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{{Use dmy dates|date=February 2021}}
{{Infobox protactinium}}
'''Protactinium''' is a [[chemical element]]; it has [[chemical symbol|symbol]] '''Pa''' and [[atomic number]] 91. It is a dense, [[radioactive]], silvery-gray [[actinide]] metal which readily reacts with [[oxygen]], water vapor, and inorganic [[acid]]s. It forms various [[chemical compound]]s, in which protactinium is usually present in the [[oxidation state]] +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some [[uraninite]] ore deposits. Because of its scarcity, high radioactivity, and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from [[spent nuclear fuel]].

The element was first identified in 1913 by [[Kazimierz Fajans]] and [[Oswald Helmuth Göhring]] and named "brevium" because of the short [[half-life]] of the specific [[isotope]] studied, [[Nuclear isomer|<sup>234m</sup>Pa]]. A more stable isotope of protactinium, <sup>231</sup>Pa, was discovered in 1917/18 by [[Lise Meitner]] in collaboration with [[Otto Hahn]], and they named the element protactinium.<ref name="meitner">{{cite journal | last=Meitner | first=Lise | title=Die Muttersubstanz des Actiniums, Ein Neues Radioaktives Element von Langer Lebensdauer | journal=Zeitschrift für Elektrochemie und angewandte physikalische Chemie | volume=24 | issue=11–12 | date=1918 | issn=0372-8323 | doi=10.1002/bbpc.19180241107 | pages=169–173}}</ref> In 1949, the [[IUPAC]] chose the name "protactinium" and confirmed Hahn and Meitner as its discoverers. The new name meant "(nuclear) [[precursor (chemistry)|precursor]] of [[actinium]],"<ref>{{cite web |title=Protactinium |url=http://hpschapters.org/northcarolina/NSDS/Protactinium.pdf |website=Human Health Fact Sheet |publisher=ANL (Argonne National Laboratory) |access-date=4 September 2023 |date=November 2001 |quote=The name comes from the Greek work protos (meaning first) and the element actinium, because protactinium is the precursor of actinium.}}</ref> 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]].<ref>[http://www.universitystory.gla.ac.uk/biography/?id=WH3023&type=P John Arnold Cranston] {{Webarchive|url=https://web.archive.org/web/20200311015550/http://www.universitystory.gla.ac.uk/biography/?id=WH3023&type=P |date=11 March 2020 }}. [[University of Glasgow]]</ref>

The longest-lived and most abundant (nearly 100%) naturally occurring [[isotope]] of protactinium, <sup>231</sup>Pa, has a [[half-life]] of 32,760 years and is a decay product of [[uranium-235]]. Much smaller trace amounts of the short-lived <sup>234</sup>Pa and its [[nuclear isomer]] <sup>234m</sup>Pa occur in the decay chain of [[uranium-238]]. <sup>233</sup>Pa 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 <sup>232</sup>Th. It is an undesired intermediate product in [[Thorium-based nuclear power | thorium-based]] [[nuclear reactor]]s, and is therefore removed from the active zone of the reactor during the breeding process. Ocean science uses the element to understand the ancient ocean's geography: analysis of the relative concentrations of various uranium, thorium, and protactinium isotopes in water and minerals is used in [[radiometric dating]] of [[sediment]]s up to 175,000 years old, and in modeling of various geological processes.<ref>{{cite journal | last1 = Negre | first1 = César | display-authors = etal | year = 2010| title = Reversed flow of Atlantic deep water during the Last Glacial Maximum | url = https://www.pure.ed.ac.uk/ws/files/11751410/ReversedATlantic_Deep.pdf| journal = Nature | volume = 468 | issue = 7320| pages = 84–8 | doi = 10.1038/nature09508 | pmid = 21048764 | bibcode = 2010Natur.468...84N }}</ref>

==History==
[[File:Periodensystem_Mendelejews.jpg|thumb|left|upright=1.5|[[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]] [[Mendeleev's predicted elements|predicted]] the existence of an element between [[thorium]] and [[uranium]].<ref name="Emsley" /> The actinide series was unknown at the time, so Mendeleev positioned [[uranium]] below [[tungsten]] in [[Group (periodic table) | 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 table]]s were published with this structure.<ref>{{cite journal|doi = 10.1007/s10698-004-5959-9|title=A Revised Periodic Table: With the Lanthanides Repositioned|author=Laing, Michael |journal=[[Foundations of Chemistry]]|volume=7|issue=3|date=2005|page=203|s2cid=97792365}}</ref> For a long time, chemists searched for [[Mendeleev's predicted elements|eka]]-tantalum<ref group=note>The prefix "eka" is derived from the [[Sanskrit]] [[wikt:एक#Sanskrit | एक]], meaning "one" or "first." In chemistry, it was formerly used to denote an element one period below the element name following it.</ref> 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.<ref name="Fessl">{{cite web |last1=Fessl |first1=Sophie |title=How Far Does the Periodic Table Go? |url=https://daily.jstor.org/how-far-does-the-periodic-table-go/ |publisher=JSTOR |access-date=9 January 2019|date=2019-01-02 }}</ref>

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).<ref name="Emsley">{{cite book|title = Nature's Building Blocks: An A-Z Guide to the Elements|last = Emsley|first = John|publisher = Oxford University Press|orig-year = 2001|location = Oxford, England, UK|isbn = 978-0-19-850340-8|chapter = Protactinium|pages = [https://archive.org/details/naturesbuildingb0000emsl/page/347 347–349]|chapter-url = https://books.google.com/books?id=j-Xu07p3cKwC&pg=PA348|date = 2003|url = https://archive.org/details/naturesbuildingb0000emsl/page/347}}</ref><ref name="google">{{cite book|title=A Glossary of Terms in Nuclear Science and Technology|author=National Research Council (U.S.). Conference on Glossary of Terms in Nuclear Science and Technology|date=1957|publisher=American Society of Mechanical Engineers|url=https://books.google.com/books?id=-zgrAAAAYAAJ&pg=PA180|page=180|access-date=25 July 2015}}</ref><ref>{{cite journal|doi = 10.1098/rspl.1899.0120|last1 = Crookes|first1 = W.|title = Radio-Activity of Uranium|url = https://books.google.com/books?id=hmZDAAAAYAAJ&pg=PA409-IA6|journal = [[Proceedings of the Royal Society of London]]|volume = 66|issue = 424–433|pages = 409–423|date = 1899|s2cid = 93563820|url-access = subscription}}</ref><!--https://www.jstor.org/pss/96048 --> Crookes dissolved [[uranium nitrate]] in [[diethyl ether|ether]], and the residual aqueous phase contained most of the {{nuclide|Th|234}} and {{nuclide|Pa|234}}. His method was used into the 1950s to isolate {{nuclide|Th|234}} and {{nuclide|Pa|234}} from uranium compounds.<ref>{{cite journal|last1 = Johansson|first1 = Sven|title = Decay of UX1, UX2, and UZ|journal = [[Physical Review]]|volume = 96|pages = 1075–1080|date = 1954|doi = 10.1103/PhysRev.96.1075|issue = 4|bibcode = 1954PhRv...96.1075J }}</ref> Protactinium was first identified in 1913, when [[Kasimir Fajans]] and [[Oswald Helmuth Göhring]] encountered the isotope <sup>234m</sup>Pa during their studies of the decay chains of [[uranium-238]]: {{nuclide|U|238}} → {{nuclide|Th|234}} → {{nuclide|Pa|234m}} → {{nuclide|U|234}}. 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 {{nuclide|Pa|234m}} (uranium&nbsp;X2).<ref name="g1250">[[#Greenwood|Greenwood]], p. 1250</ref><ref name="g1254">[[#Greenwood|Greenwood]], p. 1254</ref><ref>{{cite journal|author = Fajans, K.|author2 = Gohring, O.|name-list-style = amp|title = Über die komplexe Natur des Ur X|journal = [[Naturwissenschaften]]|date = 1913|volume =1|pages = 339|url =http://www.digizeitschriften.de/no_cache/home/jkdigitools/loader/?tx_jkDigiTools_pi1%5BIDDOC%5D=201162&tx_jkDigiTools_pi1%5Bpp%5D=425 |doi = 10.1007/BF01495360|issue = 14|bibcode = 1913NW......1..339F |s2cid = 40667401}}</ref><ref>{{cite journal|author = Fajans, K.|author2 = Gohring, O.|name-list-style = amp |title = Über das Uran X<sub>2</sub>-das neue Element der Uranreihe|journal = [[Physikalische Zeitschrift]]|date = 1913|volume = 14|pages = 877–84}}</ref><ref name="Scerri">[[Eric Scerri]], ''A tale of seven elements,'' (Oxford University Press 2013) {{ISBN|978-0-19-539131-2}}, p.67–74</ref><ref name=PaIsoDisc>{{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 |issue=5412 |pages=137–138 |doi=10.1038/244137a0|bibcode=1973Natur.244..137F |hdl=2027.42/62921 }}</ref> 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, <sup>231</sup>Pa, having a much longer half-life of 32,760 years.<ref name="meitner" /><ref name="Scerri" /><ref>[https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1918.0025 Soddy, F., Cranston, J.F. (1918) The parent of actinium]. Proceedings of the Royal Society A – Mathematical, Physical and Engineering Sciences 94: 384-403.</ref> 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 {{langx|el|πρῶτος}} ''prôtos'', meaning "first, before").<ref>{{cite web |title=Protactinium - Element information : Chemistry in its element: protactinium |url=https://www.rsc.org/periodic-table/element/91/protactinium |website=www.rsc.org |publisher=[[Royal Society of Chemistry]] |access-date=20 October 2023 |quote=At this point Fajans withdrew the name brevium since the custom was to name an element according to longest-lived isotope. Meitner than chose the name protactinium.}}</ref> The [[International Union of Pure and Applied Chemistry|IUPAC]] confirmed this naming in 1949.<ref name="CRC" /><ref name="g1251">[[#Greenwood|Greenwood]], p. 1251</ref> The discovery of protactinium completed one of the last gaps in early versions of the periodic table, and brought fame to the involved scientists.<ref>Shea, William R. (1983) [https://books.google.com/books?id=W7xyvXc-hgEC&pg=PA213 Otto Hahn and the rise of nuclear physics], Springer, p. 213, {{ISBN|90-277-1584-X}}.</ref>

[[Aristid von Grosse]] produced 2&nbsp;milligrams of Pa<sub>2</sub>O<sub>5</sub> in 1927,<ref>{{cite journal|author = von Grosse, Aristid |title = Das Element 91; seine Eigenschaften und seine Gewinnung |pages = 233–245|journal = [[Berichte der deutschen chemischen Gesellschaft]]|doi = 10.1002/cber.19280610137|volume = 61|issue = 1|date = 1928}}</ref> and in 1934 first isolated elemental protactinium from 0.1&nbsp;milligrams of Pa<sub>2</sub>O<sub>5</sub>.<ref>{{cite journal|doi = 10.1002/ange.19340473706|title = Die technische Gewinnung des Protactiniums|date = 1934|last1 = Graue|first1 = G.|last2 = Käding|first2 = H.|journal = [[Angewandte Chemie]]|volume = 47|issue = 37|pages = 650–653|bibcode = 1934AngCh..47..650G}}</ref> He used two different procedures: in the first, protactinium oxide was irradiated by 35&nbsp;keV electrons in vacuum. In the other, called the [[crystal bar process|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:<ref name="CRC" /><ref>{{cite journal| last1=Grosse| first1=A. V.| journal=[[Journal of the American Chemical Society]]| volume=56|pages=2200–2201| date=1934| doi=10.1021/ja01325a508| issue=10| title=Metallic Element 91| bibcode=1934JAChS..56R2200G}}</ref>

: 2 PaI<sub>5</sub> → 2 Pa + 5 I<sub>2</sub>

In 1961, the [[United Kingdom Atomic Energy Authority]] (UKAEA) produced 127&nbsp;grams of 99.9% pure protactinium-231 by processing 60&nbsp;tonnes of waste material in a 12-stage process, at a cost of about US$500,000.<ref name="CRC" /><ref name="Myasoedov" /> For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies.<ref name="Emsley" /> The [[Oak Ridge National Laboratory]] in the US provided protactinium at a cost of about US$280/gram.<ref>{{cite web |url=https://periodic.lanl.gov/91.shtml |title=Periodic Table of Elements: Protactinium |access-date=2013-03-21 |publisher=[[Los Alamos National Laboratory]] |url-status=dead<!-- information subsequently removed --> |archive-url=https://web.archive.org/web/20110928025549/http://periodic.lanl.gov/91.shtml |archive-date=28 September 2011}}</ref>

==Isotopes==<!-- This section is linked from [[uranium]] -->
{{Main|Isotopes of protactinium}}
Thirty [[radioisotope]]s of protactinium have been discovered, ranging from <sup>210</sup>Pa to <sup>239</sup>Pa.<ref name="Zhang">{{cite journal |last1=Zhang |first1=M. M. |last2=Wang |first2=J. G. |last3=Ma |first3=L. |last4=Gan |first4=Z. G. |last5=Zhang |first5=Z. Y. |last6=Huang |first6=M. H. |last7=Yang |first7=H. B. |last8=Yang |first8=C. L. |last9=Andreyev |first9=A. N. |last10=Yuan |first10=C. X. |last11=Tian |first11=Y. L. |last12=Wang |first12=Y. S. |last13=Wang |first13=J. Y. |last14=Qiang |first14=Y. H. |last15=Wu |first15=X. L. |last16=Xu |first16=S. Y. |last17=Zhao |first17=Z. |last18=Huang |first18=X. Y. |last19=Li |first19=Z. C. |last20=Zhou |first20=H. |last21=Zhang |first21=X. |last22=Xie |first22=G. |last23=Zhu |first23=L. |last24=Guan |first24=F. |last25=Zheng |first25=J. H. |last26=Sun |first26=L. C. |last27=Li |first27=Y. J. |last28=Yang |first28=H. R. |last29=Duan |first29=L. M. |last30=Lu |first30=Z. W. |last31=Huang |first31=W. X. |last32=Sun |first32=L. T. |last33=He |first33=Y. |last34=Xu |first34=H. S. |last35=Niu |first35=Y. F. |last36=He |first36=X. T. |last37=Ren |first37=Z. Z. |last38=Zhou |first38=S. G. |title=Discovery of the α-emitting isotope 210Pa |journal=Nature Communications |date=29 May 2025 |volume=16 |issue=1 |pages=5003 |doi=10.1038/s41467-025-60047-2 |url=https://www.nature.com/articles/s41467-025-60047-2 |access-date=1 June 2025 |language=en |issn=2041-1723|pmc=12123024 }}</ref>{{NUBASE2020|ref}} The most stable are <sup>231</sup>Pa with a half-life of 32,650 years, <sup>233</sup>Pa with a half-life of 26.975 days, and <sup>230</sup>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 six [[nuclear isomer]]s, with the most stable being <sup>234m</sup>Pa (half-life 1.159 minutes).{{NUBASE2020|ref}}

The primary [[decay mode]] for the most stable isotope <sup>231</sup>Pa and lighter isotopes (<sup>210</sup>Pa<ref name="Zhang"/> to <sup>227</sup>Pa) is [[alpha decay]], producing [[isotopes of actinium]]. The primary decay mode for <sup>228</sup>Pa to <sup>230</sup>Pa is [[electron capture]] or [[beta plus decay]], producing [[isotopes of thorium]], while the primary decay mode for the heavier isotopes (<sup>232</sup>Pa to <sup>239</sup>Pa) is [[beta decay]], producing [[isotopes of uranium]].{{NUBASE2020|ref}}

===Nuclear fission===

The longest-lived and most abundant isotope, <sup>231</sup>Pa, can fission from [[Neutron_temperature#Fast|fast neutrons]] exceeding ~1&nbsp;[[Electronvolt|MeV]].<ref name="AVG231">{{cite journal |last1=Grosse |first1=A. v. |last2=Booth |first2=E. T. |last3=Dunning |first3=J. R. |title=The Fission of Protactinium (Element 91) |journal=Physical Review |date=15 August 1939 |volume=56 |issue=4 |page=382 |doi=10.1103/PhysRev.56.382 |bibcode=1939PhRv...56..382G |url=https://journals.aps.org/pr/pdf/10.1103/PhysRev.56.382 |access-date=17 February 2023|url-access=subscription }}</ref> <sup>233</sup>Pa, the other isotope of protactinium produced in nuclear reactors, also has a fission threshold of 1&nbsp;MeV.<ref>{{cite journal |last1=Tovesson |first1=F. |last2=Hambsch |first2=F.-J |last3=Oberstedt |first3=A. |last4=Fogelberg |first4=B. |last5=Ramström |first5=E. |last6=Oberstedt |first6=S. |title=The Pa-233 Fission Cross Section |journal=Journal of Nuclear Science and Technology |date=August 2002 |volume=39 |issue=sup2 |pages=210–213 |doi=10.1080/00223131.2002.10875076 |bibcode=2002JNST...39Q.210T |s2cid=91866777 |doi-access=free }}</ref>

==Occurrence==
Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes – <sup>231</sup>Pa and <sup>234</sup>Pa, with the isotope <sup>234</sup>Pa occurring in two different energy states. Nearly all natural protactinium is <sup>231</sup>Pa. It is an [[Alpha particle|alpha emitter]] and is formed by the decay of uranium-235, whereas the [[beta particle|beta-radiating]] <sup>234</sup>Pa is produced as a result of [[:File:Decay chain(4n+2, Uranium series).PNG|uranium-238 decay]]. Nearly all uranium-238 (99.8%) decays first to the shorter-lived <sup>234m</sup>Pa isomer.<ref name="ANL">[http://www.ead.anl.gov/pub/doc/protactinium.pdf Protactinium] {{webarchive|url=https://web.archive.org/web/20080307023843/http://www.ead.anl.gov/pub/doc/protactinium.pdf |date=7 March 2008 }}, Argonne National Laboratory, Human Health Fact Sheet, August 2005</ref>

Protactinium occurs in [[uraninite]] (pitchblende) at concentrations of about 0.3-3 [[parts-per notation|parts]] <sup>231</sup>Pa per million parts (ppm) of ore.<ref name="Emsley" /> Whereas the usual content is closer to 0.3&nbsp;ppm<ref name="brit" /> (e.g. in [[Jáchymov]], [[Czech Republic]]<ref>{{cite journal|last1=Grosse|first1=A. V.|last2=Agruss|first2=M. S.|journal=[[Journal of the American Chemical Society]]|volume=56|pages=2200|date=1934|doi=10.1021/ja01325a507|issue=10|title=The Isolation of 0.1 Gram of the Oxide of Element 91 (Protactinium)|bibcode=1934JAChS..56Q2200G }}</ref>), some ores from the [[Democratic Republic of the Congo]] have about 3&nbsp;ppm.<ref name="CRC" /> 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]].<ref name="ANL" /><ref>Cornelis, Rita (2005) [https://books.google.com/books?id=1PmjurlE6KkC&pg=PA520 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}}.</ref>

===In nuclear reactors===
Two major protactinium isotopes, <sup>231</sup>Pa and <sup>233</sup>Pa, are produced from thorium in [[nuclear reactor]]s; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular, <sup>232</sup>Th, via [[Nuclear reaction#Reactions with neutrons | (''n'', 2''n'')]] reactions, produces <sup>231</sup>Th, which quickly decays to <sup>231</sup>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 [[radiotoxic]]ity of spent nuclear fuel.<ref name="b1" />

Protactinium-233 is formed upon neutron capture by <sup>232</sup>Th. It either further decays to <sup>233</sup>U, or captures another neutron and converts into the non-fissile <sup>234</sup>U.<ref>{{cite book|author=Hébert, Alain|title=Applied Reactor Physics|url=https://books.google.com/books?id=sibA5ECQ8LoC&pg=PA265|date=July 2009|publisher=Presses inter Polytechnique|isbn=978-2-553-01436-9|page=265}}</ref> <sup>233</sup>Pa has a relatively long half-life of 27 days and high [[cross section (physics)|cross section]] for neutron capture (the so-called "[[neutron poison]]"). Thus, instead of rapidly decaying to the useful <sup>233</sup>U, a significant fraction of <sup>233</sup>Pa converts to non-fissile isotopes and consumes neutrons, degrading [[neutron economy|reactor efficiency]]. To limit the loss of neutrons, <sup>233</sup>Pa is extracted from the active zone of thorium [[molten salt reactor]]s during their operation, so that it can only decay into <sup>233</sup>U. Extraction of <sup>233</sup>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&nbsp;°C, low vapor pressure, good solubility for lithium and actinides, and [[Miscibility | immiscibility]] with molten [[halide]]s.<ref name="b1">Groult, Henri (2005) [https://books.google.com/books?id=dR2DA50PUV4C&pg=PA562 Fluorinated materials for energy conversion], Elsevier, pp. 562–565, {{ISBN|0-08-044472-5}}.</ref>

==Preparation==
[[File:Uraninite-39029.jpg|thumb|right|upright|Protactinium occurs in [[uraninite]] ores.]]
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 <sup>233</sup>U:

:<chem>^{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.</chem>

The isotope <sup>231</sup>Pa can be prepared by irradiating <sup>230</sup>Th with [[Neutron temperature#Cold (slow) neutrons | slow neutrons]], converting it to the beta-decaying <sup>231</sup>Th; or, by irradiating <sup>232</sup>Th with fast neutrons, generating <sup>231</sup>Th and 2 neutrons.

Protactinium metal can be prepared by reduction of its [[fluoride]] with [[calcium]],<ref name="exp">{{cite journal|last1=Marples|first1=J. A. C.|title=On the thermal expansion of protactinium metal|journal=[[Acta Crystallographica]]|volume=18|pages=815–817|date=1965|doi=10.1107/S0365110X65001871|issue=4|bibcode=1965AcCry..18..815M }}</ref> [[lithium]], or [[barium]] at a temperature of 1300–1400&nbsp;°C.<ref name="super" /><ref name="pao2" />

==Properties==
Protactinium is an [[actinide]] positioned in the [[periodic table]] to the left of [[uranium]] and to the right of [[thorium]], and many of its physical properties are intermediate between its neighboring actinides. Protactinium is denser and more rigid than thorium, but is lighter than uranium; its melting point is lower than that of thorium, but higher than that of uranium. The thermal expansion, electrical, and thermal conductivities of these three elements are comparable and are typical of [[post-transition metal]]s. The estimated [[shear modulus]] of protactinium is similar to that of [[titanium]].<ref>Seitz, Frederick and Turnbull, David (1964) [https://books.google.com/books?id=F9V3a-0V3r8C&pg=PA289 Solid state physics: advances in research and applications], Academic Press, pp. 289–291, {{ISBN|0-12-607716-9}}.</ref> Protactinium is a metal with silvery-gray luster that is preserved for some time in air.<ref name="CRC">{{cite book |author = Hammond, C. R. |title = The Elements, in Handbook of Chemistry and Physics |edition = 81st |publisher = CRC press |isbn = 978-0-8493-0485-9 |date = 2004-06-29 |url-access = registration |url = https://archive.org/details/crchandbookofche81lide }}</ref><ref name="Myasoedov">{{cite book |last1=Myasoedov |first1=B. F. |last2=Kirby |first2=H. W.|last3=Tananaev |first3=I. G. |title=The Chemistry of the Actinide and Transactinide Elements |s2cid=93796247 |editor1-first=L. R. |editor1-last=Morss |editor2-first=N. M. |editor2-last=Edelstein |editor3-first=J. |editor3-last=Fuger |edition=3rd |date=2006 |publisher=Springer |location=Dordrecht, The Netherlands |chapter=Chapter 4: Protactinium |isbn=978-1-4020-3555-5|bibcode=2011tcot.book.....M |doi=10.1007/978-94-007-0211-0 }}</ref> Protactinium easily reacts with oxygen, water vapor, and acids, but not with alkalis.<ref name="Emsley" />

At room temperature, protactinium crystallizes in the body-centered [[Tetragonal crystal system|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&nbsp;°C.<ref name="exp" /><ref>Young, David A. (1991) [https://books.google.com/books?id=F2HVYh6wLBcC&pg=PA222 Phase diagrams of the elements], University of California Press, p. 222, {{ISBN|0-520-07483-1}}.</ref> The thermal expansion coefficient of the tetragonal phase between room temperature and 700&nbsp;°C is 9.9{{e|-6}}/°C.<ref name="exp" />

Protactinium is [[paramagnetism|paramagnetic]] and no magnetic transitions are known for it at any temperature.<ref>Buschow, K. H. J. (2005) [https://books.google.com/books?id=N9mvytGEBtwC&pg=PA129 Concise encyclopedia of magnetic and superconducting materials], Elsevier, pp. 129–130, {{ISBN|0-08-044586-1}}.</ref> It becomes [[Superconductivity|superconductive]] at temperatures below 1.4&nbsp;K.<ref name="Emsley" /><ref name="super">{{cite journal| display-authors =4| author =Fowler, R. D. |title = Superconductivity of Protactinium| journal = [[Physical Review Letters]]| volume = 15 |page = 860|date = 1965| doi = 10.1103/PhysRevLett.15.860| bibcode=1965PhRvL..15..860F| issue =22| last2 =Matthias| first2 =B.| last3 =Asprey| first3 =L.| last4 =Hill| first4 =H.| last5 =Lindsay| first5 =J.| last6 =Olsen| first6 =C.| last7 =White| first7 =R.}}</ref> Protactinium tetrachloride is paramagnetic at room temperature, but becomes [[ferromagnetism|ferromagnetic]] when cooled to 182&nbsp;K.<ref>{{cite journal|last1=Hendricks|first1=M. E.|title=Magnetic Properties of Protactinium Tetrachloride|journal=[[Journal of Chemical Physics]]|volume=55|pages=2993–2997|date=1971|doi=10.1063/1.1676528|issue=6|bibcode = 1971JChPh..55.2993H }}</ref>

Protactinium exists in two major [[oxidation state]]s: +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]5f<sup>2</sup>6d<sup>1</sup>7s<sup>2</sup>, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f<sup>0</sup> configuration. Both +4 and +5 states easily form [[hydroxide]]s in water, with the predominant ions being Pa(OH)<sup>3+</sup>, {{chem2|Pa(OH)2(2+)}}, {{chem2|Pa(OH)3(+)}}, and Pa(OH)<sub>4</sub>, all of which are colorless.<ref name="g1265">[[#Greenwood|Greenwood]], p. 1265</ref> Other known protactinium ions include {{chem2|PaCl2(2+)}}, {{chem2|PaSO4(2+)}}, PaF<sup>3+</sup>, {{chem2|PaF2(2+)}}, {{chem2|PaF6(-)}}, {{chem2|PaF7(2-)}}, and {{chem2|PaF8(3-)}}.<ref name="g1275">[[#Greenwood|Greenwood]], p. 1275</ref><ref name="trif" />

==Chemical compounds==
{{Main|Protactinium compounds}}
{| Class = "wikitable" style = "text-align: center"
! Formula
! color
! symmetry
! [[space group]]
! No
! [[Pearson symbol]]
! ''a'' (pm)
! ''b'' (pm)
! ''c'' (pm)
! ''Z''
! density (g/cm<sup>3</sup>)
|-
| Pa
| silvery-gray
| [[Tetragonal crystal system|tetragonal]]<ref name="str" />
| I4/mmm
| 139
| tI2
| 392.5
| 392.5
| 323.8
| 2
| 15.37
|-
| PaO
|
| rocksalt<ref name="pao2">{{cite journal|doi = 10.1021/ja01652a011|date = 1954|last1 = Sellers|first1 = Philip A.|last2 = Fried|first2 = Sherman|last3 = Elson|first3 = Robert E.|last4 = Zachariasen|first4 = W. H.|journal = [[Journal of the American Chemical Society]]|volume = 76|pages = 5935|title = The Preparation of Some Protactinium Compounds and the Metal|issue = 23| bibcode=1954JAChS..76.5935S |url = https://digital.library.unt.edu/ark:/67531/metadc172625/|url-access = subscription}}</ref>
| Fm{{overline|3}}m
| 225
| cF8
| 496.1
|
|
| 4
| 13.44
|-
| [[Protactinium(IV) oxide|PaO<sub>2</sub>]]
| black
| ''fcc''<ref name="pao2" />
| Fm{{overline|3}}m
| 225
| cF12
| 550.5
|
|
| 4
| 10.47
|-
| [[Protactinium(V) oxide|Pa<sub>2</sub>O<sub>5</sub>]]
| white
|
| Fm{{overline|3}}m<ref name="pao2" />
| 225
| cF16
| 547.6
| 547.6
| 547.6
| 4
| 10.96
|-
| Pa<sub>2</sub>O<sub>5</sub>
| white
| orthorhombic<ref name="pao2" />
|
|
|
| 692
| 402
| 418
|
|
|-
| PaH<sub>3</sub>
| black
| cubic<ref name="pao2" />
| Pm{{overline|3}}n
| 223
| cP32
| 664.8
| 664.8
| 664.8
| 8
| 10.58
|-
| PaF<sub>4</sub>
| brown-red
| monoclinic<ref name="pao2" />
| C2/c
| 15
| mS60
|
|
|
| 2
|
|-
| PaCl<sub>4</sub>
| green-yellow
| [[Tetragonal crystal system|tetragonal]]<ref>{{cite journal|journal=[[Journal of the Chemical Society, Dalton Transactions]]|date=1973|title=Structural parameters and unit cell dimensions for the tetragonal actinide tetrachlorides(Th, Pa, U, and Np) and tetrabromides (Th and Pa)|pages=686–691|author=Brown D.|author2=Hall T.L.|author3=Moseley P.T|doi=10.1039/DT9730000686|issue=6}}</ref>
| I4<sub>1</sub>/amd
| 141
| tI20
| 837.7
| 837.7
| 748.1
| 4
| 4.72
|-
| PaBr<sub>4</sub>
| brown
| tetragonal<ref name="pabr4">{{cite journal|display-authors=4|last1=Tahri|first1=Y.|last2=Chermette|first2=H.|last3=El Khatib|first3=N.|last4=Krupa|first4=J.|last5=Simoni|first5=E.|title=Electronic structures of thorium and protactinium halide clusters of [ThX8]4− type|journal=[[Journal of the Less Common Metals]]|volume=158|pages=105–116|date=1990|doi=10.1016/0022-5088(90)90436-N}}</ref><ref name="pabr5b" />
| I4<sub>1</sub>/amd
| 141
| tI20
| 882.4
| 882.4
| 795.7
|
|
|-
| [[Protactinium(V) chloride|PaCl<sub>5</sub>]]
| yellow
| [[Monoclinic crystal system|monoclinic]]<ref name="pacl5">{{cite journal|doi=10.1107/S0365110X67000155|last1=Dodge|first1=R. P.|last2=Smith|first2=G. S.|last3=Johnson|first3=Q.|last4=Elson|first4=R. E.|title=The crystal structure of protactinium pentachloride|journal=[[Acta Crystallographica]]|date=1967|volume=22|issue=1 |pages=85–89|bibcode=1967AcCry..22...85D }}</ref>
| C2/c
| 15
| mS24
| 797
| 1135
| 836
| 4
| 3.74
|-
| PaBr<sub>5</sub>
| red
| monoclinic<ref name="pabr5b" /><ref name="pabr5">{{cite journal|last1=Brown|first1=D.|last2=Petcher|first2=T. J.|last3=Smith|first3=A. J.|title=The crystal structure of β-protactinium pentabromide|journal=[[Acta Crystallographica B]]|volume=25|pages=178|date=1969|doi=10.1107/S0567740869007357|issue=2|bibcode=1969AcCrB..25..178B }}</ref>
| P2<sub>1</sub>/c
| 14
| mP24
| 838.5
| 1120.5
| 1214.6
| 4
| 4.98
|-
| PaOBr<sub>3</sub>
|
| monoclinic<ref name="pabr5b" />
| C2
|
|
| 1691.1
| 387.1
| 933.4
|
|
|-
| Pa(PO<sub>3</sub>)<sub>4</sub>
|
| orthorhombic<ref name="papo3">{{cite journal|doi=10.1016/j.jssc.2004.08.009|last1=Brandel|first1=V.|date=2004|pages=4743|volume=177|journal=[[Journal of Solid State Chemistry]]|last2=Dacheux|first2=N. |title=Chemistry of tetravalent actinide phosphates—Part I|issue=12|bibcode = 2004JSSCh.177.4743B }}</ref>
|
|
|
| 696.9
| 895.9
| 1500.9
|
|
|-
| Pa<sub>2</sub>P<sub>2</sub>O<sub>7</sub>
|
| cubic<ref name="papo3" />
| Pa3
|
|
| 865
| 865
| 865
|
|
|-
| Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>
| golden-yellow
| monoclinic<ref name="cene">{{cite journal|doi=10.1021/ic50136a011|last1=Starks|date=1974|first1=David F.|pages=1307|volume=13|last2=Parsons|journal=[[Inorganic Chemistry (journal)|Inorganic Chemistry]]|first2=Thomas C.|last3=Streitwieser|first3=Andrew|last4=Edelstein|first4=Norman|title=Bis(π-cyclooctatetraene) protactinium|issue=6|author-link3=Andrew Streitwieser}}</ref>
|
|
|
| 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 unit]]s per [[unit cell]]; ''fcc'' stands for the [[Cubic crystal system|face-centered cubic]] symmetry. Density was not measured directly but calculated from the lattice parameters.

===Oxides and oxygen-containing salts===
Protactinium oxides are known for the metal oxidation states +2, +4, and +5. The most stable is the white pentoxide [[Protactinium(V) oxide|Pa<sub>2</sub>O<sub>5</sub>]], which can be produced by igniting protactinium(V) hydroxide in [[air]] at a temperature of 500&nbsp;°C.<ref name="g1268">[[#Greenwood|Greenwood]], p. 1268</ref> Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO<sub>2.25</sub>. Another phase of this oxide with orthorhombic symmetry has also been reported.<ref name="pao2" /><ref name="pacl4b" /> The black dioxide [[Protactinium(IV) oxide|PaO<sub>2</sub>]] is obtained from the pentoxide by reducing it at 1550&nbsp;°C with hydrogen. It is not readily soluble in either dilute or concentrated [[nitric acid|nitric]], [[hydrochloric acid|hydrochloric]], or [[sulfuric acid]], but easily dissolves in [[hydrofluoric acid]].<ref name="pao2" /> The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100&nbsp;°C.<ref name="pacl4b">{{cite journal|last1=Elson|first1=R.|last2=Fried|first2=Sherman|last3=Sellers|first3=Philip|last4=Zachariasen|first4=W. H.|title=The tetravalent and pentavalent states of protactinium|journal=[[Journal of the American Chemical Society]]|volume=72|pages=5791|date=1950|doi=10.1021/ja01168a547|issue=12|bibcode=1950JAChS..72.5791E }}</ref> The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.<ref name="pao2" />

Protactinium forms mixed binary oxides with various metals. With alkali metals ''A'', the crystals have a chemical formula APaO<sub>3</sub> and [[perovskite structure]]; A<sub>3</sub>PaO<sub>4</sub> and distorted rock-salt structure; or A<sub>7</sub>PaO<sub>6</sub>, where oxygen atoms form a hexagonal close-packed lattice. In all of these materials, the protactinium ions are octahedrally coordinated.<ref name="g1269">[[#Greenwood|Greenwood]], p. 1269</ref><ref>{{cite journal|doi=10.1107/S056774087100284X|last1=Iyer|first1=P. N.|date=1971|pages=731|volume=27|journal=[[Acta Crystallographica B]]|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum or protactinium. IV. Further systems involving alkali metals|issue=4|bibcode=1971AcCrB..27..731I }}</ref> The pentoxide Pa<sub>2</sub>O<sub>5</sub> combines with rare-earth metal oxides R<sub>2</sub>O<sub>3</sub> to form various nonstoichiometric mixed-oxides, also of perovskite structure.<ref>{{cite journal|last1=Iyer|first1=P. N.|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum, or protactinium. III. Systems involving the rare earths|journal=[[Acta Crystallographica]]|volume=23|pages=740|date=1967|doi=10.1107/S0365110X67003639|issue=5|bibcode=1967AcCry..23..740I }}</ref>

Protactinium oxides are [[Basic oxide|basic]]; they easily convert to hydroxides and can form various salts, such as [[sulfate]]s, [[phosphate]]s, [[nitrate]]s, etc. The nitrate is usually white but can be brown due to [[radiolysis|radiolytic]] decomposition. Heating the nitrate in air at 400&nbsp;°C converts it to the white protactinium pentoxide.<ref name="target" /> The polytrioxophosphate Pa(PO<sub>3</sub>)<sub>4</sub> can be produced by reacting the difluoride sulfate PaF<sub>2</sub>SO<sub>4</sub> with [[phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>) under an inert atmosphere. Heating the product to about 900&nbsp;°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(PO<sub>3</sub>)<sub>4</sub> into the diphosphate PaP<sub>2</sub>O<sub>7</sub>, which is analogous to diphosphates of other actinides. In the diphosphate, the PO<sub>3</sub> groups form pyramids of C<sub>2v</sub> symmetry. Heating PaP<sub>2</sub>O<sub>7</sub> in air to 1400&nbsp;°C decomposes it into the pentoxides of phosphorus and protactinium.<ref name="papo3" />

===Halides===
Protactinium(V) fluoride is a white compound that forms tetragonal crystals, [[Isomorphism (crystallography)|isomorphic]] to β-[[uranium pentafluoride|UF<sub>5</sub>]].<ref>{{cite journal | last=Stein | first=Lawrence | title=Protactinium Fluorides | journal=Inorganic Chemistry | volume=3 | issue=7 | date=1964 | issn=0020-1669 | doi=10.1021/ic50017a016 | pages=995–998 }}</ref> Protactinium(V) chloride forms yellow crystals where protactinium ions are arranged in pentagonal bipyramids and [[Coordination number|coordinated]] by 7 other ions. 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.<ref name="g1270">[[#Greenwood|Greenwood]], p. 1270</ref>

[[File:PaCl5.svg|thumb|right|Coordination of protactinium (solid circles) and chlorine atoms (open circles) in protactinium(V) chloride.]]
Protactinium(V) chloride has a polymeric structure of monoclinic symmetry.<ref>{{cite journal | last=Dodge | first=R. P. | last2=Smith | first2=G. S. | last3=Johnson | first3=Q. | last4=Elson | first4=R. E. | title=The crystal structure of protactinium pentachloride | journal=Acta Crystallographica | volume=22 | issue=1 | date=1967 | issn=0365-110X | doi=10.1107/S0365110X67000155 | pages=85–89 }}</ref> There, within one polymeric chain, all chlorine atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The 7-coordination of protactinium originates from the five chlorine atoms and two bonds to protactinium atoms belonging to the nearby chains. It easily hydrolyzes in water.<ref name="g1271" /> It melts at 300&nbsp;°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&nbsp;°C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and [[hydrogen fluoride]] at 600&nbsp;°C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction.<ref name="pao2" />

Protactinium(V) chloride is prepared by reacting protactinium oxide with [[carbon tetrachloride]] at temperatures of 200–300&nbsp;°C.<ref name="pao2" /> The by-products (such as PaOCl<sub>3</sub>) are removed by fractional sublimation.<ref name="pacl5" /> Reduction of protactinium(V) chloride with hydrogen at about 800&nbsp;°C yields protactinium(IV) chloride – a yellow-green solid that sublimes in vacuum at 400&nbsp;°C. It can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400&nbsp;°C.<ref name="pao2" />

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.<ref name="pao2" /> Protactinium(V) bromide has two similar monoclinic forms: one is obtained by sublimation at 400–410&nbsp;°C, and another by sublimation at a slightly lower temperature of 390–400&nbsp;°C.<ref name="pabr5b">{{cite journal|doi=10.1038/217737a0|last1=Brown|first1=D.|last2=Petcher|first2=T. J.|last3=Smith|first3=A. J.|title=Crystal Structures of some Protactinium Bromides|date=1968|pages=737|volume=217|journal=[[Nature (journal)|Nature]]|issue=5130|bibcode = 1968Natur.217..737B |s2cid=4264482}}</ref><ref name="pabr5" />

Protactinium iodides can be produced by reacting protactinium metal with elemental iodine at 600&nbsp;°C, and by reacting Pa<sub>2</sub>O<sub>5</sub> with AlI<sub>3</sub> at elevated temperatures.<ref name="pao2" /> Protactinium(III) iodide can be obtained by heating protactinium(V) iodide in vacuum.<ref name="g1271" /> As with oxides, protactinium forms mixed halides with alkali metals. The most remarkable among these is Na<sub>3</sub>PaF<sub>8</sub>, where the protactinium ion is symmetrically surrounded by 8 F<sup>−</sup> ions, forming a nearly perfect cube.<ref name="g1275" />

More complex protactinium fluorides are also known, such as Pa<sub>2</sub>F<sub>9</sub><ref name="g1271">[[#Greenwood|Greenwood]], p. 1271</ref> and ternary fluorides of the types MPaF<sub>6</sub> (M = Li, Na, K, Rb, Cs or NH<sub>4</sub>), M<sub>2</sub>PaF<sub>7</sub> (M = K, Rb, Cs or NH<sub>4</sub>), and M<sub>3</sub>PaF<sub>8</sub> (M = Li, Na, Rb, Cs), all of which are white crystalline solids. The MPaF<sub>6</sub> formula can be represented as a combination of MF and PaF<sub>5</sub>. 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<sup>+</sup>, Rb<sup>+</sup>, Cs<sup>+</sup> or NH<sub>4</sub><sup>+</sup>. A similar variation was observed for the M<sub>2</sub>PaF<sub>7</sub> fluorides: namely, the crystal symmetry was dependent on the cation and differed for Cs<sub>2</sub>PaF<sub>7</sub> and M<sub>2</sub>PaF<sub>7</sub> (M = K, Rb or NH<sub>4</sub>).<ref name="trif">{{cite journal|last1=Asprey|first1=L. B.|last2=Kruse|first2=F. H.|last3=Rosenzweig|first3=A.|last4=Penneman|first4=R. A.|title=Synthesis and X-Ray Properties of Alkali Fluoride-Protactinium Pentafluoride Complexes|journal=[[Inorganic Chemistry (journal)|Inorganic Chemistry]]|volume=5|pages=659|date=1966|doi=10.1021/ic50038a034|issue=4}}</ref>

===Other inorganic compounds===
Oxyhalides and oxysulfides of protactinium are known. PaOBr<sub>3</sub> 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.<ref name="pabr5b" /> 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&nbsp;°C.<ref name="pao2" />

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&nbsp;°C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is thermally stable to 800&nbsp;°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&nbsp;°C.<ref name="pao2" /> Protactinium forms [[borohydride]]s, which include Pa(BH<sub>4</sub>)<sub>4</sub>. It has an unusual polymeric structure with helical chains, where the protactinium atom has coordination number of 14 and is surrounded by six BH<sub>4</sub><sup>−</sup> ions.<ref name="g1277">[[#Greenwood|Greenwood]], p. 1277</ref>

===Organometallic compounds===
[[File:Uranocene-3D-balls.png|thumb|upright|The proposed structure of the protactinocene (Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>) molecule]]
Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C<sub>5</sub>H<sub>5</sub>)<sub>4</sub>) with four [[Cyclopentadienyl complex|cyclopentadienyl]] rings, which can be synthesized by reacting protactinium(IV) chloride with Be(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>. One ring can be substituted with a halide atom.<ref name="g1278">[[#Greenwood|Greenwood]], pp. 1278–1279</ref> Another organometallic complex is the golden-yellow bis(π-cyclooctatetraene) protactinium, or [[protactinocene]] (Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>), 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 [[cyclooctatetraene|cyclooctatetraenide]] (K<sub>2</sub>C<sub>8</sub>H<sub>8</sub>) in [[tetrahydrofuran]].<ref name="cene" />

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

<sup>231</sup>Pa arises naturally from the decay of natural <sup>235</sup>U, and artificially in nuclear reactors by the reaction <sup>232</sup>Th&nbsp;+&nbsp;n&nbsp;→&nbsp;<sup>231</sup>Th&nbsp;+&nbsp;2n and the subsequent [[beta decay]] of <sup>231</sup>Th. It was once thought to be able to support a nuclear chain reaction, which could in principle be used to build [[nuclear weapon]]s; the [[physicist]] {{Interlanguage link multi|Walter Seifritz|de|3=Walter_Seifritz}} once estimated the associated [[critical mass]] as {{val|750|180|u=kg}}.<ref>Seifritz, Walter (1984) ''Nukleare Sprengkörper – Bedrohung oder Energieversorgung für die Menschheit'', Thiemig-Verlag, {{ISBN|3-521-06143-4}}.</ref> However, the possibility of criticality of <sup>231</sup>Pa has since been ruled out.<ref name=AVG231 /><ref>{{cite journal|author=Ganesan, S.|url=http://www.iisc.ernet.in/currsci/sept10/researcharticle.pdf|title=A Re-calculation of Criticality Property of <sup>231</sup>Pa Using New Nuclear Data|journal=[[Current Science]]|year=1999|volume=77|issue=5|pages=667–677|access-date=21 March 2013|archive-date=3 March 2016|archive-url=https://web.archive.org/web/20160303231501/http://www.iisc.ernet.in/currsci/sept10/researcharticle.pdf|url-status=dead}}</ref>

With the advent of highly sensitive [[Mass spectrometry|mass spectrometers]], an application of <sup>231</sup>Pa as a tracer in geology and [[paleoceanography]] has become possible. In this application, the ratio of <sup>231</sup>Pa to <sup>230</sup>Th is used for [[radiometric dating]] of sediments which are up to 175,000 years old, and in modeling of the formation of minerals.<ref name="brit" /> In particular, its evaluation in oceanic sediments helped to reconstruct the movements of [[North Atlantic]] water bodies during the last melting of [[Ice age|Ice Age]] [[glacier]]s.<ref>{{cite journal|doi = 10.1038/nature02494|display-authors = 4|author = McManus, J. F.|author2 = Francois, R.|author3 = Gherardi, J.-M.|author4 = Keigwin, L. D.|author5 = Brown-Leger, S.|name-list-style = amp|title = Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes|journal = [[Nature (journal)|Nature]]|date = 2004|volume = 428|issue = 6985|pages = 834–837|pmid = 15103371|url = http://www.seas.harvard.edu/climate/pdf/mcmanus-2004.pdf|bibcode = 2004Natur.428..834M|s2cid = 205210064|access-date = 29 November 2010|archive-date = 10 April 2013|archive-url = https://web.archive.org/web/20130410051109/http://www.seas.harvard.edu/climate/pdf/mcmanus-2004.pdf|url-status = dead}}</ref><!--10.1016/S0016-7037(98)00255-5--> 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 <sup>230</sup>Th (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.<ref name="brit">Articles "Protactinium" and "Protactinium-231 – thorium-230 dating" in Encyclopædia Britannica, 15th edition, 1995, p. 737</ref><ref>{{cite journal|last1=Cheng|first1=H.|title=Uranium-thorium-protactinium dating systematics|journal=[[Geochimica et Cosmochimica Acta]]|volume=62|pages=3437|date=1998|doi=10.1016/S0016-7037(98)00255-5|bibcode=1998GeCoA..62.3437C|issue=21–22|last2=Edwards|first2=R.Lawrence|last3=Murrell|first3=M. T.|last4=Benjamin|first4=T. M.}}</ref>

==Precautions==
Protactinium is both toxic and highly radioactive; thus, it is handled exclusively in a sealed [[glove box]]. Its major isotope <sup>231</sup>Pa has a [[specific activity]] of {{convert|0.048|Ci|GBq|lk=on}} per gram and primarily emits alpha-particles with an energy of 5&nbsp;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 <sup>227</sup>Ac, which has a specific activity of {{convert|74|Ci|GBq}} per gram, emits both alpha and beta radiation, and has a much shorter half-life of 22 years. <sup>227</sup>Ac, in turn, decays into lighter isotopes with even shorter half-lives and much greater specific activities (SA).<ref name="ANL" />

{|class="wikitable" style="text-align:center"
!Isotope
|<sup>231</sup>Pa|| <sup>227</sup>Ac|| <sup>227</sup>Th|| <sup>223</sup>Ra|| <sup>219</sup>Rn|| <sup>215</sup>Po||<sup>211</sup>Pb|| <sup>211</sup>Bi || <sup>207</sup>Tl
|-
!SA ([[Curie (unit)|Ci]]/g)
| 0.048|| 73|| 3.1{{e|4}}|| 5.2{{e|4}}|| 1.3{{e|10}}||3{{e|13}}|| 2.5{{e|7}}|| 4.2{{e|8}}||1.9{{e|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 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.<ref name="ANL" /><ref name="target" /> The maximum safe dose of Pa in the human body is {{convert|0.03|µCi|kBq|abbr=on}}, which corresponds to 0.5 micrograms of <sup>231</sup>Pa.<ref>{{cite book|author = Palshin, E.S. |display-authors = etal|title = Analytical chemistry of protactinium| place =Moscow|publisher = Nauka|date = 1968}}</ref> The maximum allowed concentrations of <sup>231</sup>Pa in the air in Germany is {{val|3|e=-4|u=Bq/m<sup>3</sup>}}.<ref name="target">{{cite journal|doi=10.1016/j.nima.2008.02.084|last1=Grossmann|date=2008|first1=R.|pages=122|volume=590|issue=1–3|last2=Maier|journal=[[Nuclear Instruments and Methods in Physics Research A]] |first2=H.|last3=Szerypo|first3=J.|last4=Friebel|first4=H.|title=Preparation of 231Pa targets|bibcode = 2008NIMPA.590..122G }}</ref>

==See also==
* [[Ada Hitchins]], who helped Soddy in discovering the element protactinium

==Notes==
<references group="note" />

==References==
{{Reflist|35em}}

==Bibliography==
*{{cite book
 |last1=Greenwood |first1=Norman N.
 |author-link1=Norman Greenwood
 |last2=Earnshaw |first2=Alan
 |date=1997
 |title=Chemistry of the Elements
 |edition=2nd|ref=Greenwood
 |publisher=[[Butterworth–Heinemann]]
 |isbn=978-0080379418
}}

==External links==
{{Commons|Protactinium}}
{{Wiktionary|protactinium}}
* [http://www.periodicvideos.com/videos/091.htm Protactinium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)

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[[Category:Protactinium| ]]
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