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In 1861, thallium was discovered by accident in a residue of a sample that had been used to study
tellurium and
selenium.
Rather than discarding the residue, it was studied with a
spectroscope. Instead of seeing the blue selenium lines and
the yellow tellurium lines, a brilliant green line was observed. The element was named thallium, meaning
green branch.
Thallium is very much like
lead in appearance. But unlike lead, thallium reacts fairly rapidly with
oxygen and moisture in the air. Thallium is found in lead and cadmium ores.
Thallium's uses in metallic form are chiefly in alloys with
bismuth,
cadmium, and
mercury. Since it is quite poisonous, its compounds (i.e. thallium(I) sulfate
(Tl2SO4)) have been used in rodenticides and various insecticides. It also
has applications in photocells, infrared detectors, and low-melting glasses. Mixture of thallium(I) iodide, TlI,
and thallium(I) bromide, TlBr, are used in the manufacture of glass for special infrared lenses.
