Thallium is a chemical element with the symbol Tl and atomic number 81.  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.  Thallium tends to form the +3 and +1 oxidation states.  The +3 state resembles that of the other elements in group 13 (boron, aluminium, gallium, indium).  However, the +1 state, which is far more prominent in thallium than the elements above it, recalls the chemistry of alkali metals, and thallium(I) ions are found geologically mostly in potassium-based ores, and (when ingested) are handled in many ways like potassium ions (K+) by ion pumps in living cells.  Commercially, thallium is produced not from potassium ores, but as a byproduct from refining of heavy-metal sulfide ores.  Approximately 65% 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.  The radioisotope thallium-201 (as the soluble chloride TlCl) is used in small amounts as an agent in a nuclear medicine scan, during one type of nuclear cardiac stress test.