Thallium is the chemical element in the periodic table that has the symbol Tl and atomic number 81.
This soft gray malleable poor metal resembles tin but discolors when exposed to air.
Thallium is highly toxic and is used in rat poisons and insecticides but since it might also cause cancer (although the EPA does not class it as carcinogen), this use has been cut back or eliminated in many countries.
It is also used in infrared detectors..
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