Protactinium is a chemical element; it has symbol Pa and atomic number 91. It is a dense, radioactive, 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.

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, protactinium-234m. A more stable isotope of protactinium, Pa, was discovered in 1917/18 by Lise Meitner in collaboration with Otto Hahn, and they named the element protactinium. In 1949, the IUPAC chose the name "protactinium" and confirmed Hahn and Meitner as its discoverers. The new name meant "(nuclear) precursor of actinium," suggesting that actinium is a product of radioactive decay of protactinium. John Arnold Cranston (working with Frederick Soddy and Ada Hitchins) is also credited with discovering the most stable isotope in 1915, but he delayed his announcement due to being called for service in the First World War.

The longest-lived and most abundant (nearly 100%) naturally occurring isotope of protactinium, protactinium-231, has a half-life of 32,760 years and is a decay product of uranium-235. Much smaller trace amounts of the short-lived protactinium-234 and its nuclear isomer protactinium-234m occur in the decay chain of uranium-238. Protactinium-233 occurs as a result of the decay of thorium-233 as part of the chain of events necessary to produce uranium-233 by neutron irradiation of thorium-232. It is an undesired intermediate product in thorium-based nuclear reactors, and is therefore removed from the active zone of the reactor during the breeding process. Ocean science utilizes the element to understand the ancient ocean's geography. Analysis of the relative concentrations of various uranium, thorium, and protactinium isotopes in water and minerals is used in radiometric dating of sediments up to 175,000 years old, and in modeling of various geological processes.

In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium. The actinide series was unknown at the time, so Mendeleev positioned uranium below tungsten in group VI, and thorium below zirconium in group IV, leaving the space below tantalum in group V empty. Until the general acceptance of the actinide concept in the late 1940s, periodic tables were published with this structure. 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.

In 1900, William Crookes isolated protactinium as an intensely radioactive material from uranium; however, he could not characterize it as a new chemical element and thus named it uranium X (UX). Crookes dissolved uranium nitrate in ether, and the residual aqueous phase contained most of the
₉₀Th
and
₉₁Pa
. His method was used into the 1950s to isolate
₉₀Th
and
₉₁Pa
from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope Pa during their studies of the decay chains of uranium-238:
₉₂U
→
₉₀Th
→
₉₁Pa
→
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
₉₁Pa
(uranium X2). 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, Pa, having a much longer half-life of 32,760 years. Meitner changed the name "brevium" to protactinium as the new element was part of the decay chain of uranium-235 as the parent of actinium (from the Greek: πρῶτος prôtos, meaning "first, before"). The IUPAC confirmed this naming in 1949. The discovery of protactinium completed one of the last gaps in early versions of the periodic table, and brought fame to the involved scientists.

Aristid von Grosse produced 2 milligrams of Pa₂O₅ in 1927, and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa₂O₅. He used two different procedures: in the first, protactinium oxide was irradiated by 35 keV electrons in vacuum. In the other, called the van Arkel–de Boer process, the oxide was chemically converted to a halide (chloride, bromide or iodide) and then reduced in a vacuum with an electrically heated metallic filament:

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

Twenty-nine radioisotopes of protactinium have been discovered. The most stable are Pa with a half-life of 32,760 years, Pa with a half-life of 27 days, and Pa with a half-life of 17.4 days. All other 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, Pa (half-life 1.2 milliseconds) and Pa (half-life 1.16 minutes).

The primary decay mode for the most stable isotope Pa and lighter (Pa to Pa) is alpha decay, producing isotopes of actinium. The primary mode for the heavier isotopes (Pa to Pa) is beta decay, producing isotopes of uranium.

The longest-lived and most abundant isotope, Pa, can fission from fast neutrons exceeding ~1 MeV. Pa, the other isotope of protactinium produced in nuclear reactors, also has a fission threshold of 1 MeV.

Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes – Pa and Pa, with the isotope Pa 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 Pa isomer.

Protactinium occurs in uraninite (pitchblende) at concentrations of about 0.3-3 parts Pa per million parts (ppm) of ore. Whereas the usual content is closer to 0.3 ppm (e.g. in Jáchymov, Czech Republic), some ores from the Democratic Republic of the Congo have about 3 ppm. Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, corresponding to a radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even when compared to water present in the same sample of soil. Much higher ratios of 2,000 and above are measured in loam soils and clays, such as bentonite.

Two major protactinium isotopes, Pa and Pa, 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, Th, via (n, 2n) reactions, produces Th, which quickly decays to Pa (half-life 25.5 hours). 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.

Protactinium-233 is formed upon neutron capture by Th. It either further decays to uranium-233, or captures another neutron and converts into the non-fissile uranium-234. 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 U, a significant fraction of Pa converts to non-fissile isotopes and consumes neutrons, degrading reactor efficiency. To limit the loss of neutrons, Pa is extracted from the active zone of thorium molten salt reactors during their operation, so that it can only decay into U. Extraction of Pa is achieved using columns of molten bismuth with lithium dissolved in it. In short, lithium selectively reduces protactinium salts to protactinium metal, which is then extracted from the molten-salt cycle, while the molten bismuth is merely a carrier, selected due to its low melting point of 271 °C, low vapor pressure, good solubility for lithium and actinides, and immiscibility with molten halides.

Before the advent of nuclear reactors, protactinium was separated for scientific experiments from uranium ores. Since reactors have become more common, it is mostly produced as an intermediate product of nuclear fission in thorium fuel cycle reactors as an intermediate in the production of the fissile uranium-233:

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

The isotope Pa can be prepared by irradiating thorium-230 with slow neutrons, converting it to the beta-decaying thorium-231; or, by irradiating thorium-232 with fast neutrons, generating thorium-231 and 2 neutrons.

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

Protactinium is an actinide positioned in the periodic table to the left of uranium and to the right of thorium, and many of its physical properties are intermediate between its neighboring actinides. Protactinium is denser and more rigid than thorium, but is lighter than uranium; its melting point is lower than that of thorium, but higher than that of uranium. The thermal expansion, electrical, and thermal conductivities of these three elements are comparable and are typical of post-transition metals. The estimated shear modulus of protactinium is similar to that of titanium. Protactinium is a metal with silvery-gray luster that is preserved for some time in air. Protactinium easily reacts with oxygen, water vapor, and acids, but not with alkalis.

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. The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9×10/°C.

Protactinium is paramagnetic and no magnetic transitions are known for it at any temperature. It becomes superconductive at temperatures below 1.4 K. Protactinium tetrachloride is paramagnetic at room temperature, but becomes ferromagnetic when cooled to 182 K.

Protactinium exists in two major oxidation states: +4 and +5, both in solids and solutions; and the +3 and +2 states, which have been observed in some solids. As the electron configuration of the neutral atom is [Rn]5f6d7s, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f configuration. Both +4 and +5 states easily form hydroxides in water, with the predominant ions being Pa(OH), Pa(OH)2+2, Pa(OH)+3, and Pa(OH)₄, all of which are colorless. Other known protactinium ions include PaCl2+2, PaSO2+4, PaF, PaF2+2, PaF−6, PaF2−7, and PaF3−8.

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

Here, a, b, and c are lattice constants in picometers, No is the space group number, and Z is the number of formula units per unit cell; fcc stands for the face-centered cubic symmetry. Density was not measured directly but calculated from the lattice parameters.

Protactinium oxides are known for the metal oxidation states +2, +4, and +5. The most stable is the white pentoxide Pa₂O₅, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500 °C. 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. 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 acid, but easily dissolves in hydrofluoric acid. The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100 °C. The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.

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

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. The polytrioxophosphate Pa(PO₃)₄ can be produced by reacting the difluoride sulfate PaF₂SO₄ with phosphoric acid (H₃PO₄) under an inert atmosphere. Heating the product to about 900 °C eliminates the reaction by-products, which include hydrofluoric acid, sulfur trioxide, and phosphoric anhydride. Heating it to higher temperatures in an inert atmosphere decomposes Pa(PO₃)₄ into the diphosphate PaP₂O₇, which is analogous to diphosphates of other actinides. In the diphosphate, the PO₃ groups form pyramids of C_2v symmetry. Heating PaP₂O₇ in air to 1400 °C decomposes it into the pentoxides of phosphorus and protactinium.

Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and coordinated by 7 other ions. The coordination is the same in protactinium(V) chloride, but the color is yellow. The coordination changes to octahedral in the brown protactinium(V) bromide, but is unknown for protactinium(V) iodide. The protactinium coordination in all its tetrahalides is 8, but the arrangement is square antiprismatic in protactinium(IV) fluoride and dodecahedral in the chloride and bromide. Brown-colored protactinium(III) iodide has been reported, where protactinium ions are 8-coordinated in a bicapped trigonal prismatic arrangement.

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

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

Protactinium bromides are produced by the action of aluminium bromide, hydrogen bromide, carbon tetrabromide, or a mixture of hydrogen bromide and thionyl bromide on protactinium oxide. They can alternatively be produced by reacting protactinium pentachloride with hydrogen bromide or thionyl bromide. Protactinium(V) bromide has two similar monoclinic forms: one is obtained by sublimation at 400–410 °C, and another by sublimation at a slightly lower temperature of 390–400 °C.

Protactinium iodides can be produced by reacting protactinium metal with elemental iodine at 600 °C, and by reacting Pa₂O₅ with AlO₃ at 600 °C. Protactinium(III) iodide can be obtained by heating protactinium(V) iodide in vacuum. As with oxides, protactinium forms mixed halides with alkali metals. The most remarkable among these is Na₃PaF₈, where the protactinium ion is symmetrically surrounded by 8 F ions, forming a nearly perfect cube.

More complex protactinium fluorides are also known, such as Pa₂F₉ and ternary fluorides of the types MPaF₆ (M = Li, Na, K, Rb, Cs or NH₄), M₂PaF₇ (M = K, Rb, Cs or NH₄), and M₃PaF₈ (M = Li, Na, Rb, Cs), all of which are white crystalline solids. The MPaF₆ formula can be represented as a combination of MF and PaF₅. These compounds can be obtained by evaporating a hydrofluoric acid solution containing both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it becomes orthorhombic for larger cations K, Rb, Cs or NH₄. A similar variation was observed for the M₂PaF₇ fluorides: namely, the crystal symmetry was dependent on the cation and differed for Cs₂PaF₇ and M₂PaF₇ (M = K, Rb or NH₄).

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. 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.

In hydrides and nitrides, protactinium has a low oxidation state of about +3. The hydride is obtained by direct action of hydrogen on the metal at 250 °C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is thermally stable to 800 °C in vacuum. Protactinium carbide (PaC) is formed by the reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400 °C. Protactinium forms 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.

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. Another organometallic complex is the golden-yellow bis(π-cyclooctatetraene) protactinium, or protactinocene (Pa(C₈H₈)₂), which is analogous in structure to uranocene. There, the metal atom is sandwiched between two cyclooctatetraene ligands. Similar to uranocene, it can be prepared by reacting protactinium tetrachloride with dipotassium cyclooctatetraenide (K₂C₈H₈) in tetrahydrofuran.

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

Protactinium-231 arises naturally from the decay of natural uranium-235, and artificially in nuclear reactors by the reaction Th + n → Th + 2n and the subsequent beta decay of Th. 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. However, the possibility of criticality of Pa has since been ruled out.

With the advent of highly sensitive mass spectrometers, an application of Pa as a tracer in geology and paleoceanography has become possible. In this application, 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. 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. Some of the protactinium-related dating variations rely on analysis of the relative concentrations of several long-living members of the uranium decay chain – uranium, protactinium, and thorium, for example. These elements have 6, 5, and 4 valence electrons, thus favoring +6, +5, and +4 oxidation states respectively, and display different physical and chemical properties. Thorium and protactinium, but not uranium compounds, are poorly soluble in aqueous solutions and precipitate into sediments; the precipitation rate is faster for thorium than for protactinium. The concentration analysis for both protactinium-231 (half-life 32,760 years) and thorium-230 (half-life 75,380 years) improves measurement accuracy compared to when only one isotope is measured; this double-isotope method is also weakly sensitive to inhomogeneities in the spatial distribution of the isotopes and to variations in their precipitation rate.

Protactinium is both toxic and highly radioactive; thus, it is handled exclusively in a sealed glove box. Its major isotope Pa 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 Ac, 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. Ac, in turn, decays into lighter isotopes with even shorter half-lives and much greater specific activities (SA).

Isotope	Pa	Ac	Th	Ra	Rn	Po	Pb	Bi	Tl
SA (Ci/g)	0.048	73	3.1×10	5.2×10	1.3×10	3×10	2.5×10	4.2×10	1.9×10
Decay	α	α, β	α	α	α	α	β	α, β	β
Half-life	33 ka	22 a	19 days	11 days	4 s	1.8 ms	36 min	2.1 min	4.8 min

As protactinium is present in small amounts in most natural products and materials, it is ingested with food or water and inhaled with air. Only about 0.05% of ingested protactinium is absorbed into the blood and the remainder is excreted. From the blood, about 40% of the protactinium deposits in the bones, about 15% goes to the liver, 2% to the kidneys, and the rest leaves the body. The biological half-life of protactinium is about 50 years in the bones, whereas its biological half-life in other organs has a fast and slow component. For example, 70% of the protactinium in the liver has a biological half-life of 10 days, and the remaining 30% for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In each affected organ, protactinium promotes cancer via its radioactivity. The maximum safe dose of Pa in the human body is 0.03 μCi (1.1 kBq), which corresponds to 0.5 micrograms of Pa. The maximum allowed concentrations of Pa in the air in Germany is 3×10 Bq/m.

• Ada Hitchins, who helped Soddy in discovering the element protactinium

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

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