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Iridium Rh ? Ir ? Mt osmium ? iridium ? platinum Appearance silvery white General properties Name, symbol, number iridium, Ir, 77 Pronunciation /?'r?di?m/ i-RID-ee-?m Element category transition metal Group, period, block 9, 6, d Standard atomic weight 192.217 Electron configuration 4f14 5d7 6s2 2, 8, 18, 32, 15, 2 Physical properties Phase solid Density (near r.t.) 22.56 g·cm-3 Liquid density at m.p. 19 g·cm-3 Melting point 2739 K, 2466 °C, 4471 °F Boiling point 4701 K, 4428 °C, 8002 °F Heat of fusion 41.12 kJ·mol-1 Heat of vaporization 563 kJ·mol-1 Molar heat capacity 25.10 J·mol-1·K-1 Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2713 2957 3252 3614 4069 4659 Atomic properties Oxidation states -3,-1, 0, 1, 2, 3, 4, 5, 6 Electronegativity 2.20 (Pauling scale) Ionization energies 1st: 880 kJ·mol-1 2nd: 1600 kJ·mol-1 Atomic radius 136 pm Covalent radius 141±6 pm Miscellanea Crystal structure face-centered cubic Magnetic ordering paramagnetic Electrical resistivity (20 °C) 47.1 nO·m Thermal conductivity 147 W·m-1·K-1 Thermal expansion 6.4 µm/(m·K) Speed of sound (thin rod) (20 °C) 4825 m·s-1 Young's modulus 528 GPa Shear modulus 210 GPa Bulk modulus 320 GPa Poisson ratio 0.26 Mohs hardness 6.5 Vickers hardness 1760 MPa Brinell hardness 1670 MPa CAS registry number 7439-88-5 Most stable isotopes Main article: Isotopes of iridium iso NA half-life DM DE (MeV) DP 188Ir syn 1.73 d e 1.64 188Os 189Ir syn 13.2 d e 0.532 189Os 190Ir syn 11.8 d e 2.000 190Os 191Ir 37.3% 191Ir is stable with 114 neutrons 192Ir syn 73.827 d ß– 1.460 192Pt e 1.046 192Os 192m2Ir syn 241 y IT 0.161 192Ir 193Ir 62.7% 193Ir is stable with 116 neutrons 193mIr syn 10.5 d IT 0.080 193Ir 194Ir syn 19.3 h ß– 2.247 194Pt 194m2Ir syn 171 d IT  ? 194Ir v t e · r Iridium ( /?'r?di?m/ i-RID-ee-?m) is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named the iridium for the goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes. 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 of the two.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 metal is employed when high corrosion resistance at high temperatures is needed, as in high-end spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Iridium radioisotopes are used in some radioisotope thermoelectric generators.Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. For this reason the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 65 million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten. Cite error: There are tags on this page, but the references will not show without a {{Reflist}} template or a tag; see the help page.
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