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Protactinium (pronounced /ˌproʊtækˈtɪniəm/, PROH-tak-TIN-ee-əm) is a chemical element with the symbol Pa and atomic number 91. Its longest-lived and most abundant naturally-occurring isotope by far, Pa-231, is a decay product of uranium-235 (U-235), and it has a half-life of 32,760 years. Much smaller trace amounts of the short-lived metastable isotope Pa-234m occur as decay products of uranium-238 (U-238). Pa-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.


Protactinium is a metallic element that belongs to the actinide group, with a bright metallic luster that it retains for some time in contact with air.[2][3] Protactinium is superconductive at temperatures below 1.4 K.[4]


Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside of scientific research.

Protactinium-231 is formed by the alpha decay of U-235 followed by beta decay of thorium-231. The physicist Walter Seifritz once estimated that protactinium might possibly be used to build a nuclear weapon with a critical mass of 750±180 kg. This possibility (of a chain reaction) has been ruled out by other nuclear physicists since then.

The ratio of protactinium-231 to thorium-230 in ocean sediments has also been used in paleoceanography to reconstruct the movements of North Atlantic water bodies during the last melting of Ice Age glaciers.[5]


In 1890, Mendeleev predicted the existence of an element between thorium and uranium. Due to the fact that the actinide element group was unknown uranium was positioned below tungsten and thorium below zirconium leaving the space below tantalum empty. Until the 1950s periodic tables were published with this structure.[6] For a long time chemists searched for eka-tantalum as an element with similar chemical properties as tantalum, making a discovery of protactinium nearly impossible.

In 1900, William Crookes isolated protactinium as a radioactive material from uranium; however, he did not identify it as a new element.[7]

Protactinium was first identified in 1913, when Kasimir Fajans and O. H. Göring encountered the short-lived isotope Pa-234m (half-life of about 1.17 minutes), during their studies of the decay chains of uranium-238 (U-238). They gave the new element the name brevium (from the Latin word, brevis, meaning brief or short);[8][9] the name was changed to protoactinium (from Greek πρῶτος + ἀκτίς meaning "first beam element") in 1918 when two groups of scientists (lead by Otto Hahn and Lise Meitner of Germany; and Frederick Soddy and John Cranston of Great Britain) independently discovered Pa-231. The name was shortened to Protactinium in 1949.

Aristid von Grosse produced 2 mg of Pa2O5 in 1927,[10] and in 1934 performed the first isolation of elemental protactinium from 0.1 mg of Pa2O5 using the iodide process: converting the oxide to an iodide and then reducing it in a vacuum with an electrically-heated metal filament:

2 PaI5 → 2 Pa + 5 I2

In 1961, the British Atomic Energy Authority (UKAEA) was able to produce 125 grams of 99.9% pure protactinium by processing 60 tons of waste material in a 12-stage process. For many years, this was the world's only significant supply of protactinium.[7]


Protactinium occurs in pitchblende to the extent of about 3.0 part 231Pa per million parts of ore.[7] name Some ores from the Democratic Republic of the Congo have about 3.0 ppm. Protactinium is one of the rarest and most expensive naturally occurring elements.[2]


Examples of protactinium compounds:

  • Fluorides: protactinium(IV) fluoride PaF4,
    protactinium(V) fluoride PaF5
  • Chlorides: protactinium(IV) chloride PaCl4,
    protactinium(V) chloride PaCl5
  • Bromides: protactinium(IV) bromide PaBr4,
    protactinium(V) bromide PaBr5
  • Iodides: protactinium(III) iodide PaI3,
    protactinium(IV) iodide PaI4,
    protactinium(V) iodide PaI5
  • Oxides: protactinium(II) oxide PaO,
    protactinium(IV) oxide PaO2,
    protactinium(V) oxide Pa2O5

See also Protactinium compounds.

Main article: Isotopes of protactinium

Twenty-nine radioisotopes of protactinium have been discovered, with the most stable being Pa-231 with a half life of 32760 years, Pa-233 with a half-life of 27.0 days, and Pa-230 with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives that are less than 1.60 days, and the majority of these have half-lives that are less than 1.8 seconds. Protactinium also has two meta states, Pa-217m (half-life 1.2 milliseconds) and Pa-234m (half-life 1.17 minutes).

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


Protactinium is both toxic and highly radioactive. It requires precautions similar to those used when handling plutonium.


1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
2. ^ a b C. R. Hammond. The Elements, in Handbook of Chemistry and Physics 81st edition. CRC press. ISBN 0849304857.
3. ^ Myasoedov, B. F.; Kirby, H. W.; Tananaev, I. G. (2006). "Chapter 4: Protactinium". in Morss, L. R.; Edelstein, N. M.; Fuger, J.. The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands: Springer. ISBN 13978-1-4020-3555-5.
4. ^ R. D. Fowler et al. (1965). "Superconductivity of Protactinium". Phys. Rev. Lett. 15: 860. doi:10.1103/PhysRevLett.15.860.
5. ^ J. F. McManus, R. Francois, J.-M. Gherardi, L. D. Keigwin, and S. Brown-Leger (2004). "Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes". Nature 428: 834–837.
6. ^ Michael Laing (2005). "A Revised Periodic Table: With the Lanthanides Repositioned". Foundations of Chemistry 7 (3): 203. doi:10.1007/s10698-004-5959-9.
7. ^ a b c Emsley, John (2001). "Protactinium". Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 347–349. ISBN 0198503407. http://books.google.com/books?id=j-Xu07p3cKwC&pg=PA348.
8. ^ K. Fajans and 0. Gohring, (1913). "Über die komplexe Natur des Ur X". Naturwissenschaften 14: 339. doi:10.1007/BF01495360.
9. ^ K. Fajans and 0. Gohring, (1913). "Über das Uran X2-das neue Element der Uranreihe". Physikalische Zeitschrift 14: 877–84.
10. ^ Aristid von Grosse (1928). "Das Element 91; seine Eigenschaften und seine Gewinnung". Berichte der deutschen chemischen Gesellschaft 61 (1): 233–245. doi:10.1002/cber.19280610137.

External links

* ANL factsheet
* WebElements.com - Protactinium
* It's Elemental - Protactinium

Chemistry Encyclopedia

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