Thallium

Uses

Thallium’s toxicity has limited most of its historical uses, but a few specialized applications remain. Thallium compounds are used in certain optical glasses and infrared-transmitting lenses because they bend light in useful ways that ordinary glass doesn’t. Thallium-doped crystals are also used inside radiation detectors, where they help convert incoming radiation into flashes of light that instruments can measure, a technique used in some medical and scientific equipment. In medicine, a radioactive isotope, thallium-201, is used in small, controlled doses for nuclear imaging tests that assess blood flow to the heart muscle.

Thallium’s older use as an ingredient in rodent and insect poisons has largely been discontinued in most countries, precisely because it proved too dangerous to handle safely and too easy to misuse.

History

Thallium was discovered in 1861 by English chemist William Crookes, who noticed an unfamiliar bright green line while examining the spectrum of residue from sulfuric acid production — a striking signature that pointed to a previously unknown element. He named it thallium after the Greek word for a green shoot, describing that distinctive spectral line. Around the same period, French chemist Claude-Auguste Lamy worked out how to isolate enough of the actual metal to study its properties directly, and the two scientists’ overlapping work helped firmly establish thallium as a new element in the years that followed.