Iridium

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IrCl3(aq)x

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Platinum symbol

Almirante Antonio de Ulloa

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, iridium is considered to be the second-densest metal (after osmium) with a density of 22.56 g/cm^3 as defined by experimental X-ray crystallography. It 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 is found in meteorites in much higher abundance than in the Earth's crust. Its presence in meteorites is one of the evidences for the theory that the dinosaurs and many other species were extinguished by a massive asteroid impact, with a high iridium content, about 65 million years ago. This event is known as the Cretaceous–Paleogene extinction event. Additionally, iridium is found in the Earth's crust only in very small quantities, with its abundance being estimated at 0.001 ppm. Therefore, iridium is one of the rarest elements in the Earth's crust.

Iridium's high melting point, hardness, and corrosion resistance make it a valuable material in a number of industrial applications. It is used in high-temperature apparatus, electrical contacts, and as a hardening agent for platinum. Due to its good resistance to corrosion, it is also used in the manufacture of crucibles and devices requiring high temperature resistance. Iridium is also used in the production of spark plugs and in radiotherapy for the treatment of cancer.

The discovery of iridium is attributed to Smithson Tennant in 1803. Tennant found iridium in the residue left when crude platinum was dissolved in aqua regia (a mixture of nitric acid and hydrochloric acid). The name iridium comes from the Latin word "iris", meaning rainbow, because of the various colors of its salts.

Properties and Compounds

Iridium is a member of the platinum group metals and has similar chemical properties to osmium. It is hard and brittle, making it difficult to machine, form, or work. Iridium compounds are highly diverse. Iridium(III) chloride (IrCl₃) and iridium(IV) oxide (IrO₂) are two of the most common compounds of iridium. The metal is resistant to attack by acids and by molten metals and is therefore used as a material for crucibles and other equipment that must withstand high temperatures.

Applications

Due to its high density and resistance to corrosion, iridium is used in high-temperature materials and devices. It is also used in durable electrical contacts, spark plugs, and as a catalyst for the chemical industry. Iridium's resistance to corrosion makes it an ideal material for the electrodes in electrolysis and for crucibles used in the growth of high-purity crystals. In addition, iridium radioisotopes are used in some types of radiotherapy for cancer treatment.

Occurrence and Production

Iridium is one of the least abundant elements in the Earth's crust. It is primarily obtained as a by-product of nickel and copper mining and processing. Due to its rarity, iridium is expensive, with its price dependent on market demand and production levels. The primary mining areas for iridium are South Africa, Russia, and Canada.

Environmental and Health Aspects

While metallic iridium is not considered harmful, iridium compounds should be handled with care as they can be toxic if ingested or inhaled. The handling of iridium compounds requires appropriate safety precautions to avoid exposure.