iridium

iridium (ĭrĭd`ēəm), metallic chemical element; symbol Ir; at. no. 77; at. wt. 192.22; m.p. about 2,410°C;; b.p. about 4,130°C;; sp. gr. 22.55 at 20°C;; valence +3 or +4. Iridium is a very hard, usually brittle, extremely corrosion-resistant silver-white metal with a face-centered cubic crystalline structure. It falls between platinum and osmium in Group 9 of the periodic table. It is not certain whether osmium or iridium is the most dense element. Iridium is found uncombined in nature as the metal and in combination with osmium and platinum. It is obtained commercially from osmiridium, a byproduct of platinum production. The metal is used in pivot bearings and in scientific and other special equipment, such as surgical tools. It is also used in making chemical crucibles. Iridium is used principally in alloys. An alloy with osmium is used to make fountain-pen nibs. Alloys with platinum are used in heavy-duty electrical contacts. An alloy of 10 parts iridium with 90 parts platinum is used in the international kilogram standard in Paris. Formerly the international meter standard was the distance between two marks on a bar made of that same alloy; it is now based on the wavelength of a line in the spectrum of an isotope of krypton. Iridium is chemically very unreactive. Pure iridium metal is not attacked by acids or acid mixtures, not even by aqua regia, which dissolves gold. Fluorine and chlorine attack it only at a red heat. It is oxidized slowly at high temperatures. It resists attack by fused bases and by most molten metals. Iridium was discovered in 1804 by Smithson Tennant; it is so named because of its various highly colored salts.

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iridium

Metallic chemical element, one of the transition elements, chemical symbol Ir, atomic number 77. A very rare, precious, silvery white, hard, brittle metal that resists most acids, it is one of the densest substances known on Earth. It probably does not occur uncombined in nature but is found in natural alloys with other noble (i.e., chemically inactive or inert) metals. The pure metal is too hard to work with to have any significant uses; alloys with platinum are used in jewelry, pen points, surgical pins and pivots, electrical contacts and sparking points, and extrusion dies. The international prototype kilogram, the primary standard (see weights and measures) for weight, is made of an alloy comprising 90% platinum and 10% iridium. The discovery of abnormally high amounts of iridium in rocks dating to the boundary between the Cretaceous and Tertiary periods led to a much-debated hypothesis that an iridium-containing asteroid striking Earth led to a catastrophic chain of events including the extinction of dinosaurs and many other forms of life.

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(Iridium Satellite LLC, Bethesda, MD, www.iridium.com) A phone and pager service that provides global coverage using handheld phones and low-earth orbit (LEO) satellites. Data access is available at 2.4 Kbps and Internet access at 10 Kbps. Originally announced in 1990 and completed in 1998 at a cost of $6 billion, Iridium was the first global satellite phone service. It originally used cellular partners to provide standard cellphone service when satellites were not required. Iridium LLC went into Chapter 11 in August 1999, and in late 2000, Iridium Satellite LLC was formed to resurrect the system. See LEO and Globalstar.