Editing Period 6 element (section)

===Iridium===
{{main|Iridium}}
'''Iridium''' is the [[chemical element]] with [[atomic number]] 77, and is represented by the symbol '''Ir'''. A very hard, brittle, silvery-white [[transition metal]] of the [[platinum group|platinum family]], iridium is the second-[[density|densest]] element (after [[osmium]]) and is the most [[corrosion]]-resistant metal, even at temperatures as high as 2000&nbsp;°C. Although only certain molten salts and [[halogen]]s are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.

Iridium was discovered in 1803 among insoluble impurities in natural [[platinum]]. [[Smithson Tennant]], the primary discoverer, named the iridium for the goddess [[Iris (mythology)|Iris]], personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is [[Abundance of elements in Earth's crust|one of the rarest elements]] in the [[Crust (geology)#Earth's crust|Earth's crust]], with annual production and consumption of only three [[tonne]]s. {{chem|191|Ir}} and {{chem|193|Ir}} are the only two naturally occurring [[isotope]]s of iridium as well as the only [[stable isotope]]s; the latter is the more abundant of the two.

The most important iridium compounds in use are the salts and acids it forms with [[chlorine]], though iridium also forms a number of [[organometallic compound]]s used in industrial [[catalysis]], and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-end [[spark plug]]s, [[crucible]]s for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the [[chloralkali process]]. Iridium radioisotopes are used in some [[radioisotope thermoelectric generator]]s.

Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. For this reason the unusually high abundance of iridium in the clay layer at the [[Cretaceous–Paleogene boundary]] gave rise to the [[Alvarez hypothesis]] that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66&nbsp;million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum group metals, the high density and [[Goldschmidt classification#Siderophile elements|tendency]] of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.