from The American Heritage® Dictionary of the English Language, 5th Edition.

  • noun A soft, malleable, highly toxic metallic element, used in photocells, infrared detectors, low-melting glass, and formerly in rodent and ant poisons. A radioactive isotope, Tl-201, is used in medical imaging. Atomic number 81; atomic weight 204.38; melting point 304°C; boiling point 1,473°C; specific gravity 11.85 (20°C); valence 1, 3. cross-reference: Periodic Table.

from The Century Dictionary.

  • noun Chemical symbol, Tl; atomic weight, 204.2. A rare metal which was discovered in the residuum left from the distillation of selenium by Crookes, in 1861, and was first supposed to contain tellurium, but afterward proved, by the aid of the spectroscope, to be new.

from the GNU version of the Collaborative International Dictionary of English.

  • noun (Chem.) A rare metallic element of the aluminium group found in some minerals, as certain pyrites, and also in the lead-chamber deposit in the manufacture of sulphuric acid. It is isolated as a heavy, soft, bluish white metal, easily oxidized in moist air, but preserved by keeping under water. Symbol Tl. Atomic weight 203.7.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

  • noun A metallic chemical element (symbol Tl) with atomic number 81.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • noun a soft grey malleable metallic element that resembles tin but discolors on exposure to air; it is highly toxic and is used in rodent and insect poisons; occurs in zinc blende and some iron ores


from The American Heritage® Dictionary of the English Language, 4th Edition

[thall(o)– (from its green spectral line) + –ium.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

Coined based on Ancient Greek θαλλός (thallos, "green branch") (after the color of the radiation spectra).


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  • Tl

    December 1, 2007

  • The toxic properties of thallium compounds were recognized soon after the discovery of the element in the 19th century. Thallium compounds in toxic doses cause lack of coordination, trembling, hair loss, and finally respiratory distress, which results in death by asphyxia.

    February 27, 2015