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(prō'tăktĭn`ēəm), radioactive chemical element; symbol Pa; at. no. 91; mass number of most stable isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 231; m.p. greater than 1,600°C;; b.p. 4,026°C;; sp. gr. 15.37 (calculated); valence +4, +5. Protactinium is a malleable, shiny silver-gray radioactive metal. It does not tarnish rapidly in air. Known compounds include a chloride (PaCl4), a fluoride (PaF4), a dioxide (PaO2), and a pentoxide (Pa2O5). Protactinium has 24 isotopes of which only three are found in nature. The most stable is protactinium-231 (half-lifehalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
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 about 32,500 years); it is also the most common, being found in nature in all uranium ores in about the same abundance as radium.

Protactinium has been called the "mother" of actinium, which is formed by the alpha decay of protactinium. The first discovery of protactinium was in 1913 by Kasimir Fajans and O. Göhring, who found the isotope protactinium-234m (half-life 1.2 min), a decay product of uranium-238; they named it brevium for its short life. Protactinium-231 was first identified in 1918 by Otto HahnHahn, Otto
, 1879–1968, German chemist and physicist. His important contributions in the field of radioactivity include the discovery of several radioactive substances, the development of methods of separating radioactive particles and of studying chemical problems by the
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 and Lisa MeitnerMeitner, Lise
, 1878–1968, Austrian-Swedish physicist and mathematician. She was professor at the Univ. of Berlin (1926–33). A refugee from Germany after 1938, she became associated with the Univ. of Stockholm and with the Nobel Institute at Stockholm.
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 and independently by Frederick SoddySoddy, Frederick
, 1877–1956, English chemist. He worked under Lord Rutherford at McGill Univ. and with Sir William Ramsay at the Univ. of London. After serving (1910–14) as lecturer in physical chemistry and radioactivity at the Univ.
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 and John A. Cranston; the name protoactinium was adopted at this time. In 1927, Aristid V. Grosse prepared the pentoxide, and in 1934 isolated the metal from a purified sample of oxide. The name protactinium was adopted in 1949 by the International Union of Pure and Applied Chemistry.



(Pa), a radioactive chemical element; one of the actinides. Atomic number, 91.

The first protactinium isotope (more precisely, nuclear isomer), the short-lived 234mPa (half-life T½ = 1.18 min), was discovered in 1913 by K. Fajans and the German physicist O. Göhring in the radioactive uranium-radium series. In 1918, O. Hahn and L. Meitner and, independently, F. Soddy and the British chemist J. Cranston obtained the long-lived isotope 231Pa (T½ = 32,400 yr), which is a member of the radioactive actinium decay series. In this series, protactinium is the parent of actinium (the isotope 227Ac is formed upon α-decay of 231Pa), hence the name protactinium (from the Greek protos, “first”). Protactinium isotopes with mass numbers 224–237 and the nuclear isomer 234mPa are known, the most stable being 231Pa, with a mass of 231.0359 atomic mass units. The isotopes 231Pa, 234Pa (uranium Z, symbol UZ), and 234mPa (uranium X2, symbol UX2) occur in nature as members of natural radioactive decay series.

Protactinium is one of the least abundant elements, constituting only about 1 × 10-10 percent of the mass of the earth’s crust. Very little is known about the occurrence and migration of protactinium in the biosphere. The concentration of 234Pa in sea water amounts to only about 1 × 10-19 gram per liter; substantially larger amounts are found in soils. The synthetic species 233Pa is rapidly accumulated in plants and animals; for example, its accumulation coefficient, that is, the ratio of protactinium concentration in an organism to the concentration in the environment, is 1 million in the planktonic alga Coscinodiscus janischii, 2,000 in the benthic alga Ulva rigida, and 3,000 in the crustaceans Brachyura and Mytilus.

Protactinium is a lustrous, light-gray metal existing in two modifications. At low temperatures, it is tetragonal and stable up to 1170°C; at high temperatures, it is body-centered cubic, with a melting point of 1560°C and a boiling point of approximately 4280°C. Protactinium has a density of 15.4 g/cm3, and at temperatures below 2°K it is superconducting. Its surface is usually coated with a film of the oxide PaO. Protactinium has an outer electronic configuration of 5f26d17s2. In compounds, it exhibits oxidation states varying from +2 to +5 (+5 is most characteristic, followed by + 4). Given the oxidation state + 5, protactinium atoms do not contain any electrons in the 5f sub-shell, and the element behaves more like niobium and tantalum than like one of the actinides. Depending on the conditions of oxidation, it is possible to obtain the oxides PaO2, Pa6O14, and Pa2O5 as well as three phases of variable composition. The oxide Pa2O5 may be united with acid potassium sulfate (basic properties of Pa2O5) as well as with the oxides of alkali and alkaline-earth elements (acidic properties of Pa2O5). Halides and oxyhalides of protactinium are known, as are the carbide PaC and hydride PaH3. In aqueous solutions, ions containing Pa (V) or Pa (IV) tend to hydrolyze and polymerize, and their behavior often cannot be reproduced, which makes it very difficult to conduct studies of the element. The considerable interest in protactinium is related to the possible use of thorium as a means of producing atomic energy (the absorption of neutrons by 232Th nuclei produces the isotope 233Th, which rapidly decays to yield β-emitting 233Pa).


Pal’shin, E. S., B. F. Miasoedov, and A. V. Davydov. Analiticheskaia khimiia protaktiniia. Moscow, 1968.
Formy elementov i radionuklidov v morskoi vode. Moscow, 1974.
Radioactivity in the Marine Environment. Washington, D.C., 1971.



A chemical element, symbol Pa, atomic number 91; the third member of the actinide group of elements; all the isotopes are radioactive; the longest-lived isotope is protactinium-231.


a toxic radioactive metallic element that occurs in uranium ores and is produced by neutron irradiation of thorium. Symbol: Pa; atomic no.: 91; half-life of the most stable isotope, 231Pa: 32 500 years; valency: 4 or 5; relative density: 15.37 (calc.); melting pt.: 1572°C