It is a dense, radioactive, silvery-gray actinide metal which readily reacts with oxygen, water vapor, and inorganic acids.

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.

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.

[10] 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.

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.

[13] For a long time, chemists searched for eka-tantalum[note 1] as an element with similar chemical properties to tantalum, making a discovery of protactinium nearly impossible.

[17] 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: 23892U → 23490Th → 234m91Pa → 23492U.

[18][19][20][21][22][23] 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, 231Pa, having a much longer half-life of 32,760 years.

[8][22][24] 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").

[26][27] The discovery of protactinium completed one of the last gaps in early versions of the periodic table, and brought fame to the involved scientists.

[26] 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.

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.

Thus, instead of rapidly decaying to the useful 233U, a significant fraction of 233Pa converts to non-fissile isotopes and consumes neutrons, degrading reactor efficiency.

To limit the loss of neutrons, 233Pa is extracted from the active zone of thorium molten salt reactors during their operation, so that it can only decay into 233U.

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.

[61] The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.

[62][63] The pentoxide Pa2O5 combines with rare-earth metal oxides R2O3 to form various nonstoichiometric mixed-oxides, also of perovskite structure.

[64] Protactinium oxides are basic; they easily convert to hydroxides and can form various salts, such as sulfates, phosphates, nitrates, etc.

[65] 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.

There, within one polymeric chain, all halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions.

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).

PaOBr3 has a monoclinic structure composed of double-chain units where protactinium has coordination 7 and is arranged into pentagonal bipyramids.

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.

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.

[69] Another organometallic complex is the golden-yellow bis(π-cyclooctatetraene) protactinium, or protactinocene (Pa(C8H8)2), which is analogous in structure to uranocene.

[38] 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.

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.

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.

[37] As protactinium is present in small amounts in most natural products and materials, it is ingested with food or water and inhaled with air.