Bruger:Christian75/GS-77-Ir
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Iridium er det 77. grundstof i det periodiske system, og har det kemiske symbol Ir.. Det er et hårdt, skørt, sølvhvidt overgangsmetal og hører til platinmetalerne. Iridium er det grundstof med den næsthøjeste densitt (efter osmium) og det mest korrosions resistente metal, selv ved høje temperaturer som 2000 °C. Selvom kun visse smelter (smeltede alkali salte) og halogener er korosive overfor iridium på fast form, så er findelt iridium støv lang mere reaktivt og kan endog brænde. De vigtigste iridiumforbindelser er de salte og syrer som det danner med chlor, og iridium danner også et antal organometalliske forbindelser som bruges som katalysatorer og inden for forskning. 191Ir og 193Ir er de eneste stabile og de eneste naturligt forekommende isotoper af iridium. Den sidstnævnte er den mest udbredte af de to.
Iridium blev opdaget i 1803 af Smithson Tennant i London, England, blandt uopløselige urenheder fra platin
Iridium was discovered in 1803 by Smithson Tennant in London, England, among insoluble impurities in natural platinum from South America. Although it is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes, it has a number of specialized industrial and scientific applications. Iridium is employed when high corrosion resistance at high temperatures is needed, as in spark plugs, crucibles for recrystallization of semiconductors at high temperatures, electrodes for the production of chlorine in the chloralkali process, and radioisotope thermoelectric generators used in unmanned spacecraft. Iridium compounds also find applications as catalysts for the production of acetic acid.
An unusually high abundance of iridium in a clay layer of the K–T geologic boundary was a crucial clue that led to the theory that the extinction of dinosaurs and many other species 65 million years ago was caused by the impact of a massive extraterrestrial object—the so-called Alvarez hypothesis. Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. It is thought that the amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but because of the high density and tendency of iridium to bond with iron, most iridium descended below the crust and into the Earth's core when the planet was young and still molten.