The Element Iridium
Atomic Number: 77
Atomic Weight: 192.217
Melting Point: 2719 K (2446°C or 4435°F)
Boiling Point: 4701 K (4428°C or 8002°F)
Density: 22.42 grams per cubic centimeter
Phase at Room Temperature: Solid
Element Classification: Metal
Period Number: 6
Group Number: 9
Group Name: none
What's in a name? From the Latin word for rainbow, iris.
Say what? Iridium is pronounced as i-RID-ee-em.
History and Uses:
Iridium and osmium were discovered at the same time by the British chemist Smithson Tennant in 1803. Iridium and osmium were identified in the black residue remaining after dissolving platinum ore with aqua regia, a mixture of 25% nitric acid (HNO3) and 75% hydrochloric acid (HCl). Today, iridium is still obtained from platinum ores and as a by-product of mining nickel.
Pure iridium is very brittle and is nearly impossible to machine. It is primarily used as a hardening agent for platinum. Platinum-iridium alloys are used to make crucibles and other high temperature equipment. Iridium is also alloyed with osmium to make the tips of fountain pens and compass bearings.
Iridium is the most corrosive resistant metal known. For this reason, the standard meter bar was created from an alloy of 90% platinum and 10% iridium. This bar was replaced as the definition of the meter in 1960 when the meter was redefined in terms of the orange-red spectral line of krypton-86.
A thin, worldwide layer of iridium exists in a layer of sediment that was put down at the end of the Cretaceous period. Since meteors and asteroids contain a higher percentage of iridium than the earth's crust, this iridium enriched layer is seen as evidence that the earth was struck by a large meteor or asteroid at that time. Dust from the impact would have spread around the globe, depositing the iridium. The dust also would have blocked the sun for a time, resulting in the extinction of many plant and animal species, including the dinosaurs.
Estimated Crustal Abundance: 1×10-3 milligrams per kilogram
Estimated Oceanic Abundance: Not Applicable
Number of Stable Isotopes: 2 (View all isotope data)
Ionization Energy: 9.1 eV
Oxidation States: +4, +3
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d7