thallium, Metallic chemical element, chemical symbol Tl, atomic number 81. It was discovered in 1861 through its green spectral line, and its name derives from the Greek word for “green shoot.” Thallium is a soft, low-melting element of low tensile strength. It dulls to bluish gray upon exposure to air. Thallium metal has no commercial use, and thallium compounds have no major commercial application. Thallium poisoning, which may be fatal, causes nervous and gastrointestinal disorders and rapid loss of hair.