Rhodium
transition metalProperties
| Property | Value |
|---|---|
| Atomic Mass | 102.91 amu |
| Category | transition metal |
| Group | 9 |
| Period | 5 |
| Electron Configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d8 |
| Electronegativity | 2.28 (Pauling) |
| Oxidation States | 3, 1, 2, 4 |
| Melting Point | 2237 K (1963.8 °C) |
| Boiling Point | 3968 K (3694.8 °C) |
| Density | 12.41 g/cm³ |
| Discovered By | William Hyde Wollaston (1803) |
About Rhodium
Rhodium is one of the platinum-group metals William Hyde Wollaston pulled out of crude South American platinum in 1803-1804, naming it for the rose-pink color of Rh₂(SO₄)₃ solutions (Greek rhodon, rose). It's the rarest non-radioactive metal in the crust at about 1 part per billion and effectively never occurs alone — global production sits near 30 tonnes/year, almost entirely as a byproduct of platinum and nickel mining in the Bushveld Complex. The metal is brilliantly reflective, refuses to tarnish in air below 600 °C, and resists aqua regia at room temperature, which is why thin Rh electroplate sits over white-gold jewelry. The chemistry is dominated by Rh(I) and Rh(III): Wilkinson's catalyst RhCl(PPh₃)₃ revolutionized homogeneous hydrogenation, and the Monsanto and Cativa methanol-carbonylation processes that make most industrial acetic acid run on cis-[Rh(CO)₂I₂]⁻. The biggest tonnage application by far is three-way auto catalysts, where Rh handles the NOₓ-reduction half of the reaction.
Fun Fact
Rhodium is the most expensive precious metal commonly traded on commodity markets, having reached over $29,000 per ounce in 2021, approximately 15 times the price of gold at the time.
Common Uses
- NOₓ-reduction component in three-way automotive catalytic converters
- Wilkinson's catalyst RhCl(PPh₃)₃ for homogeneous alkene hydrogenation
- Monsanto/Cativa rhodium-iodide catalysts for methanol carbonylation to acetic acid
- Rhodium electroplate over white gold and platinum jewelry to prevent tarnish
- Pt/Rh thermocouples (Type B, R, S) for furnace temperatures up to 1700 °C
- Reflective coatings on optical instruments and searchlight mirrors