Finely divided iridium dust is much more reactive and can be flammable. The bulk metal can tolerate temperatures as high as 2000 ☌ in a reducing atmosphere, although certain molten salts and halogens are corrosive to to it. Considering the abundance of oxygen in the universe, iridium is more likely to be incorporated into cosmic dust. Iridium does form two oxides with negative free energy of formation (exothermic), while gold's only known oxide has a positive free energy of formation (1),(2). It is also called the most corrosion-resistant metal, although there is a delicate point here. Iridium radioisotopes are used in some radioisotope thermoelectric generators. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in industrial catalysis and in research. Iridium is one of the rarest elements in Earth's crust, with annual production and consumption of only three tonnes. Smithson Tennant, the primary discoverer, named iridium for the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium was discovered in 1803 among insoluble impurities in natural platinum. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is generally credited with being the second densest element (after osmium) based on measured density, although calculations involving the space lattices of the elements show that iridium is denser. Iridium is a chemical element with symbol Ir and atomic number 77.
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