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[thal-ee-uh m] /ˈθæl i əm/
noun, Chemistry
a soft, malleable, rare, bluish-white metallic element: used in the manufacture of alloys and, in the form of its salts, in rodenticides. Symbol: Tl; atomic weight: 204.37; atomic number: 81; specific gravity: 11.85 at 20°C.
Origin of thallium
1860-65; < New Latin, equivalent to thall- (< Greek thallós green stalk) + -ium -ium; named after green line in its spectrum Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for thallium
  • Statistics and information on the worldwide supply, demand, and flow of thallium.
  • Small amounts of thallium are used to make certain medical agents and electronics.
  • Exposure to higher levels of thallium may occur in the workplace.
British Dictionary definitions for thallium


a soft malleable highly toxic white metallic element used as a rodent and insect poison and in low-melting glass. Its compounds are used as infrared detectors and in photoelectric cells. Symbol: Tl; atomic no: 81; atomic wt: 204.3833; valency: 1 or 3; relative density: 11.85; melting pt: 304°C; boiling pt: 1473±10°C
Word Origin
C19: from New Latin, from Greek thallos a green shoot; referring to the green line in its spectrum
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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Word Origin and History for thallium

rare metallic element, 1861, Modern Latin, from Greek thallos "young shoot, green branch" (see thallus) + element name ending -ium. So called by its discoverer, Sir William Crookes (1832-1919), from the green line in its spectrum by which he detected it.

Online Etymology Dictionary, © 2010 Douglas Harper
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thallium in Medicine

thallium thal·li·um (thāl'ē-əm)
Symbol Tl
A soft, malleable, highly toxic metallic element, used in photocells, infrared detectors, and low-melting glass. Atomic number 81; atomic weight 204.38; melting point 303.5°C; boiling point 1,473°C; specific gravity 11.85; valence 1, 3.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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thallium in Science
Symbol Tl
A soft, malleable, very poisonous metallic element that is used in photography, in making low-melting and highly refractive glass, and in treating skin infections. Atomic number 81; atomic weight 204.38; melting point 303.5°C; boiling point 1,457°C; specific gravity 11.85; valence 1, 3. See Periodic Table.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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