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Name , symbol , atomic number Iridium, Ir, 77
Element category Transition metals
Group , period , block 9 , 6 , d
Appearance silvery white
CAS number 7439-88-5
EC number 231-095-9
ECHA InfoCard 100.028.269
Mass fraction of the earth's envelope 0.001 ppm
Atomic mass 192.217 (3) and
Atomic radius (calculated) 135 (180) pm
Covalent radius 141 pm
Electron configuration [ Xe ] 4 f 14 5 d 7 6 s 2
1. Ionization energy 8th.96702 (22) eV865.19 kJ / mol
2. Ionization energy 17th.0 (3) eV1 640 kJ / mol
3. Ionization energy 28.0 (1.6 eV)2 700 kJ / mol
4. Ionization energy 40.0 (1.7 eV)3 860 kJ / mol
5. Ionization energy 57.0 (1.9) eV5 500 kJ / mol
Physical state firmly
Crystal structure Cubic area-centered
density 22.56 g / cm 3
Mohs hardness 6.5
magnetism paramagnetic ( Χ m = 3.8 10 −5 )
Melting point 2739 K (2466 ° C)
boiling point 4403 K (4130 ° C)
Molar volume 8.52 · 10 −6 m 3 · mol −1
Heat of evaporation 564 kJ / mol
Heat of fusion 26 kJ mol −1
Speed ​​of sound 4825 m s −1
Electric conductivity 19.7 · 10 6 A · V −1 · m −1
Thermal conductivity 150 W m −1 K −1
Oxidation states −3, −1, 0, 1, 2, 3 , 4 , 5, 6, 8, 9
Normal potential 1.156 V (Ir 3+ + 3e - → Ir)
Electronegativity 2.20 ( Pauling scale )
isotope NH t 1/2 ZA ZE (M eV ) ZP
189 Ir {syn.} 13.2 d ε 0.532 189 Os
190 Ir {syn.} 11.78 d ε 2,000 190 Os
191 Ir 37.3% Stable
192 Ir {syn.} 73.830 d β - 1,460 192 pt
ε 1.046 192 Os
192 m Ir {syn.} 241 a IT 0.155 192 Ir
193 Ir 62.7  % Stable
194 Ir {syn.} 19.15 h β - 2.247 194 pt
195 Ir {syn.} 2.5 h β - 1.120 195 pt
For other isotopes see list of isotopes
NMR properties
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
191 Ir 3/2 4.812 · 10 6 1.09 · 10 −5 1.72
193 Ir 3/2 5.227 · 10 6 2.34 · 10 −5 1.87
safety instructions
GHS labeling of hazardous substances


02 - Highly / extremely flammable 07 - Warning


H and P phrases H: 228
P: 210
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Iridium is a chemical element with the symbol Ir and the atomic number 77. It is one of the transition metals , in the periodic table it is in group 9 (in the older counting part of the 8th subgroup ) or cobalt group . The very heavy, hard, brittle, silver-white shiny precious metal from the group of platinum metals is considered to be the most corrosion-resistant element. Below 0.11 Kelvin it changes to a superconductive state.


Iridium ( ancient Greek ἰριοειδής irio-eides "rainbow -like " after the many colors of its compounds) was discovered in London in 1804 by Smithson Tennant together with osmium . When raw platinum was dissolved in aqua regia , both platinum metals were found in the insoluble black residue. The variety of colors of the iridium salts inspired Tennant to the name Iridium. The “ original kilogram ” and the “ original meter ” also consist of an iridium alloy; Both have been kept in Paris in the Bureau International des Poids et Mesures since 1898 .


Iridium is rarer than gold or platinum . After rhenium, together with rhodium and ruthenium, it is the rarest non-radioactive metal. Its proportion in the continental crust is only 1  ppb . In nature it occurs elementally in the form of small grains or accompanied by platinum. With osmium it forms two naturally occurring minerals:

Important deposits are in South Africa , the Urals , North and South America , Tasmania , Borneo and Japan .

Free iridium as well as other elements of the platinum group are found in river sands. In addition, iridium is produced in the smelting of nickel ores .


Iridium 8.3 g arc melted
Iridium foil

Physical Properties

Because of its hardness and brittleness , iridium is difficult to work with. In the naturally occurring isotopic composition, iridium is the second most dense element after osmium .

Chemical properties

In red heat, iridium oxidizes incompletely to black IrO 2 , which breaks down again above 1140  ° C. Like osmium, iridium is also volatile when exposed to heat and especially when the oxygen content is higher than oxide IrO 3 . In contrast to osmium, however, it is deposited again as metal or IrO 2 in cold spots . In powder form, it is a flammable solid that can be easily ignited by brief exposure to an ignition source. The more finely the substance is distributed, the greater the risk of ignition. In compact form, it is not flammable. It is stable in mineral acids , including aqua regia . In chloride melts, however, it is converted to double chlorides in the presence of chlorine , e.g. B. Na 2 [IrCl 6 ].


There are two natural isotopes of iridium, 34 radioisotopes and 21 core isomers , of which the core isomer 192m2 Ir is the most stable with a half-life of 241 years. It decays through internal conversion to 192 Ir, which is the isotope with the longest half-life with a half-life of 73.831 days. 192 Ir decays as a beta emitter to the platinum isotope 192m Pt, most of the others to osmium . The remaining isotopes and core isomers have half-lives between 300 µs at 165 Ir and 11.78 days at 190 Ir.

Because of its gamma radiation with an energy of about 550 keV (kiloelectron volt), 192 Ir is suitable for radiographic testing of components. For workpieces with a wall thickness of more than 20 mm, this isotope is usually used (regulated by standards, see e.g. DIN EN ISO 5579).

For non-destructive testing (NDT), the iridium emitter is usually welded in the form of a 2–3 mm large tablet in an emitter holder, and this is housed in a lockable type B work container, which is lined with depleted uranium to shield the gamma radiation .

Work containers for iridium emitters have the following dimensions: 20 cm long, 10 cm wide and 15 cm high. Due to the uranium jacket, the weight is around 13 to 20 kg, depending on the activity.

List of iridium isotopes


Iridium is often a component of alloys , which it gives hardness and / or brittleness. Platinum-iridium alloys are used for precision measurements, in medicine and mechanical engineering .

It is also used:

Cretaceous-Paleogene Impact

Iridium occurs in a relatively high concentration in that 66 million year old sediment layer that separates the chronostratigraphic systems of the Cretaceous and the Paleogene , and is considered an indication of a large meteorite impact, which, according to predominant scientific opinion, about 75 percent of the species of that time in addition to the dinosaurs fell victim; see also Iridium anomaly , Chicxulub crater and Cretaceous-Paleogene boundary .

safety instructions

Metallic iridium is non-toxic because of its durability. As a powder or dust it is highly flammable, in compact form it is non-combustible. Iridium compounds must be classified as toxic.


Many iridium salts are colored: with chlorine it forms olive-green iridium (III) chloride or dark blue-black, not quite defined iridium (IV) chloride . It reacts with fluorine to form yellow, volatile iridium (VI) fluoride or yellow-green iridium (V) fluoride . The oxidation state + IX predicted for Iridium years ago was also confirmed experimentally in 2014 by the synthesized [IrO 4 ] + . It is the only element in which this oxidation state is known in a compound. In addition to the oxides and halides of iridium, numerous octahedral, diamagnetic iridium (III) complexes, such as amine complexes and chloro complexes, are known. The reduction of iridium (III) chloride in alcohol in the presence of π-acceptor ligands leads to square planar iridium (I) complexes, of which the Vaskas complex is best studied. By reducing iridium (III) chloride with carbon monoxide , one obtains the iridium carbonyls like the canary yellow [Ir 4 (CO) 12 ] and the red [Ir 6 (CO) 16 ]. Iridium complexes can be used in homogeneous catalysis, especially in hydrogenation reactions.


Web links

Wiktionary: Iridium  - explanations of meanings, word origins, synonyms, translations
Commons : Iridium  - album with pictures, videos and audio files

Individual evidence

  1. ^ Harry H. Binder: Lexicon of the chemical elements. S. Hirzel Verlag, Stuttgart 1999, ISBN 3-7776-0736-3 .
  2. The values ​​for the properties (info box) are taken from (Iridium) , unless otherwise stated .
  3. CIAAW, Standard Atomic Weights Revised 2013 .
  4. a b c d e Entry on iridium in Kramida, A., Ralchenko, Yu., Reader, J. and NIST ASD Team (2019): NIST Atomic Spectra Database (ver. 5.7.1) . Ed .: NIST , Gaithersburg, MD. doi : 10.18434 / T4W30F ( ). Retrieved June 13, 2020.
  5. a b c d e entry on iridium at WebElements, , accessed on June 13, 2020.
  6. JW Arblaster: Densities of osmium and iridium. In: Platinum Metals Review. 33, 1, 1989, pp. 14-16 ( full text ; PDF; 209 kB).
  7. Robert C. Weast (Ed.): CRC Handbook of Chemistry and Physics . CRC (Chemical Rubber Publishing Company), Boca Raton 1990, ISBN 0-8493-0470-9 , pp. E-129 to E-145. Values ​​there are based on g / mol and given in cgs units. The value specified here is the SI value calculated from it, without a unit of measure.
  8. a b Yiming Zhang, Julian RG Evans, Shoufeng Yang: Corrected Values ​​for Boiling Points and Enthalpies of Vaporization of Elements in Handbooks. In: Journal of Chemical & Engineering Data . 56, 2011, pp. 328-337, doi: 10.1021 / je1011086 .
  9. a b Guanjun Wang, Mingfei Zhou, James T. Goettel, Gary G. Schrobilgen, Jing Su, Jun Li, Tobias Schlöder, Sebastian Riedel: Identification of an iridium-containing compound with a formal oxidation state of IX . In: Nature . tape 514 , August 21, 2014, p. 475-477 , doi : 10.1038 / nature13795 .
  10. a b c Entry on Iridium, powder in the GESTIS substance database of the IFA , accessed on April 12, 2020(JavaScript required) .
  11. David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 85th edition. CRC Press, Boca Raton, Florida, 2005. Section 14, Geophysics, Astronomy, and Acoustics; Abundance of Elements in the Earth's Crust and in the Sea.
  12. CERN Neutrino Factory Working Group Technical Note, accessed February 1, 2011.
  13. Michael J. Henehan, Andy Ridgwell, Ellen Thomas, Shuang Zhang, Laia Alegret, Daniela N. Schmidt, James WB Rae, James D. Witts, Neil H. Landman, Sarah E. Greene, Brian T. Huber, James R. Super, Noah J. Planavsky, Pincelli M. Hull: Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact . In: PNAS . 116, No. 43, October 2019. doi : 10.1073 / pnas.1905989116 .
  14. Steve Ritter: Iridium Dressed To The Nines - Periodic Table: IrO 4 + is the first molecule with an element in the +9 oxidation state . In: Chemical & Engineering News . 2014.
  15. Lexicon of Chemistry: Iridium Compounds - Lexicon of Chemistry , accessed on February 20, 2018