) bromide has two similar monoclinic forms, one is obtained by sublimation at 400–410 °C and another by sublimation at slightly lower temperature of 390–400 °C.[45][47]

Protactinium iodides result from the oxides and aluminium iodide or ammonium iodide heated to 600 °C.[35] Protactinium(III) iodide was obtained by heating protactinium(V) iodide in vacuum.[57] As with oxides, protactinium forms mixed halides with alkali metals. Among those, most remarkable is Na3PaF8 where protactinium ion is symmetrically surrounded by 8 F ions which form a nearly perfect cube.[41]

More complex protactinium fluorides are also known such as Pa2F9[57] 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 being 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 these both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it lowers to orthorphombic 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).[42]

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.[45] 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.[35]

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 stable to heating to 800 °C in vacuum. Protactinium carbide PaC is formed by reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400 °C.[35] 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.[58]

Organometallic compounds

Uranocene-3D-balls
The proposed structure of the protactinocene (Pa(C8H8)2) molecule

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.[59] Another organometallic complex is 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.[49]

Applications

Although protactinium is located in the periodic table between uranium and thorium, which both have numerous applications, there are currently no uses for protactinium outside scientific research owing to its scarcity, high radioactivity and high toxicity.[27]

Protactinium-231 arises from the decay of uranium-235 formed in nuclear reactors, and by the reaction 232Th + n → 231Th + 2n and subsequent beta decay. 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.[60] However, the possibility of criticality of 231Pa has been ruled out since then.[61]

With the advent of highly sensitive mass spectrometers, an application of 231Pa as a tracer in geology and paleoceanography has become possible. So, the ratio of protactinium-231 to thorium-230 is used for radiometric dating of sediments which are up to 175,000 years old and in modeling of the formation of minerals.[28] In particular, its evaluation in oceanic sediments allowed to reconstruct the movements of North Atlantic water bodies during the last melting of Ice Age glaciers.[62] Some of the protactinium-related dating variations rely on the analysis of the relative concentrations for several long-living members of the uranium decay chain – uranium, protactinium, and thorium, for example. These elements have 6, 5 and 4 valence electrons and thus favor +6, +5 and +4 oxidation states, respectively, and show different physical and chemical properties. So, 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. Besides, the concentration analysis for both protactinium-231 (half-life 32,760 years) and thorium-230 (half-life 75,380 years) allows to improve the 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.[28][63]

Precautions

Protactinium is both toxic and highly radioactive and thus all manipulations with it are performed in a sealed glove box. Its major isotope 231Pa has a specific activity of 0.048 curies (1.8 GBq) per gram and primarily emits alpha-particles with an energy of 5 MeV, which can be stopped by a thin layer of any material. However, it slowly decays, with a half-life of 32,760 years, into 227Ac, which has a specific activity of 74 curies (2,700 GBq) per gram, emits both alpha and beta radiation, and has a much shorter half-life of 22 years. 227Ac, in turn, decays into lighter isotopes with even shorter half-lives and much greater specific activities (SA), as summarized in the table below showing the decay chain of protactinium-231.[27]

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 in other organs the kinetics has a fast and slow component. So in the liver 70% of protactinium have a half-life of 10 days and 30% remain for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In all these organs, protactinium promotes cancer via its radioactivity.[27][55] 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. This isotope is 2.5×108 times more toxic than hydrocyanic acid.[64] The maximum allowed concentrations of 231Pa in the air in Germany is 3×10−4 Bq/m3.[55]

See also

  • Ada Hitchins, who helped Soddy to discover the element protactinium

References

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Bibliography

External links

Actinide

The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.Strictly speaking, both actinium and lawrencium have been labeled as group 3 elements, but both elements are often included in any general discussion of the chemistry of the actinide elements. Actinium is the more often omitted of the two, because its placement as a group 3 element is somewhat more common in texts and for semantic reasons: since "actinide" means "like actinium", it has been argued that actinium cannot logically be an actinide, even though IUPAC acknowledges its inclusion based on common usage.The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, with the exception being either actinium or lawrencium. The series mostly corresponds to the filling of the 5f electron shell, although actinium and thorium lack any f-electrons, and curium and lawrencium have the same number as the preceding element. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. They all have very large atomic and ionic radii and exhibit an unusually large range of physical properties. While actinium and the late actinides (from americium onwards) behave similarly to the lanthanides, the elements thorium, protactinium, and uranium are much more similar to transition metals in their chemistry, with neptunium and plutonium occupying an intermediate position.

All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.

Of the actinides, primordial thorium and uranium occur naturally in substantial quantities. The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium and plutonium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table inserts the lanthanide and actinide series in their proper places, as parts of the table's sixth and seventh rows (periods).

Ada Hitchins

Ada Florence Remfry Hitchins (26 June 1891 – 4 January 1972) was the principal research assistant of British chemist Frederick Soddy, who won the Nobel prize in 1921 for work on radioactive elements and the theory of isotopes. Hitchins isolated samples from uranium ores, taking precise and accurate measurements of atomic mass that provided the first experimental evidence for the existence of different isotopes. She also helped to discover the element protactinium, which Dmitri Mendeleev had predicted should occur in the periodic table between uranium and thorium.

Aristid von Grosse

Aristid von Grosse was a German nuclear chemist. During his work with Otto Hahn, he got access to waste material from radium production, and with this starting material he was able in 1927 to isolate protactinium oxide and was later able to produce metallic protactinium by decomposition of protactinium iodide.

From 1948 to 1969, he was president of the Research Institute of Temple University. He was later affiliated with the laboratories of the Franklin Institute in Philadelphia until his retirement in 1979.Aristid was born in Riga in January 1905. He died of pneumonia in Laguna Hills, California on July 21, 1985.

Extended periodic table

An extended periodic table theorizes about chemical elements beyond those currently known in the periodic table and proven up through oganesson, which completes the seventh period (row) in the periodic table at atomic number (Z) 118.

If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional so-called g-block, containing at least 18 elements with partially filled g-orbitals in each period. An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969. The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Despite many searches, no elements in this region have been synthesized or discovered in nature.According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals, but spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number. While Seaborg's version of the extended period had the heavier elements following the pattern set by lighter elements, as it did not take into account relativistic effects, models that take relativistic effects into account do not. Pekka Pyykkö and Burkhard Fricke used computer modeling to calculate the positions of elements up to Z = 172, and found that several were displaced from the Madelung rule. As a result of uncertainty and variability in predictions of chemical and physical properties of elements beyond 120, there is currently no consensus on their placement in the extended periodic table.

Elements in this region are likely to be highly unstable with respect to radioactive decay and undergo alpha decay or spontaneous fission with extremely short half-lives, though element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. Other islands of stability beyond the known elements may also be possible, including one theorized around element 164, though the extent of stabilizing effects from closed nuclear shells is uncertain. It is not clear how many elements beyond the expected island of stability are physically possible, whether period 8 is complete, or if there is a period 9. The International Union of Pure and Applied Chemistry (IUPAC) defines an element to exist if its lifetime is longer than 10−14 seconds (0.01 picoseconds, or 10 femtoseconds), which is the time it takes for the nucleus to form an electron cloud.As early as 1940, it was noted that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at Z > 1/α ≈ 137, suggesting that neutral atoms cannot exist beyond element 137, and that a periodic table of elements based on electron orbitals therefore breaks down at this point. On the other hand, a more rigorous analysis calculates the analogous limit to be Z ≈ 173 where the 1s subshell dives into the Dirac sea, and that it is instead not neutral atoms that cannot exist beyond element 173, but bare nuclei, thus posing no obstacle to the further extension of the periodic system. Atoms beyond this critical atomic number are called supercritical atoms.

Isotopes of neptunium

Neptunium (93Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was 239Np in 1940, produced by bombarding 238U with neutrons to produce 239U, which then underwent beta decay to 239Np.

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.

Twenty-three neptunium radioisotopes have been characterized, with the most stable being 237Np with a half-life of 2.14 million years, 236Np with a half-life of 154,000 years, and 235Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has 4 meta states, with the most stable being 236mNp (t1/2 22.5 hours).

The isotopes of neptunium range from 219Np to 244Np, though the intermediate isotopes 220-222Np have not yet been observed. The primary decay mode before the most stable isotope, 237Np, is electron capture (with a good deal of alpha emission), and the primary mode after is beta emission. The primary decay products before 237Np are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Uranium-237 and neptunium-239 are regarded as the leading hazardous radioisotopes in the first hour-to-week period following nuclear fallout from a nuclear detonation, with Np-239 dominating "the spectrum for several days".

Isotopes of protactinium

Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard mass to be given.

Twenty-nine radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32,760 years, 233Pa with a half-life of 26.967 days, and 230Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives less than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. This element also has five meta states, 217mPa (t1/2 1.15 milliseconds), 220m1Pa (t1/2 308 nanoseconds), 220m2Pa (t1/2 69 nanoseconds), 229mPa (t1/2 420 nanoseconds), and 234mPa (t1/2 1.17 minutes).

The only naturally occurring isotopes are 231Pa, which occurs as an intermediate decay product of 235U, 234Pa and 234mPa, both of which occur as intermediate decay products of 238U. 231Pa makes up nearly all natural protactinium.

The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope 231Pa is alpha decay, except for 228Pa to 230Pa, which primarily decay by electron capture to isotopes of thorium. The primary mode for the heavier isotopes is beta minus (β−) decay. The primary decay products of 231Pa and isotopes of protactinium lighter than and including 227Pa are isotopes of actinium and the primary decay products for the heavier isotopes of protactinium are isotopes of uranium.

Kazimierz Fajans

Kazimierz (Kasimir in many American publications; 27 May 1887 – 18 May 1975) was a Polish American physical chemist of Polish-Jewish origin, a pioneer in the science of radioactivity and the discoverer of chemical element protactinium.

Major actinide

Major actinides is a term used in the nuclear power industry that refers to the plutonium and uranium present in used nuclear fuel, as opposed to the minor actinides neptunium, americium, curium, berkelium, and californium.

Mendeleev's predicted elements

Dmitri Mendeleev published a periodic table of the chemical elements in 1869 based on properties that appeared with some regularity as he laid out the elements from lightest to heaviest. When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that as-then-unknown elements existed with properties appropriate to fill those gaps. He named them eka-boron, eka-aluminium and eka-silicon, with respective atomic masses of 44, 68, and 72.

Mononuclidic element

A mononuclidic element or monotopic element is one of the 22 chemical elements that is found naturally on Earth essentially as a single nuclide (which may, or may not, be a stable nuclide). This single nuclide will have a characteristic atomic mass. Thus, the element's natural isotopic abundance is dominated either by one stable isotope or by one very long-lived isotope. There are 19 elements in the first category (which are both monoisotopic and mononuclidic), and 3 (bismuth, thorium and protactinium) in the second category (mononuclidic but not monoisotopic, since they have zero, not one, stable nuclides). A list of the 22 mononuclidic elements is given at the end of this article.

Of the 26 monoisotopic elements that, by definition, have only one stable isotope, there exist 7 (26 minus 19 = 7) which are nevertheless not considered mononuclidic, due to the presence of a significant fraction of a very long-lived (primordial) radioisotope occurring in their natural abundance. These elements are vanadium, rubidium, indium, lanthanum, europium, rhenium, and lutetium.

Oswald Helmuth Göhring

Oswald Helmuth Göhring, also known as Otto Göhring, (1889 - c. 1915) was a German chemist who, with his teacher Kasimir Fajans, co-discovered the chemical element protactinium in 1913.

Protactinium(IV) oxide

Protactinium(IV) oxide is a chemical compound with the formula PaO2. The black oxide is formed by reducing Pa2O5 with hydrogen at 1 550 °C. Protactinium(IV) oxide does not dissolve in H2SO4, HNO3, or HCl solutions, but reacts with the HF.As protactinium(IV) oxide, like other protactinium compounds, is radioactive, toxic and very rare, it has no known technological use.

Protactinium(V) chloride

Protactinium(V) chloride is the chemical compound composed of protactinium and chlorine with the formula PaCl5. It forms yellow monoclinic crystals and has a unique structure composed of chains of 7 coordinate, pentagonal bipyramidal, protactinium atoms sharing edges.

Protactinium(V) oxide

Protactinium(V) oxide is a chemical compound with the formula Pa2O5. When it is reduced with hydrogen, it forms PaO2. Aristid V. Grosse was first to prepare 2 mg of Pa2O5 in 1927. Pa2O5 does not dissolve in concentrated HNO3, but dissolves in HF and in a HF + H2SO4 mixture and reacts at high temperatures with solid oxides of alkali metala and alkaline earth metals.As protactinium(V) oxide, like other protactinium compounds, is radioactive, toxic and very rare, it has very limited technological use. Mixed oxides of Nb, Mg, Ga and Mn, doped with 0.005–0.52% Pa2O5, have been

used as high temperature dielectrics (up to 1300 °C) for ceramic capacitors.

Protactinium oxide

Protactinium oxide may refer to:

Protactinium(II) oxide, PaO

Protactinium(IV) oxide, PaO2

Protactinium(V) oxide, Pa2O5

Symbol (chemistry)

In relation to the chemical elements, a symbol is a code for a chemical element. Symbols for chemical elements normally consist of one or two letters from the Latin alphabet and are written with the first letter capitalised. (Many functional groups have their own chemical symbol, e.g. Ph for the phenyl group, and Me for the methyl group.)

Earlier symbols for chemical elements stem from classical Latin and Greek vocabulary. For some elements, this is because the material was known in ancient times, while for others, the name is a more recent invention. For example, Pb is the symbol for lead (plumbum in Latin); Hg is the symbol for mercury (hydrargyrum in Greek); and He is the symbol for helium (a new Latin name) because helium was not known in ancient Roman times. Some symbols come from other sources, like W for tungsten (Wolfram in German) which was not known in Roman times.

A 3-letter temporary symbol may be assigned to a newly synthesized (or not-yet synthesized) element. For example, "Uno" was the temporary symbol for hassium (element 108) which had the temporary name of unniloctium, based on its atomic number being 8 greater than 100. There are also some historical symbols that are no longer officially used.

In addition to the letter(s) for the element itself, additional details may be added to the symbol as superscripts or subscripts a particular isotope, ionization or oxidation state, or other atomic detail. A few isotopes have their own specific symbols rather than just an isotopic detail added to their element symbol.

Attached subscripts or superscripts specifying a nuclide or molecule have the following meanings and positions:

The nucleon number (mass number) is shown in the left superscript position (e.g., 14N). This number defines the specific isotope. Various letters, such as "m" and "f" may also be used here to indicate a nuclear isomer (e.g., 99mTc). Alternately, the number here can represent a specific spin state (e.g., 1O2). These details can be omitted if not relevant in a certain context.

The proton number (atomic number) may be indicated in the left subscript position (e.g., 64Gd). The atomic number is redundant to the chemical element, but is sometimes used to emphasize the change of numbers of nucleons in a nuclear reaction.

If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Ca2+).

The number of atoms of an element in a molecule or chemical compound is shown in the right subscript position (e.g., N2 or Fe2O3). If this number is one, it is normally omitted - the number one is implicitly understood if unspecified.

A radical is indicated by a dot on the right side (e.g., Cl• for a neutral chlorine atom). This is often omitted unless relevant to a certain context because it is already deducible from the charge and atomic number, as generally true for nonbonded valence electrons in skeletal structures.In Chinese, each chemical element has a dedicated character, usually created for the purpose (see Chemical elements in East Asian languages). However, Latin symbols are also used, especially in formulas.

A list of current, dated, as well as proposed and historical signs and symbols is included here with its signification. Also given is each element's atomic number, atomic weight or the atomic mass of the most stable isotope, group and period numbers on the periodic table, and etymology of the symbol.

Hazard pictographs are another type of symbols used in chemistry.

Uranium-232

Uranium-232 (23292U, 232U, U-232) is an isotope of uranium. It has a half-life of 68.9 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamma radiation emitted by 208Tl (a daughter of 232U, produced relatively quickly) makes the 233U contaminated with it more difficult to handle.

Production of 233U (through the neutron irradiation of 232Th) invariably produces small amounts of 232U as an impurity, because of parasitic (n,2n) reactions on uranium-233 itself, or on protactinium-233, or on thorium-232:

232Th (n,γ) 233Th (β−) 233Pa (β−) 233U (n,2n) 232U

232Th (n,γ) 233Th (β−) 233Pa (n,2n) 232Pa (β−) 232U

232Th (n,2n) 231Th (β−) 231Pa (n,γ) 232Pa (β−) 232UAnother channel involves neutron capture reaction on small amounts of thorium-230, which is a tiny fraction of natural thorium present due to the decay of uranium-238:

230Th (n,γ) 231Th (β−) 231Pa (n,γ) 232Pa (β−) 232UThe decay chain of 232U quickly yields strong gamma radiation emitters:

232U (α, 68.9 years)

228Th (α, 1.9 year)

224Ra (α, 3.6 day, 0.24 MeV) (from this point onwards, the decay chain is identical to that of 232Th; thorium-232 is nevertheless much less dangerous because its extremely long half-life of about 14 billion years means that not as much of its dangerous daughters builds up)

220Rn (α, 55 s, 0.54 MeV)

216Po (α, 0.15 s)

212Pb (β−, 10.64 h)

212Bi (α, 61 min, 0.78 MeV)

208Tl (β−, 3 min, 2.6 MeV) (35.94% branching ratio)

208Pb (stable)This makes manual handling in a glove box with only light shielding (as commonly done with plutonium) too hazardous, (except possibly in a short period immediately following chemical separation of the uranium from thorium-228, radium-224, radon-220, and polonium-216) and instead requiring remote manipulation for fuel fabrication.

Unusually for an isotope with even mass number, 232U has a significant neutron absorption cross section for fission (thermal neutrons 75 barns (b), resonance integral 380 b) as well as for neutron capture (thermal 73 b, resonance integral 280 b).

Uranium-233

Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur, to maintain the neutron economy (if it misses the 233U window, the next fissile target is 235U, meaning a total of 4 neutrons needed to trigger fission).

233U usually fissions on neutron absorption, but sometimes retains the neutron, becoming uranium-234. The capture-to-fission ratio of uranium-233 is smaller than those of the other two major fissile fuels, uranium-235 and plutonium-239.

Uranium-234

Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, U-234 occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of U-238. The primary path of production of U-234 via nuclear decay is as follows: U-238 nuclei emit an alpha particle to become thorium-234 (Th-234). Next, with a short half-life, Th-234 nuclei emit a beta particle to become protactinium-234 (Pa-234), or more likely a nuclear isomer denoted Pa-234m. Finally, Pa-234 or Pa-234m nuclei emit another beta particle to become U-234 nuclei.

U-234 nuclei decay by alpha emission to thorium-230, except for the tiny fraction (parts per billion) of nuclei which undergo spontaneous fission.

Extraction of rather small amounts of U-234 from natural uranium would be feasible using isotope separation, similar to that used for regular uranium-enrichment. However, there is no real demand in chemistry, physics, or engineering for isolating U-234. Very small pure samples of U-234 can be extracted via the chemical ion-exchange process - from samples of plutonium-238 that have been aged somewhat to allow some decay to U-234 via alpha emission.

Enriched uranium contains more U-234 than natural uranium as a byproduct of the uranium enrichment process aimed at obtaining U-235, which concentrates lighter isotopes even more strongly than it does U-235. The increased percentage of U-234 in enriched natural uranium is acceptable in current nuclear reactors, but (re-enriched) reprocessed uranium might contain even higher fractions of U-234, which is undesirable. This is because U-234 is not fissile, and tends to absorb slow neutrons in a nuclear reactor - becoming U-235.

U-234 has a neutron-capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral - the average over neutrons having various intermediate energies. In a nuclear reactor non-fissile isotopes capture a neutron breeding fissile isotopes. U-234 is converted to U-235 more easily and therefore at a greater rate than U-238 is to Pu-239 (via neptunium-239) because U-238 has a much smaller neutron-capture cross-section of just 2.7 barns.

However, (n, 2n) reactions with fast neutrons also convert small amounts of U-235 to U-234, so that spent nuclear fuel may contain about 0.010% U-234, a much higher fraction than in non-irradiated uranium.Depleted uranium contains much less U-234 (around 0.001%) which makes the radioactivity of depleted uranium about one-half of that of natural uranium. Natural uranium has an "equilibrium" concentration of U-234 at the point where an equal number of decays of U-238 and U-234 will occur.

Protactinium compounds
Pa(IV)
Pa(V)

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