thallium

Thallium is a chemical element with the symbol Tl and atomic number 81. It is a soft, silvery-white post-transition metal in group 13. Thallium was discovered in 1861 by William Crookes and Claude-Auguste Courtois in residues from zinc ore; its name derives from the Greek thallos, meaning green shoot, in reference to a characteristic emerald-green spectral line observed in its flame tests.

Occurrence and production: Thallium occurs only in trace amounts in the Earth's crust and is primarily recovered as a byproduct of zinc refining and from certain copper and lead ores. It is found in minerals such as crookesite and lorandite. Industrial production is dominated by recovering thallium from zinc smelting operations.

Properties: Tl is soft and highly malleable, and it tarnishes in air. It has a melting point of about 304 °C and a boiling point of about 1,473 °C. It forms oxides Tl2O and Tl2O3 and commonly exists in the +1 oxidation state, although +3 is known. Thallium forms a variety of salts, including TlCl and Tl2S3. It is toxic to humans and deeply persistent in the environment.

Applications: Historically used in rodenticides and insecticides, but now restricted in many countries. Today it is used as a dopant in scintillator crystals such as NaI(Tl) for gamma-ray detection and in certain semiconductor and infrared detector materials. The radionuclide thallium-201 is used in nuclear medicine for myocardial perfusion imaging.

Safety and regulation: Thallium and its compounds are highly toxic if ingested or inhaled. Chronic exposure can cause nervous system damage, hair loss, and organ failure. Handling requires strict controls, ventilation, and protective equipment; disposal is regulated.