Atomic number: 77

Symbol: Ir

Category: transition metal

Density at 25°C: solid: density: 22.56 g·cm−3

The salts of this element are brightly coloured which led to the name iridium, the rainbow goddess. Iridium is mostly used in corrosion resistant alloys and high temperature situations such as spark plugs. It is slightly less dense than osmium.

Pieces of iridium. image: adapted from

More Iridium is found in meteorites than its average abundance in the Earth's crust. This makes it useful in geology suggesting a meteor impacted in Earth past causing a layer of iridium clay - the so-called Cretaceous–Paleogene boundary.

Use, natural applications or common combinations: 

  • spark plugs
  • alloys
  • high temperature crucibles
  • prototype of the standard metre built in  1889. 

Basic characteristics:

  • Standard atomic weight: 192.217(3)
  • Melting point: 2466 °C
  • Boiling point: 4428 °C
  • Radioactivity: almost all naturally stable
  • 192Ir is used for some radiotherapy and for checking metals. It has a halflife of 73 days emitting gamma rays.