ChemSphere
77Ir192.217

Iridium

Transition metal · Period 6 · Group 9 · d-block

The asteroid metal that marks the end of the dinosaurs

Atomic mass
192.217 u
State at 25 °C
Solid
Electronegativity
2.2
Melting point
2,445.9 °C
Boiling point
4,129.9 °C
Configuration
[Xe] 4f14 5d7 6s2
Building atom…
77 protons 115 neutrons 77 electrons

Nucleus drawn at reduced count for clarity. Drag to rotate.

Iridium, explained

Video lesson in production

Bhavya’s explainer for this element will play here.

Overview

Iridium is the most corrosion-resistant metal known. Even boiling aqua regia, which dissolves gold and platinum, barely affects the solid metal. Its configuration is [Xe]4f14 5d7 6s2, and its tightly bound 5d electrons and strong metallic bonding make it very hard, brittle and dense, about 22.56 g/cm3, second only to osmium. It melts at 2446 °C and keeps its strength at high temperature, so iridium crucibles are used to grow single crystals of laser and electronic materials from melts.

Iridium's oxidation states range from −3 to +9. The +9 state, found in the ion IrO₄⁺ and detected in 2014, is the highest known for any element. In the laboratory, iridium complexes are excellent catalysts and phosphorescent emitters used in OLED displays. Iridium is very rare in Earth's crust because it is siderophile, meaning it dissolved into iron and sank to the core as the planet formed. Meteorites are richer in it, which is why a thin iridium-rich clay layer pointed to an asteroid impact.

Discovery

Smithson Tennant discovered iridium in 1803 in London, in the same black residue left after dissolving crude platinum in aqua regia that yielded osmium. He named it after Iris, the Greek goddess of the rainbow, because its salts display a striking range of colours. Pure iridium was hard to work, and it was first melted in quantity in 1860 by Henri Sainte-Claire Deville and Jules Henri Debray.

Where it’s found
About 1 part per billion of Earth's crust, recovered as a by-product of nickel and platinum mining, mainly in South Africa.
What it’s used for
  • Spark-plug electrodes
  • Crucibles for crystal growth
  • OLED phosphorescent emitters
  • Iridium-192 radiography and cancer therapy
  • Electrolysis electrodes
In living things
No biological role. Iridium metal is inert and non-toxic; some of its compounds are being studied as anticancer drugs.
Safety
Bulk metal is safe; iridium-192 is a strong gamma emitter and must be shielded.

How it compares

Where Iridium sits among all 118 elements on the trends that shape the table.

  • Electronegativity2.2Pauling

    #16 of 100 elements with data

  • Ionization energy880kJ/mol

    #24 of 104 elements with data

  • Atomic radius135pm

    #59 of 95 elements with data

  • Electron affinity151kJ/mol

    #14 of 109 elements with data

  • Density22.6g/cm³

    #10 of 114 elements with data

  • Melting point2,720K

    #8 of 110 elements with data

Density
22.6 g/cm³
Atomic radius
135 pm
Covalent radius
122 pm
Molar heat
25.1 J/(mol·K)
Appearance
silvery white
Discovered by
Smithson Tennant

Inside the atom

77 electrons arranged in 6 shells: 2 · 8 · 18 · 32 · 15 · 2.

Orbital diagram

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d7 · shorthand [Xe] 4f14 5d7 6s2

  1. 1s2
  2. 2s2
  3. 2p6
  4. 3s2
  5. 3p6
  6. 4s2
  7. 3d10
  8. 4p6
  9. 5s2
  10. 4d10
  11. 5p6
  12. 6s2
  13. 4f14
  14. 5d7

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Iridium bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. Each electron is harder to remove than the last, because the remaining ones feel more of the nucleus.

Signature reactions

The chemistry Iridium is known for.

  1. Redox · Synthesis

    Chlorination

    Hot chlorine is one of the few reagents that attacks iridium, forming iridium(III) chloride. The anhydrous chloride is a starting point for iridium catalysts.

    2Ir(s)+3Cl2(g)2IrCl3(s)

    Conditions: about 600 °C

  2. Redox · Oxidation

    Oxidation at high temperature

    Iridium slowly forms a dark oxide film when heated in air. Above about 1100 °C the oxide decomposes again, so iridium survives in air at temperatures that would oxidise most metals.

    Ir(s)+O2(g)IrO2(s)

    Conditions: heating in air, roughly 600–1000 °C

  3. Nuclear · Neutron capture

    Making iridium-192

    Stable iridium-191 captures a neutron to form iridium-192, which has a half-life of about 74 days. Its gamma rays are used to check welds in pipelines and to deliver radiation directly to tumours in brachytherapy.

    191Ir+10n192Ir

    Conditions: neutron flux in a nuclear reactor

Where you’ll meet it

  • Cretaceous–Paleogene boundary

    Cretaceous–Paleogene boundary

    This thin clay layer, rich in iridium, records the asteroid impact that ended the age of dinosaurs.

    Photo: Wikimedia Commons
  • Chicxulub crater

    Chicxulub crater

    The buried crater in Mexico identified as the source of the worldwide iridium anomaly.

    Photo: Wikimedia Commons
  • International Prototype of the Kilogram

    International Prototype of the Kilogram

    This cylinder of platinum-iridium alloy defined the kilogram for 130 years.

    Photo: Wikimedia Commons
  • Spark plug

    Spark plug

    Long-life spark plugs use fine iridium tips that resist erosion by the spark.

    Photo: Wikimedia Commons

Research

No research on Iridium yet

When Bhavya publishes a project that studies Iridium, it will be linked here.

See all research