Iridium (Ir)

Iridium is a chemical element with atomic number 77 in the periodic table. In Earth’s crust, this precious metal occurs in scarce amounts. As a member of the platinum family of periodic table elements, iridium is characterized by a high density and hardness. It has nine valence electrons that can occur in the oxidation states +1, +3, and +4. 

Chemical and Physical Properties of Iridium

Atomic number77
Atomic weight192.22 g.mol-1
GroupTransition Metal
ColorA lustrous, silver-white metal with a yellowish hue
Physical stateSolid at room temperature
Density22.42 at 20°C
Melting point2450°C
Boiling point4527°C
Ionic radius0.066 nm (+4)
Van der Waals radius0.126 nm
Most characteristic isotope192Ir
Electronic shell[Xe] 4f14 5d7 6s2
The energy of the first ionization886 kJ.mol-1
Crystal StructureCubic: Face centered
Covalent Radius1.27 Å
Atomic Radius1.87 Å
Atomic Volume08.54 cm³/mol
Name OriginLatin: iris (rainbow)
DiscovererS.Tenant, A.F.Fourcory, L.N.Vauquelin, H.V.Collet-Descoltils
Oxidation States2,3,(4),6
UsesUsed with osmium to tip gold pen points, to make crucible and special containers, and in heat-resistant alloys
DescriptionRare, very heavy, soft, silvery-white metal

Located between osmium and platinum in the periodic table, the chemical element iridium is labeled with the symbol Ir and the atomic number 77. It has an atomic mass of 192.2 g.mol-1 and electron configuration [Xe] 4f14 5d7 6s2. Iridium is the hardest substance according to the classification of elements within Mendeleev’s system.

This member of the platinum family is also very brittle and dense in its metal form. However, when exposed to temperatures between 1,200° and 1,500° C (2,200° to 2,700° F), iridium becomes ductile and malleable. Iridium reaches a boiling point at 4527°C, while its melting point is at 2450°C. 

Iridium has a specific gravity of 22.42 (17°C), an electronegativity of 2.2 according to Pauling, and the atomic radius according to van der Waals is 0.126 nm. Its crystal structure is cubic-face centered. Being non-reactive with water, acids, silicates, molten metals, or air, this transition metal is able to resist corrosion better than any other metal in the periodic table of elements. Iridium is unaffected even by aqua regia. Ruthenium and osmium are the only elements that show lower reactivity than iridium.  


How Was Iridium Discovered?

Ancient Ethiopians and various peoples from the South American continent were familiar with a type of platinum ore that contained traces of several chemical elements that are classified in the platinum family of the periodic table, such as ruthenium, rhodium, palladium, osmium, and iridium.

In the 17th century, this ore was introduced in Europe by the Spanish conquistadors, who had learned about – what was locally referred to as platina or “silverette” – on their South American exploits on the territory of today’s western Colombia.

Smithson Tennant and the Discovery of Iridium

The story of iridium’s discovery takes place in London in the year 1803. The English chemist Smithson Tennant (1761 – 1815) made a revolutionary discovery of two new chemical elements – iridium (Ir) and osmium (Os) – by conducting a chemical analysis on the acid-insoluble black residues from these platinum ore solutions. 

The 300-Pound Cape of Good Hope Iron Meteorite 

Prior to this, Tennant’s curiosity was sparked by the analysis of a 300-pound Cape of Good Hope iron meteorite in 1801. In the meteorite analysis he conducted, Tennant exposed the sample to acid which resulted in graphite (plumbago). 

In 1803, Tennant attempted an individual experiment on a crude platinum sample. By diluting it in aqua regia, the British scientist obtained black residue from the platinum ore solution. Since he previously managed to isolate graphite during the meteorite experiment, Tennant disagreed with his French colleague Joseph Louis Proust (1754 – 1826) who thought that the black residue from the platinum ore submerged in aqua regia was graphite, i.e. the crystalline form of carbon. In order to prove his claim, Tennant exposed the solution to alkalis and marine (hydrochloric) acid and purified the platinum ore, which helped him isolate two new chemicals – iridium and osmium. 

On June 21, 1804, Tennant submitted his scientific evidence in a letter addressed to the Royal Society of London for Improving Natural Knowledge (today’s Royal Society) after which the new element iridium was officially recognized and assigned the atomic number 77 in the periodic table. 

It should be noted that the French chemists H.V. Collet-Descotils, A.F. Fourcroy, and N.L. Vauquelin attempted performing similar experiments to Tennant’s. However, they failed in isolating iridium from the platinum ore solution residue. 

Robert Hare’s Contribution

Robert Hare (1781 – 1858), an American chemist and professor of chemistry at the University of Pennsylvania, is considered to be the first scientist who managed to distill iridium in its pure form. 

How Did Iridium Get Its Name?

The name of this chemical element was inspired by the Greek goddess of the rainbow, Iris (also derived from the Latin word for rainbow, ‘iris’). The British chemist Smithson Tennant, who was a professor at Cambridge, gave the name iridium to his newly discovered element in 1804 after finding a striking resemblance of the vivid colors of the salts that were produced under bright lights with the colors of the rainbow. 

Where Can You Find Iridium?

In the Universe, a large percent of iridium occurs in meteors and asteroids. In fact, they contain larger amounts of the 77th element than Earth’s crust. The sediment in the layers that do contain some iridium are traced to the end of the Cretaceous period

In fact, some scientists believe that during the asteroid impact which led to the extinction of the dinosaurs, element 77 was disseminated throughout our planet along with an incredible amount of climate-changing gases that were released into the atmosphere upon impact. In this way, iridium was deposited into the alluvium formations. The very thin layer of iridium in Earth’s crust that dates from the Cretaceous period is considered to be evidence of a collision between Earth and an asteroid. 

In the alluvial iridium deposits of the Earth’s thinnest layer (i.e. its crust), this chemical element may either occur in its free elemental form or compounded with platinum or other noble metals that share similar chemical properties (like the iridium-osmium alloy, i.e.osmiridium). 

For commercial use, iridium is obtained as a by-product in the mining of nickel (sulfides), platinum, copper, and nickel ores. Brazil, Australia, the United States, Myanmar, Russia, and South Africa are where the world’s largest iridium ore mines are located. 

Iridium in Everyday Life

The pure, elemental form of iridium metal is not easy to be worked with due to its hardness and brittleness. That’s why this substance is mainly used in platinum alloys, which are used for various applications in different industries, in medicine, and in scientific research:

  • Platinum-iridium alloys are used to make crucibles resistant to extremely high temperatures, as well as other high-temperature equipment.  Their high tolerance to heat allows them to serve industrial processes and chemical experiments in which the substances are to be exposed to high heat; 
  • When alloyed with osmium, iridium is used in the manufacturing of the tips of fountain pens (working on the capillary pipe principle), pen points, as well as compass gadgets;
  • Compounded with platinum, iridium makes the metal even harder and more resistant to corrosion. It’s often used to make jewelry, surgical pins, compass bearings, and pivots in this form;
  • Due to its high melting point and low reactivity, iridium is also applied in the production of spark plugs and electrical contacts which are further used as devices that ignite the fuel to combustion;
  • When used in the manufacturing of optical lenses, iridium helps with the reduction of glare;
  • The radioactive isotope iridium-192 finds its application in medicine as a part of a type of radiotherapy called brachytherapy, as well as in industrial radiography;
  • The original standard meter bar (mètre étalon) that had been installed in Paris after the French Revolution in order for people to practice the new measurement system was made of 90% platinum and 10% iridium alloy. The standard meter bars were used until the 1960s to measure the SI standard of length. After that, an international agreement was made that the meter, as the fundamental unit of length, should be defined in terms of the orange-red spectral line of the krypton-86 isotope. 

The Use of Platinum-Iridium Alloy

The platinum-iridium alloy is a compound of the two noble metals. The addition of iridium to the alloy allows it to possess platinum’s chemical stability while gaining more hardness from iridium. This makes the alloy suitable for use in the production of jewelry, especially in the manufacturing of watch cases.

The combination of these two precious metals is also beneficial for medicinal purposes. Namely, due to the excellent mechanical and electrochemical properties of this alloy, it is used in the production of metal microelectrodes for electrical stimulation of the nervous tissue during medical examinations that require electrophysiological recordings. 

The Use of Iridium Nanorods in Medicine

Cyclo-metallic iridium nanorods also have significant uses in medicine. They’re used in chemotherapy and photodynamic therapy in order to act against the spreading of tumorous cells, thus eliminating the need for larger doses of cancer-fighting drugs. 

How Dangerous Is Iridium?

This member of the platinum family of elements is not considered to have a highly toxic effect. Plus, the iridium-192 isotope is used in nuclear medicine, as a part of radiotherapy of cervical, prostate, head, and neck cancers. 

Upon mishandling of the elemental form of iridium during industrial processes, uncontrolled exposure to this chemical may cause eye and skin irritation, or irritation of the digestive tract if ingested. Also, the powdered form of iridium is easily flammable and thus poses a significant fire hazard. 

Environmental Effects of Iridium

Iridium is not considered to be a biologically hazardous substance since it’s not reactive when it comes to contact with water, soil, or air. 

Isotopes of Iridium

The natural, elemental form of iridium consists of two stable isotopes – the iridium-191 (37.3 percent) and iridium-193 (62.7 percent). The other isotopes of iridium have an atomic mass that ranges from 164Ir to 202Ir. Among them, there are 34 radioactive isotopes of the element and many nuclear isomers. Having a half-life of 241 years, 192m2Ir is the most stable nuclear isomer of iridium.

Iridium-192 (192Ir) is an iridium radioisotope with a half-life of 73.83 days. It decays by emission of beta (β) particles and gamma (γ) radiation into 192Pt. Some of its β particles turn into 192Os during the decay process. This radioactive isotope of iridium is a product of neutron activation of the elemental form of the 77th element that occurs in nature. 

Additionally, the iridium-191 isotope is the first radioactive isotope of any known chemical element that has displayed the physical phenomenon called the Mössbauer effect, i.e. recoilless nuclear resonance fluorescence. 

Tabular representation of the iridium isotopes


[n 1]

ZNIsotopic mass (Da)[3]

[n 2][n 3]


[n 4]



[n 5]



[n 6][n 7]

Spin and


[n 8][n 4]

Natural abundance (mole fraction)
Excitation energy[n 4]Normal proportionRange of variation
164Ir7787163.99220(44)#1# ms  2−#  
165Ir7788164.98752(23)#50# ns (<1 µs)p164Os1/2+#  
α (rare)161Re
166Ir7789165.98582(22)#10.5(22) msα (93%)162Re(2−)  
p (7%)165Os
167Ir7790166.981665(20)35.2(20) msα (48%)163Re1/2+  
p (32%)166Os
β+ (20%)167Os
168Ir7791167.97988(16)#161(21) msα164Re(2-)  
β+ (rare)168Os
169Ir7792168.976295(28)780(360) ms

[0.64(+46−24) s]

β+ (rare)169Os
170Ir7793169.97497(11)#910(150) ms

[0.87(+18−12) s]

β+ (64%)170Oslow#  
α (36%)166Re
171Ir7794170.97163(4)3.6(10) s

[3.2(+13−7) s]

α (58%)167Re1/2+  
β+ (42%)171Os
172Ir7795171.970610(30)4.4(3) sβ+ (98%)172Os(3+)  
α (2%)168Re
173Ir7796172.967502(15)9.0(8) sβ+ (93%)173Os(3/2+,5/2+)  
α (7%)169Re
174Ir7797173.966861(30)7.9(6) sβ+ (99.5%)174Os(3+)  
α (.5%)170Re
175Ir7798174.964113(21)9(2) sβ+ (99.15%)175Os(5/2−)  
α (.85%)171Re
176Ir7799175.963649(22)8.3(6) sβ+ (97.9%)176Os   
α (2.1%)172Re
177Ir77100176.961302(21)30(2) sβ+ (99.94%)177Os5/2−  
α (.06%)173Re
178Ir77101177.961082(21)12(2) sβ+178Os   
179Ir77102178.959122(12)79(1) sβ+179Os(5/2)−  
180Ir77103179.959229(23)1.5(1) minβ+180Os(4,5)(+#)  
181Ir77104180.957625(28)4.90(15) minβ+181Os(5/2)−  
182Ir77105181.958076(23)15(1) minβ+182Os(3+)  
183Ir77106182.956846(27)57(4) minβ+ ( 99.95%)183Os5/2−  
α (.05%)179Re
184Ir77107183.95748(3)3.09(3) hβ+184Os5−  
185Ir77108184.95670(3)14.4(1) hβ+185Os5/2−  
186Ir77109185.957946(18)16.64(3) hβ+186Os5+  
187Ir77110186.957363(7)10.5(3) hβ+187Os3/2+  
188Ir77111187.958853(8)41.5(5) hβ+188Os1−  
189Ir77112188.958719(14)13.2(1) dEC189Os3/2+  
190Ir77113189.9605460(18)11.78(10) dβ+190Os4−  
192Ir77115191.9626050(18)73.827(13) dβ (95.24%)192Pt4+  
EC (4.76%)192Os
194Ir77117193.9650784(18)19.28(13) hβ194Pt1−  
195Ir77118194.9659796(18)2.5(2) hβ195Pt3/2+  
196Ir77119195.96840(4)52(1) sβ196Pt(0−)  
197Ir77120196.969653(22)5.8(5) minβ197Pt3/2+  
198Ir77121197.97228(21)#8(1) sβ198Pt   
199Ir77122198.97380(4)7(5) sβ199Pt3/2+#  
200Ir77123199.976800(210)#43(6) sβ200Pt(2-, 3-)  
201Ir77124200.978640(210)#21(5) sβ201Pt(3/2+)  
202Ir77125201.981990(320)#11(3) sβ202Pt(2-)  

Source: Wikipedia

List of Iridium Compounds

Most often, iridium forms oxides, hydrides, and chlorides, where this chemical element adopts the oxidation states of +1, +3, or +4. However, compounds with oxidation states ranging from 0 to +6 are also observed. Hexachloroiridate, [IrCl6]2− and hexabromo-iridate [IrBr6]2− are the only iridium compounds that occur in a +4 oxidation state. 

The salts of this chemical are characterized by vivid colors. In direct reaction with sulfur at atmospheric pressure, iridium produces the inorganic compound iridium disulfide that adopts the structure of the ‘fool’s gold’, i.e. pyrite crystal structure. 

Some of the most common iridium compounds include:

  • Ammonium Chloroiride
  • Ammonium Hexachloroiridate(III) Monohydrate
  • Ammonium Hexachloroiridate(III) Hydrate
  • Ammonium Hexachloroiridate(IV)
  • Cesium Iridium Sulfate Dodecahydrate
  • Chloroiridic Acid Hexahydrate
  • Chloropentaammineiridium(III) Chloride
  • Dihydrogen Hexabromoiridate(IV) Hexahydrate
  • Dipotassium Hexachloroiridate
  • Hydrogen Hexachloroiridate(IV) Hydrate
  • Iridium Acetate
  • Iridium Acetate Solution
  • Iridium(III) Acetate
  • Iridium(III) Bromide
  • Iridium(IV) Bromide
  • Iridium(III) Bromide Hydrate
  • Iridium(III) Bromide Tetrahydrate
  • Iridium Carbide
  • Iridium(III) Chloride IrCl3
  • Iridium Chloride Solution
  • Iridium(III) Chloride Hydrate
  • Iridium(III) Chloride Hydrochloride Hydrate
  • Iridium(III) Chloride Trihydrate
  • Iridium(IV) Chloride
  • Iridium(IV) Chloride Hydrate
  • Iridium(III) Fluoride
  • Iridium(IV) Fluoride
  • Iridium(VI) Fluoride
  • Iridium(III) Iodide Ir3
  • Iridium(IV) Iodide Ir4
  • Iridium(IV) Oxide Hydrate
  • Iridium Oxide
  • Iridium Potassium Cyanide
  • Iridium Ruthenium Oxide
  • Iridium Sulfate
  • Iridium Sulfate Solution
  • Iridium Trichloride
  • Iridium Tetrachloride
  • Lithium Hexachloriridate
  • Potassium Hexabromoiridate(IV)
  • Potassium Hexachloroiridate(III)
  • Potassium Hexacyanoiridate(III) Hydrate
  • Potassium Hexanitroiridate(III)
  • Sodium Hexabromoiridate(IV)
  • Sodium Hexachloroiridate(III) Hydrate
  • Sodium Hexachloroiridate Hexahydrate
  • Sodium Hexanitroiridate(III)
  • Strontium Iridium Oxide

Iridium Acetate Solution (C12H18Ir3O15)

The iridium(III) acetate solution has been used in industrial processes as a precursor of both homogeneous and supported catalysts. It’s worth noting that this is a highly toxic iridium compound. 

5 Interesting Facts and Explanations

  1. Aqua regia is a highly corrosive and concentrated fuming liquid with a distinct yellow-orange color. It’s made by mixing nitric acid  (HNO3) into hydrochloric acid (HCl). This mixture is able to attack gold and platinum, but not iridium. 
  2. The Iridium satellite constellation is an L-band network of 66 crosslinked LEO satellites located in low Earth orbit. During the formation process of Iridium communications, it was estimated that 77 satellites would be needed to fulfill the company’s goal. Hence, the satellite communication company was named after the 77th element – iridium. They provide global coverage to satellite phones, communications, and integrated transceivers throughout the entire surface of our planet.  
  3. The mineral pyrite, or iron pyrite, (FeS2 – iron(II) disulfide) is the most abundant sulfide mineral which is also referred to as ‘fool’s gold’. Its yellow-metallic, polished shine resembles the noble metal gold, hence its name. 
  4. A Mössbauer effect refers to the physical phenomenon discovered by Rudolf Mössbauer (1929 – 2011). The German physicist and winner of the Nobel Prize for Physics in 1961 for the aforementioned discovery was the first to observe the process in which some isotopes emit or absorb gamma rays without being deprived of their energy.  
  5. The point of the famous Parker 51 fountain pen is made with ruthenium and 3.8% iridium alloy.