![]() 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. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium, as well as the only stable isotopes the latter is the more abundant. Iridium is one of the rarest elements in Earth's crust, with annual production and consumption of only 3 tonnes (6.6 thousand pounds). Smithson Tennant, the primary discoverer, named it after 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. However, corrosion-resistance is not quantifiable in absolute terms although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable, whereas gold dust is not flammable but can be attacked by substances that iridium resists, such as aqua regia. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 ☌ (3,630 ☏). A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of 22.56 g/cm 3 (0.815 lb/cu in) as defined by experimental X-ray crystallography. Iridium is a chemical element with the symbol Ir and atomic number 77.
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