1. A soft grey malleable metallic element that resembles tin but discolours on exposure to air; it is highly toxic and is used in rodent and insect poisons; occurs in zinc blende and some iron ores
   "Thallium was once used in rat poison but is now banned in many countries";
   - Tl, atomic number 81

Derived forms: thalliums

Type of: metal, metallic element

Part of: blende, sphalerite, zinc blende

Encyclopedia: Thallium