Protactinium

Protactinium (formerly protoactinium) is a radioactive chemical element with the 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, ₉₁Pa

Microscope image of a sample of protactinium-233
Protactinium
Pronunciation	/ˌproʊtækˈtɪniəm/ ​(PROH-tak-TIN-ee-əm)
Appearance	bright, silvery metallic luster
Standard atomic weight Ar°(Pa)
	231.03588±0.00001 231.04±0.01 (abridged)^[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) thorium ← protactinium → uranium
Atomic number (Z)	91
Group	f-block groups (no number)
Period	period 7
Block	f-block
Electron configuration	[Rn] 5f^2 6d^1 7s^2
Electrons per shell	2, 8, 18, 32, 20, 9, 2
Physical properties
Phase at STP	solid
Melting point	1841 K ​(1568 °C, ​2854 °F)
Boiling point	4300 K ​(4027 °C, ​7280 °F) (?)
Density (near r.t.)	15.37 g/cm^3
Heat of fusion	12.34 kJ/mol
Heat of vaporization	481 kJ/mol
Atomic properties
Oxidation states	+2, +3, +4, +5 (a weakly basic oxide)
Electronegativity	Pauling scale: 1.5
Ionization energies	1st: 568 kJ/mol
Atomic radius	empirical: 163 pm
Covalent radius	200 pm
Spectral lines of protactinium
Other properties
Natural occurrence	from decay
Crystal structure	​body-centered tetragonal^[2]
Thermal expansion	~9.9 µm/(m⋅K)^[3] (at r.t.)
Thermal conductivity	47 W/(m⋅K)
Electrical resistivity	177 nΩ⋅m (at 0 °C)
Magnetic ordering	paramagnetic^[4]
CAS Number	7440-13-3
History
Prediction	Dmitri Mendeleev (1869)
Discovery and first isolation	Kasimir Fajans and Oswald Helmuth Göhring (1913)
Named by	Otto Hahn and Lise Meitner (1917–8)
Isotopes of protactinium v e
Main isotopes^[5] Decay abun­dance half-life (t1/2) mode pro­duct ^229Pa synth 1.5 d ε ^229Th ^230Pa synth 17.4 d β^+ ^230Th β^− ^230U α ^227Ac ^231Pa 100% 3.265×10^4 y α ^227Ac ^232Pa synth 1.32 d β^− ^232U ^233Pa trace 26.975 d β^− ^233U ^234Pa trace 6.70 h β^− ^234U ^234mPa trace 1.159 min β^− ^234U
Category: Protactinium view talk edit | references

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, i.e. protactinium-234m. A more stable isotope of protactinium, ²³¹Pa, was discovered in 1917/18 by Lise Meitner in collaboration with Otto Hahn, and they chose the name protactinium.^[6] The IUPAC chose the name "protactinium" in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant "(nuclear) precursor^[7] 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.^[8]

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. Ocean science utilizes the element to understand the ancient ocean. 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.^[9]

History

 

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.^[10] The actinide series 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; until the acceptance of the actinide concept in the late 1940s, periodic tables were published with this structure.^[11] 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 – although dubnium is more chemically similar to protactinium, not tantalum.^[12]

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).^[10][13][14] Crookes dissolved uranium nitrate in ether, and the residual aqueous phase contains most of the ²³⁴
₉₀Th
and ²³⁴
₉₁Pa
. His method was still used in the 1950s to isolate ²³⁴
₉₀Th
and ²³⁴
₉₁Pa
from uranium compounds.^[15] 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: ²³⁸
₉₂U
→ ²³⁴
₉₀Th
→ ^234m
₉₁Pa
→ ²³⁴
₉₂U
. 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 ^234m
₉₁Pa
(uranium X2).^{[16][17][18][19][20][21]} 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 of protactinium, ²³¹Pa, having a much longer half-life of about 33,000 years.^[6][20][22] Thus the name "brevium" was changed to protactinium^[6] as the new element was part of the decay chain of uranium-235 as the parent of actinium (from Greek: πρῶτος prôtos "first, before"). The IUPAC confirmed this naming in 1949.^[23][24] 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.^[25]

Aristid von Grosse produced 2 milligrams of Pa₂O₅ in 1927,^[26] and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa₂O₅.^[27] 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:^[23][28]

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.^[23][29] For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies.^[10] Oak Ridge National Laboratory in the US provided protactinium at a cost of about US$280/gram.^[30]

Isotopes

Main article: Isotopes of protactinium

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).^[31]

The primary decay mode for the most stable isotope ²³¹Pa and lighter (²¹¹Pa to ²³¹Pa) is alpha decay, producing isotopes of actinium. The primary mode for the heavier isotopes (²³²Pa to ²³⁹Pa) is beta decay, producing isotopes of uranium.^[31]

Nuclear fission

The longest-lived and most abundant isotope, ²³¹Pa, can fission from fast neutrons exceeding ~1 MeV.^{[32] 233}Pa, the other isotope of protactinium produced in nuclear reactors, also has a fission threshold of 1 MeV.^[33]

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.^[34]

Protactinium occurs in uraninite (pitchblende) at concentrations of about 0.3-3 parts ²³¹Pa per million parts (ppm) of ore.^[10] Whereas the usual content is closer to 0.3 ppm^[35] (e.g. in Jáchymov, Czech Republic^[36]), some ores from the Democratic Republic of the Congo have about 3 ppm.^[23] 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.^[34][37]

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.^[38]

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.^{[39] 233}Pa 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.^[38]

Preparation

 

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 given are half-lives):

${\displaystyle {\ce {^{232}_{90}Th + ^{1}_{0}n -> ^{233}_{90}Th ->[\beta^-][22.3\ {\ce {min}}] ^{233}_{91}Pa ->[\beta^-][26.967\ {\ce {d}}] ^{233}_{92}U}}}$ 

The 231 isotope can be prepared by radiating 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,^[40] lithium or barium at a temperature of 1300–1400 °C.^[41][42]

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.^[43] Protactinium is a metal with silvery-gray luster that is preserved for some time in air.^[23][29] Protactinium easily reacts with oxygen, water vapor and acids, but not with alkalis.^[10]

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.^[40][44] The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9×10⁻⁶/°C.^[40]

Protactinium is paramagnetic and no magnetic transitions are known for it at any temperature.^[45] It becomes superconductive at temperatures below 1.4 K.^[10][41] Protactinium tetrachloride is paramagnetic at room temperature but turns ferromagnetic upon cooling to 182 K.^[46]

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.^[47] Other known protactinium ions include PaCl2+2, PaSO2+4, PaF³⁺, PaF2+2, PaF−6, PaF2−7 and PaF3−8.^[48][49]

Chemical compounds

Formula	color	symmetry	space group	No	Pearson symbol	a (pm)	b (pm)	c (pm)	Z	density (g/cm^3)
Pa	silvery-gray	tetragonal^[2]	I4/mmm	139	tI2	392.5	392.5	323.8	2	15.37
PaO		rocksalt^[42]	Fm3m	225	cF8	496.1			4	13.44
PaO2	black	fcc^[42]	Fm3m	225	cF12	550.5			4	10.47
Pa2O5	white		Fm3m^[42]	225	cF16	547.6	547.6	547.6	4	10.96
Pa2O5	white	orthorhombic^[42]				692	402	418
PaH3	black	cubic^[42]	Pm3n	223	cP32	664.8	664.8	664.8	8	10.58
PaF4	brown-red	monoclinic^[42]	C2/c	15	mS60				2
PaCl4	green-yellow	tetragonal^[50]	I41/amd	141	tI20	837.7	837.7	748.1	4	4.72
PaBr4	brown	tetragonal^[51]^[52]	I41/amd	141	tI20	882.4	882.4	795.7
PaCl5	yellow	monoclinic^[53]	C2/c	15	mS24	797	1135	836	4	3.74
PaBr5	red	monoclinic^[52]^[54]	P21/c	14	mP24	838.5	1120.5	1214.6	4	4.98
PaOBr3		monoclinic^[52]	C2			1691.1	387.1	933.4
Pa(PO3)4		orthorhombic^[55]				696.9	895.9	1500.9
Pa2P2O7		cubic^[55]	Pa3			865	865	865
Pa(C8H8)2	golden-yellow	monoclinic^[56]				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 white pentoxide Pa₂O₅, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500 °C.^[57] Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO_2.25. Another phase of this oxide with orthorhombic symmetry has also been reported.^[42][58] 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.^[42] The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100 °C.^[58] The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.^[42]

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.^[59][60] The pentoxide Pa₂O₅ combines with rare-earth metal oxides R₂O₃ to form various nonstoichiometric mixed-oxides, also of perovskite structure.^[61]

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.^[62] 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 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 C_2v symmetry. Heating PaP₂O₇ in air to 1400 °C decomposes it into the pentoxides of phosphorus and protactinium.^[55]

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.^[63]

 

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.^[64] 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.^[42]

Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachloride at temperature of 200–300 °C.^[42] The by-products (such as PaOCl₃) are removed by fractional sublimation.^[53] 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.^[42]

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.^[42] 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.^[52][54]

Protactinium iodides result from the oxides and aluminium iodide or ammonium iodide heated to 600 °C.^[42] Protactinium(III) iodide was obtained by heating protactinium(V) iodide in vacuum.^[64] 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.^[48]

More complex protactinium fluorides are also known such as Pa₂F₉^[64] 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).^[49]

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.^[52] 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.^[42]

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.^[42] 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.^[65]

Organometallic compounds

 

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.^[66] 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.^[56]

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.^[34]

Protactinium-231 arises from the decay of natural uranium-235, and in nuclear reactors 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 [de] once estimated the associated critical mass as 750±180 kg.^[67] However, the possibility of criticality of ²³¹Pa has been ruled out since then.^[68]

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.^[35] In particular, its evaluation in oceanic sediments helped to reconstruct the movements of North Atlantic water bodies during the last melting of Ice Age glaciers.^[69] 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) would 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.^[35][70]

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.^[34]

Isotope	^231Pa	^227Ac	^227Th	^223Ra	^219Rn	^215Po	^211Pb	^211Bi	^207Tl
SA (Ci/g)	0.048	73	3.1×10^4	5.2×10^4	1.3×10^10	3×10^13	2.5×10^7	4.2×10^8	1.9×10^8
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. 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 all these organs, protactinium promotes cancer via its radioactivity.^[34][62] 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.^[71] The maximum allowed concentrations of ²³¹Pa in the air in Germany is 3×10⁻⁴ Bq/m³.^[62]

See also

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

References

1. "Standard Atomic Weights: Protactinium". CIAAW. 2017.
2. Donohue, J. (1959). "On the crystal structure of protactinium metal". Acta Crystallographica. 12 (9): 697. doi:10.1107/S0365110X59002031.
3. Cverna, Fran, ed. (2002). "Chapter 2. Thermal Expansion". ASM Ready Reference: Thermal Properties of Metals (PDF). ASM International. p. 11. ISBN 0871707683.
4. Lide, D. R., ed. (2005). "Magnetic susceptibility of the elements and inorganic compounds". (PDF) (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
5. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
6. Meitner, L. (1918) Die Muttersubstanz des Actiniums, ein Neues Radioaktives Element von Langer Lebensdauer. Zeitschrift für Elektrochemie und angewandte physikalische Chemie 24: 169-173.
7. http://hpschapters.org/northcarolina/NSDS/Protactinium.pdf^{[bare URL PDF]}
8. John Arnold Cranston. University of Glasgow
9. Negre, César et al. “Reversed flow of Atlantic deep water during the Last Glacial Maximum.” Nature, vol. 468,7320 (2010): 84-8. doi:10.1038/nature09508
10. Emsley, John (2003) [2001]. "Protactinium". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 347–349. ISBN 978-0-19-850340-8.
11. Laing, Michael (2005). "A Revised Periodic Table: With the Lanthanides Repositioned". Foundations of Chemistry. 7 (3): 203. doi:10.1007/s10698-004-5959-9. S2CID 97792365.
12. Fessl, Sophie (2 January 2019). "How Far Does the Periodic Table Go?". JSTOR. Retrieved 9 January 2019.
13. National Research Council (U.S.). Conference on Glossary of Terms in Nuclear Science and Technology (1957). A Glossary of Terms in Nuclear Science and Technology. American Society of Mechanical Engineers. p. 180. Retrieved 25 July 2015.
14. Crookes, W. (1899). "Radio-Activity of Uranium". Proceedings of the Royal Society of London. 66 (424–433): 409–423. doi:10.1098/rspl.1899.0120. S2CID 93563820.
15. Johansson, Sven (1954). "Decay of UX1, UX2, and UZ". Physical Review. 96 (4): 1075–1080. Bibcode:1954PhRv...96.1075J. doi:10.1103/PhysRev.96.1075.
16. Greenwood, p. 1250
17. Greenwood, p. 1254
18. Fajans, K. & Gohring, O. (1913). "Über die komplexe Natur des Ur X". Naturwissenschaften. 1 (14): 339. Bibcode:1913NW......1..339F. doi:10.1007/BF01495360. S2CID 40667401.
19. Fajans, K. & Gohring, O. (1913). "Über das Uran X₂-das neue Element der Uranreihe". Physikalische Zeitschrift. 14: 877–84.
20. Eric Scerri, A tale of seven elements, (Oxford University Press 2013) ISBN 978-0-19-539131-2, p.67–74
21. {{cite journal }url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/62921/244137a0.pdf |title=Discovery and Naming of the Isotopes of Element 91 |last1=Fajans |first1=K. |last2=Morris |first2=D. F. C. |date=1973 |journal=Nature |volume=244 |pp=137–138 |doi=10.1038/244137a0}}
22. Soddy, F., Cranston, J.F. (1918) The parent of actinium. Proceedings of the Royal Society A – Mathematical, Physical and Engineering Sciences 94: 384-403.
23. Hammond, C. R. (29 June 2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
24. Greenwood, p. 1251
25. Shea, William R. (1983) Otto Hahn and the rise of nuclear physics, Springer, p. 213, ISBN 90-277-1584-X.
26. von Grosse, Aristid (1928). "Das Element 91; seine Eigenschaften und seine Gewinnung". Berichte der deutschen chemischen Gesellschaft. 61 (1): 233–245. doi:10.1002/cber.19280610137.
27. Graue, G.; Käding, H. (1934). "Die technische Gewinnung des Protactiniums". Angewandte Chemie. 47 (37): 650–653. Bibcode:1934AngCh..47..650G. doi:10.1002/ange.19340473706.
28. Grosse, A. V. (1934). "Metallic Element 91". Journal of the American Chemical Society. 56 (10): 2200–2201. doi:10.1021/ja01325a508.
29. Myasoedov, B. F.; Kirby, H. W.; Tananaev, I. G. (2006). "Chapter 4: Protactinium". In Morss, L. R.; Edelstein, N. M.; Fuger, J. (eds.). The Chemistry of the Actinide and Transactinide Elements. Bibcode:2011tcot.book.....M. doi:10.1007/978-94-007-0211-0. ISBN 978-1-4020-3555-5. S2CID 93796247. {{cite book}}: |journal= ignored (help)
30. . Los Alamos National Laboratory. Archived from the original on 28 September 2011. Retrieved 21 March 2013.
31. Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
32. Grosse, A. v.; Booth, E. T.; Dunning, J. R. (15 August 1939). "The Fission of Protactinium (Element 91)". Physical Review. 56 (4): 382. Bibcode:1939PhRv...56..382G. doi:10.1103/PhysRev.56.382. Retrieved 17 February 2023.
33. Tovesson, F.; Hambsch, F.-J; Oberstedt, A.; Fogelberg, B.; Ramström, E.; Oberstedt, S. (August 2002). "The Pa-233 Fission Cross Section". Journal of Nuclear Science and Technology. 39 (sup2): 210–213. Bibcode:2002JNST...39Q.210T. doi:10.1080/00223131.2002.10875076. S2CID 91866777. Retrieved 17 February 2023.
34. Protactinium 7 March 2008 at the Wayback Machine, Argonne National Laboratory, Human Health Fact Sheet, August 2005
35. Articles "Protactinium" and "Protactinium-231 – thorium-230 dating" in Encyclopædia Britannica, 15th edition, 1995, p. 737
36. Grosse, A. V.; Agruss, M. S. (1934). "The Isolation of 0.1 Gram of the Oxide of Element 91 (Protactinium)". Journal of the American Chemical Society. 56 (10): 2200. doi:10.1021/ja01325a507.
37. 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, ISBN 0-470-85598-3.
38. Groult, Henri (2005) Fluorinated materials for energy conversion, Elsevier, pp. 562–565, ISBN 0-08-044472-5.
39. Hébert, Alain (July 2009). Applied Reactor Physics. Presses inter Polytechnique. p. 265. ISBN 978-2-553-01436-9.
40. Marples, J. A. C. (1965). "On the thermal expansion of protactinium metal". Acta Crystallographica. 18 (4): 815–817. doi:10.1107/S0365110X65001871.
41. Fowler, R. D.; Matthias, B.; Asprey, L.; Hill, H.; et al. (1965). "Superconductivity of Protactinium". Physical Review Letters. 15 (22): 860. Bibcode:1965PhRvL..15..860F. doi:10.1103/PhysRevLett.15.860.
42. Sellers, Philip A.; Fried, Sherman; Elson, Robert E.; Zachariasen, W. H. (1954). "The Preparation of Some Protactinium Compounds and the Metal". Journal of the American Chemical Society. 76 (23): 5935. doi:10.1021/ja01652a011.
43. Seitz, Frederick and Turnbull, David (1964) Solid state physics: advances in research and applications, Academic Press, pp. 289–291, ISBN 0-12-607716-9.
44. Young, David A. (1991) Phase diagrams of the elements, University of California Press, p. 222, ISBN 0-520-07483-1.
45. Buschow, K. H. J. (2005) Concise encyclopedia of magnetic and superconducting materials, Elsevier, pp. 129–130, ISBN 0-08-044586-1.
46. Hendricks, M. E. (1971). "Magnetic Properties of Protactinium Tetrachloride". Journal of Chemical Physics. 55 (6): 2993–2997. Bibcode:1971JChPh..55.2993H. doi:10.1063/1.1676528.
47. Greenwood, p. 1265
48. Greenwood, p. 1275
49. Asprey, L. B.; Kruse, F. H.; Rosenzweig, A.; Penneman, R. A. (1966). "Synthesis and X-Ray Properties of Alkali Fluoride-Protactinium Pentafluoride Complexes". Inorganic Chemistry. 5 (4): 659. doi:10.1021/ic50038a034.
50. Brown D.; Hall T.L.; Moseley P.T (1973). "Structural parameters and unit cell dimensions for the tetragonal actinide tetrachlorides(Th, Pa, U, and Np) and tetrabromides (Th and Pa)". Journal of the Chemical Society, Dalton Transactions (6): 686–691. doi:10.1039/DT9730000686.
51. Tahri, Y.; Chermette, H.; El Khatib, N.; Krupa, J.; et al. (1990). "Electronic structures of thorium and protactinium halide clusters of [ThX8]4− type". Journal of the Less Common Metals. 158: 105–116. doi:10.1016/0022-5088(90)90436-N.
52. Brown, D.; Petcher, T. J.; Smith, A. J. (1968). "Crystal Structures of some Protactinium Bromides". Nature. 217 (5130): 737. Bibcode:1968Natur.217..737B. doi:10.1038/217737a0. S2CID 4264482.
53. Dodge, R. P.; Smith, G. S.; Johnson, Q.; Elson, R. E. (1967). "The crystal structure of protactinium pentachloride". Acta Crystallographica. 22: 85–89. doi:10.1107/S0365110X67000155.
54. Brown, D.; Petcher, T. J.; Smith, A. J. (1969). "The crystal structure of β-protactinium pentabromide". Acta Crystallographica B. 25 (2): 178. doi:10.1107/S0567740869007357.
55. Brandel, V.; Dacheux, N. (2004). "Chemistry of tetravalent actinide phosphates—Part I". Journal of Solid State Chemistry. 177 (12): 4743. Bibcode:2004JSSCh.177.4743B. doi:10.1016/j.jssc.2004.08.009.
56. Starks, David F.; Parsons, Thomas C.; Streitwieser, Andrew; Edelstein, Norman (1974). "Bis(π-cyclooctatetraene) protactinium". Inorganic Chemistry. 13 (6): 1307. doi:10.1021/ic50136a011.
57. Greenwood, p. 1268
58. Elson, R.; Fried, Sherman; Sellers, Philip; Zachariasen, W. H. (1950). "The tetravalent and pentavalent states of protactinium". Journal of the American Chemical Society. 72 (12): 5791. doi:10.1021/ja01168a547.
59. Greenwood, p. 1269
60. Iyer, P. N.; Smith, A. J. (1971). "Double oxides containing niobium, tantalum or protactinium. IV. Further systems involving alkali metals". Acta Crystallographica B. 27 (4): 731. doi:10.1107/S056774087100284X.
61. Iyer, P. N.; Smith, A. J. (1967). "Double oxides containing niobium, tantalum, or protactinium. III. Systems involving the rare earths". Acta Crystallographica. 23 (5): 740. doi:10.1107/S0365110X67003639.
62. Grossmann, R.; Maier, H.; Szerypo, J.; Friebel, H. (2008). "Preparation of 231Pa targets". Nuclear Instruments and Methods in Physics Research A. 590 (1–3): 122. Bibcode:2008NIMPA.590..122G. doi:10.1016/j.nima.2008.02.084.
63. Greenwood, p. 1270
64. Greenwood, p. 1271
65. Greenwood, p. 1277
66. Greenwood, pp. 1278–1279
67. Seifritz, Walter (1984) Nukleare Sprengkörper – Bedrohung oder Energieversorgung für die Menschheit, Thiemig-Verlag, ISBN 3-521-06143-4.
68. Ganesan, S. (1999). (PDF). Current Science. 77 (5): 667–677. Archived from the original (PDF) on 3 March 2016. Retrieved 21 March 2013.
69. McManus, J. F.; Francois, R.; Gherardi, J.-M.; Keigwin, L. D.; et al. (2004). (PDF). Nature. 428 (6985): 834–837. Bibcode:2004Natur.428..834M. doi:10.1038/nature02494. PMID 15103371. S2CID 205210064. Archived from the original (PDF) on 10 April 2013. Retrieved 29 November 2010.
70. Cheng, H.; Edwards, R.Lawrence; Murrell, M. T.; Benjamin, T. M. (1998). "Uranium-thorium-protactinium dating systematics". Geochimica et Cosmochimica Acta. 62 (21–22): 3437. Bibcode:1998GeCoA..62.3437C. doi:10.1016/S0016-7037(98)00255-5.
71. Palshin, E.S.; et al. (1968). Analytical chemistry of protactinium. Moscow: Nauka.

Bibliography

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth–Heinemann. ISBN 978-0080379418.

External links

 

Wikimedia Commons has media related to Protactinium.

 

Look up protactinium in Wiktionary, the free dictionary.

• Protactinium at The Periodic Table of Videos (University of Nottingham)