thallium

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.

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.

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.

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

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