- noun a metallic element which is poisonous and used in pesticides.
- chemical symbolTl
(written as Thallium)
- A lustrous silver-gray metal whose atomic number is 81. It is soft, malleable, when exposed to dry air quickly forms a heavy oxide coating, and when in moist air or water rapidly forms a hydroxide. It has about 35 known isotopes, of which 2 are stable. It is used, for instance, in semiconductors, lasers, cryogenic switches, and optics. Its chemical symbol is TI.
- chemical symbolTI