Table Of Elements

Iridium, a chemical element, one of the platinum metals of Group 8, of the periodic table. It is very dense and rare and is used in platinum alloys. A precious, silver-white metal, it is hard and very brittle, but it becomes ductile and can be worked at a white heat, from 1,200 degrees to 1,500 degrees Celsius. It is one of the densest terrestrial substances. In the massive state the metal is practically unbreakable in acids and is not attacked even by aquaregia. It can be dissolved in concentrated hydrochloric acid in the presence of sodium perch lorate at 125 degrees to 150 degrees Celsius.

      The element was discovered in 1802 in the acid-insoluble residues of platinum ores by the English chemist Smithson Tennant. The name iridium, comes from the Greek word iris "rainbow", refers to the various colors of its compounds. Natural iridium consists of a mixture of two stable isotopes. The chemistry of iridium centers on the oxidation state, though compounds of all states from 0 to 6 are known with perhaps the exception of +2. Iridium-containing ores are found in South Africa, Alaska, USA, as well as in Myonnor (Buram), Brarit, Russia, and Australia. In the late 20th century South Africa was the world's major producer of the element iridium.

      Because of difficulties in preparation and fabrication, the pure metal has few applications. lridium is mainly, used in the form of platinum alloys. Platinum-itidium alloys (5 to 10 percent iridium) are already workable metals that are much harder and stiffer, and also more resistant to chemical attack than the soft pure platinum. Such alloys are used for jewelry, pen points, surgical pens, and electrical contacts and sparking points.

      It has a face-centered cubic crystalline structure. The atomic number is 77, the atomic weight is 192.2, the melting point is 2,410 degrees Celsius, the boiling point is 4,527 ceases, the specific gravity is 22.4. Pure iridium probably does not occur in nature; its abundance in the earth's crust is very low, about 0.001 parts per mmbw. Though very rare, iridium does occur in natural alloys with other platnium metals. Iridium generally is produced commercially along with the other platinum metals as a by-product of nickel of copper production.

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