iridium (Ir)chemical element, one of the platinum metals of Groups 8–10 (VIIIb), Periods 5 and 6, of the periodic table. It is very dense and rare and is used in platinum alloys. A precious, silver-white metal, iridium is hard and brittle, but it becomes ductile and can be worked at a white heat, from 1,200° to 1,500° C (2,200° to 2,700° F). It is one of the densest terrestrial substances. In the massive state the metal is practically insoluble in acids and is not attacked even by aqua regia. It can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate at 125° to 150° C (257° to 302° F).

Because of difficulties in preparation and fabrication, the pure metal has few applications. Iridium is chiefly used in the form of platinum alloys. Platinum-iridium alloys (5 to 10 percent iridium) are readily workable metals that are much harder and stiffer and more resistant to chemical attack than the soft pure platinum. Such alloys are used for jewelry, pen points, surgical pins and pivots, and electrical contacts and sparking points. The international prototype standard kilogram of mass is made from an alloy containing 90 percent platinum and 10 percent iridium.

Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal with a density of 22.56 g/cm³ as defined by experimental X-ray crystallography.

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