Sir William Crookes discovered thallium in 1861, positively identifying it by a bright green line in its spectrum (hence the name, which is from the Greek, thallos, for "green twig"). Although in appearance thallium resembles lead, it does not have the corrosion resistance of lead and so has few commercial applications.
Thallium has the chemical symbol Tl and atomic number 81. It has the electron configuration [Xe] 6s26p1 and has a +3 or +1 oxidation state. As stated above, because thallium is heavy, it has a greater stability in the +1 oxidation state (inert pair effect). Therefore, it is found more commonly in its +1 oxidation state. Thallium is soft and malleable.
Thallium compounds are quite toxic and some have been used as rat poisons. A few compounds are used in glasses for special infra-red lenses. Because of its toxicity, thallium was widely used in insecticide and rat poison until this usage was prohibited in 1975 in the U.S.
Stephen R. Marsden (ChemTopics)