iridium, 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.

Atomic Number:	Atomic Number: 77
Atomic Symbol:	Atomic Symbol: Ir
	Name of Element: Iridium
Atomic Weight:	Atomic Weight: 192.22
Electron Configuration:	Electron Configuration: 2 · 8 · 1832 · 15 · 2

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 90percnt platinum and 10percnt 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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