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 weight | 192.217(2) |
| Discoverer (year) | Tennant, Smithson (1803) |
| Natural form | metallic solid (face centered cubic) |
| Electron configuration | [Xe] 4f 14 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/cm3 | 22.4 |
| 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 atm | 0.13 |
| Thermal conductivity (W/cm·K) at 25ºC and 1 atm | 1.470 |
| Oxidation state | +4, +3 |
| Electron affinity (eV) | 1.56 |
| 1st Ionization potential (eV) | 8.9670 |
