Iridium is the rarest naturally occurring element on Earth. There is one iridium atom in every billion atoms in the rocks of the Earth. This dense metal can be found in its pure form in nature, as well as in other common minerals containing nickel and copper. Iridium is present in meteorites and other space rocks. A layer of iridium-rich clay is found in the Earth's crust around the world, especially in the Badlands of South Dakota, U.S.A. Scientists believe that this small amount of iridium in our planet's crust was deposited by dust from an explosion 66 million years ago when a large meteorite impacted our planet. Applications for this element include the mirror coating of orbiting X-ray telescopes. Iridium is more durable than platinum and copper and is therefore preferred over these metals for use in spark plugs. Iridium is also mixed with osmium to make an alloy called osmiridium, which is used in compasses and put on the tips of some fountain pens to make them strong.

Summary of properties (Ir)

Atomic weight192.217(2)
Discoverer (year)Tennant, Smithson (1803)
Natural formmetallic solid (face centered cubic)
Electron configuration[Xe] 414 5d7 6s2
Melting point (ºC)2410
Boiling point (ºC)4527
Abundance in earth's crust (ppm)0.001
Isotope (abundance)191Ir (37.3), 193Ir (62.7)
Density g/cm322.4
Van der Waals radius (pm)213
Covalent radius (pm)132
Electronegativity (Pauling)2.2
Vaporization enthalpy (kJ/mol)563.60
Enthalpy of fusion (kJ/mol)41.12
Specific heat capacity (J/g·K) at 25ºC and 1 atm0.13
Thermal conductivity (W/cm·K) at 25ºC and 1 atm1.470
Oxidation state+4, +3
Electron affinity (eV)1.56
1st Ionization potential (eV)8.9670

Back to the Periodic Table of the Elements.