Iridium is a chemical element that has the symbol Ir and atomic number 77. A dense, very hard, brittle, silvery-white transition metal of the platinum family, iridium is used in high-strength alloys that can withstand high temperatures and occurs in natural alloys with platinum or osmium. Iridium is notable for being the most corrosion-resistant element known and for its significance in the determination of the probable cause of the extinction, by a meteorite strike, of the dinosaurs. It is used in high-temperature apparatuses, electrical contacts, and as a hardening agent for platinum.
A platinum group metal, iridium is white, resembling platinum, but with a slight yellowish cast. Due to its extreme hardness and brittleness, iridium is difficult to machine, form, or work. It is the most corrosion-resistant metal known: iridium cannot be attacked by any acids or by aqua regia, but it can be attacked by molten salts, such as NaCl and NaCN.
The measured density of iridium is only slightly lower than that of osmium, which is often listed as the densest element known. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22,650 kg/m³ for iridium versus 22,610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time.
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