Iridium
[Xe] 4f¹⁴ 5d⁷ 6s² · 2 · 8 · 18 · 32 · 15 · 2
The most corrosion-resistant metal in the table: iridium dissolves in neither acid nor aqua regia. Its anomaly in the Cretaceous-Palaeogene boundary layer became the evidence for the asteroid that killed the dinosaurs.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 77
- Atomic mass
- 192.217 а.е.м.
- Electron configuration
- [Xe] 4f¹⁴ 5d⁷ 6s²
- Electrons per shell
- 2 · 8 · 18 · 32 · 15 · 2
- Block
- d-block
- Group
- Cobalt group
- Period
- 6
- Electronegativity (Pauling)
- 2.2
- Electronegativity (Allen)
- 1.68
- Atomic radius
- 180 pm
- Covalent radius
- 141 pm
- Van der Waals radius
- 202 pm
- Ionisation energy 1
- 880 kJ/mol
- Ionisation energy 2
- 1 600 kJ/mol
- Electron affinity
- 151 kJ/mol
- Common oxidation states
- +3, +4
- Oxidation states
- -3, -1, +1, +2, +3, +4, +5, +6, +7, +8, +9
Physical properties
- State at 25 °C
- solid
- Density
- 22.56 g/cm³
- Melting point
- 2 719 K · 2 445.9 °C
- Boiling point
- 4 403 K · 4 129.9 °C
- Speed of sound
- 4 825 m/s
Thermal properties
- Heat of fusion
- 41.12 kJ/mol
- Heat of vaporisation
- 564 kJ/mol
- Specific heat capacity
- 0.131 J/(g·K)
- Thermal conductivity
- 147 W/(m·K)
Mechanical properties
- Young's modulus
- 528 GPa
- Shear modulus
- 210 GPa
- Bulk modulus
- 320 GPa
- Poisson ratio
- 0.26
- Mohs hardness
- 6.5
- Brinell hardness
- 1 670 MPa
Electrical and magnetic properties
- Electrical resistivity
- 47.1 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- 0.11 K
Crystal structure
- Crystal structure
- face-centred cubic (fcc)
- Lattice constants
- a = 383.9 pm
Abundance
- In the crust
- 0.001 mg/kg
- In the ocean
- no data
- In the universe
- 0.002 mg/kg
- In the human body
- no data
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 191Ir | 37.3 % | stable | — |
| 193Ir | 62.7 % | stable | — |
Iridium-192, half-life 74 days, is among the most widely used sources for industrial radiography.
Discovery
- Year of discovery
- 1803
- Discovered by
- Smithson Tennant
- Where
- England
- Origin of the name
- after Iris, goddess of the rainbow: its salts span every colour
History
Tennant isolated iridium together with osmium in 1803. The many-coloured salts gave it the name of Iris, goddess of the rainbow.
Where it occurs
0.001 mg/kg of the crust — iridium is strongly depleted here because it went into the core with the iron as Earth formed. Meteorites hold hundreds of times more.
How it is produced
From platinum concentrates and nickel slimes. World output is about 7 tonnes a year.
Role in living things
No biological role.
Safety
The metal is inert and harmless. Iridium-192 is a powerful gamma source, and lost radiography sources have caused severe radiation injuries.
Uses
- Spark plugs with iridium electrodes, which last many times longer than ordinary ones
- Crucibles for growing single crystals at 2000 °C
- Electrodes for chlorine and oxygen production
- Iridium-192 in industrial gamma radiography and brachytherapy
- The kilogram standard: a platinum-iridium alloy held at Sèvres until 2019
Curiosities
- The international prototype kilogram was 90 % platinum and 10 % iridium, and defined the unit of mass for 130 years.
- The iridium anomaly in a 66-million-year-old clay layer is direct evidence of an asteroid impact.
- Iridium will not dissolve even in aqua regia — the only metals sharing that are ruthenium and rhodium.
- Iridium and osmium were both found in the same black residue in 1803.