Rhodium
[Kr] 4d⁸ 5s¹ · 2 · 8 · 18 · 16 · 1
The dearest of the platinum metals and the most effective catalyst for reducing nitrogen oxides. Practically all the rhodium mined goes into car catalysts, where nothing else will substitute for it.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 45
- Atomic mass
- 102.906 а.е.м.
- Electron configuration
- [Kr] 4d⁸ 5s¹
- Electrons per shell
- 2 · 8 · 18 · 16 · 1
- Block
- d-block
- Group
- Cobalt group
- Period
- 5
- Electronegativity (Pauling)
- 2.28
- Electronegativity (Allen)
- 1.56
- Atomic radius
- 173 pm
- Covalent radius
- 142 pm
- Van der Waals radius
- 195 pm
- Ionisation energy 1
- 719.7 kJ/mol
- Ionisation energy 2
- 1 740 kJ/mol
- Ionisation energy 3
- 2 997 kJ/mol
- Electron affinity
- 109.7 kJ/mol
- Common oxidation states
- +3
- Oxidation states
- -3, -1, +1, +2, +3, +4, +5, +6
Physical properties
- State at 25 °C
- solid
- Density
- 12.41 g/cm³
- Melting point
- 2 237 K · 1 963.9 °C
- Boiling point
- 3 968 K · 3 694.9 °C
- Speed of sound
- 4 700 m/s
Thermal properties
- Heat of fusion
- 26.59 kJ/mol
- Heat of vaporisation
- 493 kJ/mol
- Specific heat capacity
- 0.243 J/(g·K)
- Thermal conductivity
- 150 W/(m·K)
Mechanical properties
- Young's modulus
- 275 GPa
- Shear modulus
- 150 GPa
- Bulk modulus
- 380 GPa
- Poisson ratio
- 0.26
- Mohs hardness
- 6
- Brinell hardness
- 1 100 MPa
Electrical and magnetic properties
- Electrical resistivity
- 43.3 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- 0.0003 K
Crystal structure
- Crystal structure
- face-centred cubic (fcc)
- Lattice constants
- a = 380.34 pm
Abundance
- In the crust
- 0.001 mg/kg
- In the ocean
- 0 mg/L
- In the universe
- 0.06 mg/kg
- In the human body
- 0 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 103Rh | 100 % | stable | — |
Rhodium-103 is the only stable isotope. Rhodium-102 serves as a source in research applications.
Discovery
- Year of discovery
- 1803
- Discovered by
- William Hyde Wollaston
- Where
- England
- Origin of the name
- from Greek rhodon (rose), the colour of its salt solutions
History
William Wollaston discovered rhodium in 1803 while examining the residues from dissolving crude platinum in aqua regia. The rose colour of its salts gave the name, from the Greek for rose.
Where it occurs
0.001 mg/kg of the crust — among the rarest non-radioactive elements. It is won only alongside platinum and nickel; over 80 % comes from South Africa.
How it is produced
By separating platinum concentrates. World output is about 20 tonnes a year — less gold than is mined in a single day.
Role in living things
No biological role.
Safety
The metal is inert. Rhodium compounds are mildly toxic, but data are sparse given how little is used.
Uses
- Three-way automotive catalysts: rhodium reduces NOx to nitrogen
- Reflective mirror coatings and rhodium plating on white gold
- Thermocouples for very high temperatures
- Catalysts for acetic acid production and hydroformylation
Curiosities
- In 2021 rhodium briefly topped 20 000 dollars an ounce — ten times the price of gold.
- That price is why catalytic converters became a target for theft worldwide.
- Rhodium-plated white gold in jewellery is a microscopically thin rhodium layer over an alloy.
- Rhodium reflects over 80 % of visible light and never tarnishes, which is why searchlight mirrors are coated with it.