Thallium's atomic number is 81 and its symbol is Tl. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Discovered using flame spectroscopy, in which thallium produces a notable green spectral line. Thallium, from Greek, thallós, meaning "green shoot" or "twig". 

Commercially, thallium is produced not from potassium ores, but as a byproduct from refining of heavy-metal sulfide ores. Approximately 60–70% of thallium production is used in the electronics industry, and the remainder is used in the pharmaceutical industry and in glass manufacturing. It is also used in infrared detectors.

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