Thallium

**Semantic type:** Element, Ion, or Isotope

**Definition:** A naturally occurring trace metalloid element with atomic symbol Tl, atomic number 81, and atomic weight 204.383. Thallium is used in manufacturing, as well as in some chemical reactions and medical procedures. Use as a pesticide has been banned in the United States due to toxicity from accidental exposure. It is characterized as an odorless, bluish-white, metallic soft solid that turns to a gray powder with an aromatic odor when exposed to air. Exposure occurs by inhalation of fumes, ingestion, or contact.

**Synonyms:** - THALLIUM - THALLIUM - Thallium Metallicum - Tl