protactinium prōˌtăktĭnˈēəm [key], radioactive chemical element; symbol Pa; at. no. 91; mass number of most stable isotope 231; m.p. greater than 1,600℃; b.p. 4,026℃; 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-life 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 Hahn and Lisa Meitner and independently by Frederick Soddy 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.

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