Iridium is a chemical element with the chemical symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of 22. 56 g/cm3 (0. 815 lb/cu in) as defined by experimental X-ray crystallography. It is one of the most corrosion-resistant metals, even at temperatures as high as 2,000 °C (3,630 °F). However, corrosion-resistance is not quantifiable in absolute terms; although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable, whereas gold dust is not flammable but can be attacked by substances that iridium resists, such as aqua regia.

Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named it after the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in Earth's crust, with estimated annual production and consumption of only 7. 3 tonnes (16 thousand pounds) in 2018. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium, as well as the only stable isotopes; the latter is the more abundant.

The dominant uses of iridium are the metal itself and its alloys, as in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Important compounds of iridium are chlorides and iodides in industrial catalysis. Iridium is a component of some OLEDs.

Iridium is found in meteorites in much higher abundance than in the Earth's crust. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago, now known to be produced by the impact that formed the Chicxulub crater. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2. 5 million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.

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Atomic properties

Standard atomic weight192.217 ±0.002
Atomic mass192.217 u

Atomic radii

Radius (empirical)136 pm
Radius (calculated)180 pm
Covalent radius141 ±6 pm

Atomic shell

Electron configurationXe 4f14 5d7 6s2
Ionization energy(1st) 8.96702 eV
(2nd) 17 eV
(3rd) 28 eV
(4th) 40 eV
(5th) 57 eV
Shell model

Physical properties

Density22.56 g·cm−3
Molar volume8.52·10-6 m3·mol−1
Speed of sound4,825 m·s−1


Melting point2,719 K
Boiling point4,403 K
Liquid range1,684 K
Transition temperature0.1 K


Melting enthalpy26 kJ·mol-1
Enthalpy of vaporization560 kJ·mol-1
Binding energy671 kJ·mol-1

Heat and conductivity

Thermal conductivity150 W·m-1·K-1
Expansion coefficient6.4·10-6 K-1


Mohs hardness6.5
Brinell hardness1,670 NM·m-2
Vickers hardness1,760 NM·m-2

Elastic properties

Young’s modulus528 GPa
Shear modulus210 GPa
Bulk modulus320 GPa
Poisson’s ratio0.26

Electrical properties

Electrical conductivity1.97·107 S·m-1
Resistance4.699·10-8 Ωm


Magnetic susceptibility2.56·10-5 cm3·mol−1 (298 K)

Optical properties

Reflectivity78 %

Chemical properties

Basicityslightly basic
Oxidation state-3, -1, 0, 1, 2, 3, 4, 5, 6, 8, 9
Standard potential1.156 V (Ir3+ + 3e- → Ir)


Pauling scale2.2
Allred-Rochow scale1.55
Nagle scale1.43
Pearson absolute negativity5.4 eV

Other properties

Natural occurrenceprimordial
Crystal structureFace-centered cubic
Goldschmidt Classificationsiderophile
Superconductorwith transition tempperature (solid body, normal pressure)
Price/kg55,850 ±350 USD

Natural abundances

2 ppb ≈ 2.01·1010 M☉
2 ppb ≈ 3.97·109 Mt
550 ppb ≈ 550 mg
Earth’s crust
0.4 ppb ≈ 11 Mt