Iridium
Symbol: Ir
Atomic Number: 77
Atomic Mass: 192.217 amu (atomic mass units)
Melting Point: 2410.0 C (2683.15 K, 4370.0 F)
Boiling Point: 4527.0 C (4800.15 K, 8180.6 F)
Number of Protons: 77
Number of Electrons: 77
Number of Neutrons: 115
Classification: Transition Metal or Platinum Group Metal or Precious Metal
Crystal Structure: Cubic
Density @ 293 K: 22.5 grams per cubic centimeter
Color: White with a slight yellowish cast
Iridium was discovered at the same time as another transition metal element osmium. In 1803 the British chemist Smithson Tennant was examining the black residue left after dissolving platinum ore with a mixture of hydrochloric and nitric acids known as aqua regia. This residue was found to contain both iridium and osmium. Iridium salts are multicolored and it was for this reason the element was named after the Latin name for rainbow “iris”.
Iridium and osmium both belong to a group of elements known as the Platinum Group Metals (PGM). The other metals belonging to this group are platinum, ruthenium, palladium and rhodium. Iridium is a very hard and brittle metal which means it is very difficult to machine or mold. It is not attacked by acids but can be attacked by molten salts such sodium chloride and sodium cyanide.
There are two stable isotopes of iridium which make up the total of the naturally occurring element. Iridium-193 is the most abundant at 62.7% the remainder being made up of iridium-191. There are over fifty unstable isotopes of iridium that have been recognized. The half lives of these unstable isotopes range from less than one microsecond to 241 years.
Iridium is obtained as a by-product from the refining of nickel and platinum. It has a few industrial uses.
* It is used as a hardening alloy for platinum.
* Platinum-iridium alloys are used to make laboratory crucibles and other equipment used at high temperatures.
* The standard meter rule kept in Paris was made of 90% platinum and 10% iridium. This rule was replaced in 1960 by a measurement of the spectral lines produced by krypton-86.
* Iridium-osmium alloys are used to make nibs for fountain pens and compass bearings.
* It is used to make electrical contacts.
There is a thin layer of iridium rich soils world wide found at the top of the cretaceous period in geological history. Paleontologists use this as support of the theory that the dinosaurs and many other life forms were wiped out by a meteorite hitting the earth at that time. Crustal rocks on earth contain on average 0.4 parts per billion of iridium by weight while carbonaceous meteorites can contain 550 parts per billion of iridium.
