noun Chemistry.
a radioactive, metallic element. Symbol: Pa; atomic number: 91.

1915–20; prot- + actinium Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
protactinium (ˌprəʊtækˈtɪnɪəm)
Former name: protoactinium 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

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American Heritage
Medical Dictionary

protactinium pro·tac·tin·i·um (prō'tāk-tĭn'ē-əm)
Symbol Pa
A rare, extremely toxic radioactive element having 13 known isotopes, the most stable of which is Pa 231 with a half-life of 32,700 years. Atomic number 91; melting point 1,572°C; specific gravity 15.37; valence 4, 5.

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American Heritage
Science Dictionary
protactinium   (prō'tāk-tĭn'ē-əm)  Pronunciation Key 
Symbol Pa
A rare, extremely toxic, radioactive metallic element of the actinide series that occurs in uranium ores. It has 13 known isotopes, the most stable of which is protactinium 231 with a half-life of 32,760 years. Atomic number 91; approximate melting point 1,550°C; specific gravity 15.37; valence 4, 5. See Periodic Table.
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Encyclopedia Britannica


radioactive chemical element of the actinoid series of the periodic table, rarer than radium; its atomic number is 91. It occurs in all uranium ores to the extent of 0.34 part per million of uranium and was first isolated (1934) in metallic form by Aristid V. Grosse. The first isotope, protactinium-234, was discovered (1913) by Kasimir Fajans and O.H. Gohring and named brevium, afterward uranium X2, because it was a short-lived member of the uranium radioactive decay series. The long-lived isotope protactinium-231 (originally called protoactinium) was discovered (1917) independently by Otto Hahn and Lise Meitner in pitchblende, by Fajans, and by Frederick Soddy, John Cranston, and Sir Alexander Fleck. This isotope decays to actinium-227 with a half-life of 32,500 years.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
They discovered the first long-lived isotope of the element protactinium.
The electrodeposition of protactinium was investigated by using mainly ammonium formate-sulfuric acid aqueous solution.
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