Iridium (i-rid-ee-əm) 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. 191 Ir and 193 Ir 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. 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. A member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high melting point (the ninth highest of all elements), solid iridium is difficult to machine, form, or work, and thus powder metallurgy is commonly employed instead.It is the only metal to maintain good mechanical properties in air at temperatures above 1600 °C. Iridium has a very high boiling point (10th among all elements) and becomes a superconductor at temperatures below 0.14 K. Iridium's modulus of elasticity is the second highest among the metals, only being surpassed by osmium.This,together with a high modulus of rigidity and a very low figure for Poisson's ratio (the relationship of longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty. Iridium is the most corrosion-resistant metal known:it is not attacked by almost any acid, aqua regia, molten metals or silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanide and potassium cyanide,as well as oxygen and the halogens (particularly fluorine)at higher temperatures. Iridium has two naturally occurring, stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3% and 62.7%, respectively. At least 34 radioisotopes have also been synthesized, ranging in mass number from 164 to 199. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, with a half-life of 73.827 days, and finds application in brachytherapy. The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Native platinum used by ancient Ethiopians and by South American cultures always contained a small amount of the other platinum group metals, including iridium. Platinum reached Europe as platina ("small silver"), found in the 17th century by the Spanish conquerors in a region today known as the department of Chocó in Colombia. The discovery that this metal was not an alloy of known elements, but instead a distinct new element, did not occur until 1748. Iridium is one of the least abundant elements in the Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; gold is 40 times more abundant, platinum is 10 times more abundant, and silver and mercury are 80 times more abundant.Tellurium is about as abundant as iridium, and only three naturally occurring elements are less abundant: rhenium, ruthenium, and rhodium, iridium being 10 times more abundant than the last two. Iridium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich). Iridium–osmium alloys were used to tip fountain pen nibs. The first major use of iridium was in 1834 in nibs mounted on gold. Since 1944, the famous Parker 51 fountain pen was fitted with a nib tipped by a ruthenium and iridium alloy (with 3.8% iridium). The tip material in modern fountain pens is still conventionally called "iridium," although there is seldom any iridium in it; other metals such as tungsten have taken its place. References: ^ J. W. Arblaster: Densities of Osmium and Iridium, in: Platinum Metals Review, 1989, 33, 1, S. 14–16; Volltext. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth– Heinemann. pp. 1113–1143, 1294. ISBN 0-7506- 3365-4. OCLC 37499934 41901113 213025882 37499934 41901113. ^ Hunt, L. B. (1987). "A History of Iridium". Platinum Metals Review 31 (1): 32–41. ^ Kittel, C. (2004). Introduction to Solid state Physics, 7th Edition. Wiley-India. ISBN 81-265-1045-5. ^ Arblaster, J. W. (1995). "Osmium, the Densest Metal Known". Platinum Metals Review 39 (4): 164. ^ Cotton, Simon (1997). Chemistry of Precious Metals. 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