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[proh-tak-tin-ee-uh m] /ˌproʊ tækˈtɪn i əm/
noun, Chemistry
a radioactive, metallic element. Symbol: Pa; atomic number: 91.
Also, protoactinium.
Origin of protactinium
1915-20; prot- + actinium Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for protactinium
  • 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.
British Dictionary definitions for protactinium


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 Former name protoactinium
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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protactinium in Medicine

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.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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protactinium in Science
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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