Rhodium
Rhodium is a chemical element with symbol Rh and atomic number 45. It is a rare, silvery-white, hard, and chemically inert transition metal. It is a member of the platinum group.
Identity
- Atomic mass
- 102.906 u
- Category
- Transition metals
- Group
- 9
- Period
- 5
- Block
- d
- Oxidation States
- +1, +3
- Phase (25 °C)
- Solid
- Appearance
- silvery white metallic
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d8
- Short configuration
- [Kr] 4d8 5s1
- Electrons per shell
- 2, 8, 18, 16, 1
- Electronegativity (Pauling)
- 2.28
- Electron affinity
- 110 kJ/mol
- 1st ionization energy
- 720 kJ/mol
- Ionization energies
- 720 · 1740 · 2997 kJ/mol
Physical properties
- Melting point
- 1963.8 °C (2237 K)
- Boiling point
- 3694.8 °C (3968 K)
- Density
- 12.41 g/cm³
- Molar heat capacity
- 25.0 J/(mol·K)
Chemical behavior of rhodium
Rhodium is a group 9 transition metal with symbol Rh and atomic number 45. It is a hard silvery platinum-group metal, highly resistant to heat and corrosion with high reflectivity. Its main state is +3, with +1, +2 and +4 in complexes. One of the scarcest crustal metals, key to car catalysts, mirrors and chemical synthesis.
Reactivity
Rh + 3 Cl2 -> RhCl4
Rhodium reacts with chlorine to form rhodium tetrachloride, a bluish-gray solid that is the most stable compound.
Rh + 4 HNO3 -> RhO4 + 4 NO2 + H2O
Rhodium reacts with concentrated nitric acid, releasing nitrogen dioxide and forming rhodium tetroxide.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Rhodium(III) chloride | RhCl3 | Bluish-gray solid; getter and reagents precursor |
| Rhodium tetrachloride | RhCl4 | Bluish-gray solid; getter precursor and color agent |
| Rhodium tetroxide | RhO4 | Orange solid; getter precursor and oxidant |
Electron shell distribution
View in the atom builderIsotopes
Natural rhodium is monoisotopic: rhodium-103 (100 %). Over a dozen radioisotopes are known but all short-lived, the most stable being rhodium-101 (~3.3 years) and rhodium-102 (~207 days). Isotopic purity aids analysis but not supply, which rests on mining.
Occurrence and production
Among the rarest metals (~0.1 ppb) in cooperite, braggite and nickel-copper sulfide ores. Mainly a by-product of platinum mining in South Africa and Russia, plus Canada and Zimbabwe, recovered from refinery slimes.
History and origin of the name
William Hyde Wollaston isolated it in 1803 in London from crude platinum residues, naming it from Greek rhodon (rose) for the pink color of its salts. Found alongside palladium, it launched platinum-group chemistry.
Applications and relevance of rhodium
Uses of the element
About 80 % purifies car exhaust in three-way catalysts cutting nitrogen oxides. Bright plate coats white gold, silver and optics; alloys stiffen high-temperature glass-fiber bushings and thermocouples.
Uses of its compounds
Complexes catalyze carbonylation to acetic acid, hydrogenation and hydroformylation, including chiral synthesis of drugs. Oxide-derived coatings serve mirrors and specialty reflectors.
Health, safety and environment
Biological role
Rhodium is not essential and has low toxicological risk as metal. Soluble salts irritate skin, eyes and airways. Jewelry plate is safe in normal use, but industrial compounds need careful handling.
Safety
Metal is stable and low toxicity, yet inorganic rhodium compounds and plating mists irritate respiratory tract and skin. Use ventilation, gloves and eye protection; fine powder needs dust control. No nutritional role.
Environment
Mining and refining carry heavy footprint; metal is persistent but inert. High value drives strong recycling of spent car catalysts, recovering rhodium for new products through complex separation plants.
Frequently asked questions about Rhodium
Why is rhodium essential in gasoline-car catalysts?
It uniquely reduces nitrogen oxides to nitrogen while platinum/palladium oxidize CO and hydrocarbons. Tiny washcoat loadings meet emission limits over long hot duty. Hence price swings with car output and recycling.
What makes rhodium useful in chemical catalysts?
Phosphine and carbonyl complexes steer carbonylation, hydrogenation and hydroformylation selectively, even chirally for drugs. Ligand design around rhodium picks one pathway or mirror image over others. Conditions and ligands set outcome.
How is rhodium used in the glass industry?
Rhodium-platinum alloys form bushings, stirrers and fiber-drawing parts facing molten glass: hot strength plus attack resistance extends life. Cost restricts it to critical hot-zone parts with tuned alloy composition.
Why recycle rhodium from used catalysts?
Scarce and pricey, yet concentrated enough in honeycombs to recover profitably. Specialized smelting and refining return it to new catalysts, cutting primary mining. Separation is chemically complex and needs proper plants.
Why is jewelry dipped in rhodium?
A thin electroplate gives a bright white mirror finish and evens tone, especially over white gold. Layer is microns thin and wears on rings, so pieces need replating to restore look. Base alloy sets durability.
Why use rhodium in mirrors and reflectors?
High reflectivity in key bands plus corrosion resistance keep precision optics stable in harsh service. Used in instruments and specialty reflectors where performance outweighs very high cost. Finish and stack design matter.
Why does rhodium matter for chiral molecules?
Chiral rhodium complexes favor one mirror image (enantiomer) in hydrogenation and related steps, vital because drug enantiomers act differently in the body. Tailored ligands build the asymmetric pocket selecting the path.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.