Thallium is a metal element denoted by the atomic symbol Tl. It has an atomic number of 81 and the average atomic weight of its isotopes is 204.3833. It is normally found as a bluish-gray metal that is so soft it can be cut with a knife. It has a melting point of 579.2°F (304°C) and a boiling point of about 2683.4°F (1473°C).

Thallium was discovered by the British physicist William Crookes while he was examining selenium and other compounds using a spectroscope. While examining the spectra of these elements he noticed a bright, lime green band that did not correspond to any known element. He named the element thallium after the Greek word thallos meaning "green twig." At about the same time, this element was isolated by the French chemist C. A. Lamy. Lamy announced his findings shortly after Crookes, and a brief controversy over priority ensued.

In its pure form, thallium is dangerous, particularly when in contact with skin. It is a suspected carcinogen. It is often used in electronic equipment, and one of its compounds, thallium sulfate, is used as a rodenticide and ant poison. Its use as a biocide, however, has been prohibited in the United States since 1975. Another compound, thallium bromide-iodide, is used in crystalline form as an infrared detector. Other thallium compounds are used in the manufacture of imitation diamonds, glass, and as a catalyst.