• synonyms


[proh-tak-tin-ee-uh m]
noun Chemistry.
  1. a radioactive, metallic element. Symbol: Pa; atomic number: 91.
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Origin of protactinium

First recorded in 1915–20; prot- + actinium
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for protactinium


  1. 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, 231 Pa: 32 500 years; valency: 4 or 5; relative density: 15.37 (calc.); melting pt: 1572°CFormer name: protoactinium
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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

protactinium in Medicine


n. Symbol Pa
  1. 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.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

protactinium in Science


  1. 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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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.