ThalliumI

Thallium(I), abbreviated Tl+, is the monovalent cation of the element thallium. It is the most common oxidation state for thallium in inorganic compounds and forms a variety of Tl+ salts and oxides. In solids, Tl+ is a soft, highly polarizable cation that tends to form bonds with electronegative elements such as halogens and oxygen.

Common Tl+ compounds include thallium(I) chloride (TlCl), thallium(I) iodide (TlI), thallium(I) nitrate (TlNO3), and thallous oxide (Tl2O). Tl+ salts often exhibit low to moderate solubility in water, and their chemistry is distinguished by a tendency to be oxidized to thallium(III) under suitable oxidizing conditions. Thallium(III) chemistry is more oxidizing and less extensive in aqueous media, with oxides such as Tl2O3 known but less common in practice.

Natural occurrence and production are limited by thallium’s rarity and toxicity. Thallium is found only at trace levels in minerals such as crookesite and lorandite, and is typically extracted as a byproduct of sn and base-metal refining. Industrial use of Tl+ centers on specialized applications, including thallium-doped sodium iodide (NaI(Tl)) scintillators, where Tl+ acts as an activator producing light in response to ionizing radiation.

Toxicology and safety are major considerations: thallium compounds are highly toxic and can interfere with essential biological processes. Handling requires appropriate safety measures, and use is tightly regulated in many jurisdictions.