Rhodium
Rhodium is a hard, silvery-white metal from the platinum group and one of the rarest and most valuable precious metals in the world. It takes its name from the rose-colored compounds it forms, and its price is famously volatile compared to metals like gold.
- Group · Period
- 9 · 5
- At room temp
- solid
- Melts at
- 2237 K
- Density
- 12.4 g/cm³
- Discovered
- 1803
Uses
By far the largest use of rhodium is inside catalytic converters, where it works alongside platinum and palladium to reduce harmful nitrogen oxide emissions from vehicle exhaust — a role that consumes the majority of the rhodium produced worldwide each year. In jewelry, a thin layer of rhodium is commonly electroplated onto white gold and silver pieces to give them a bright, reflective white finish that also resists tarnishing and scratching far better than the base metal alone.
Rhodium also serves as a catalyst in various industrial chemical processes, including reactions used to manufacture certain plastics and other organic compounds, and its extreme rarity and resistance to corrosion have made it valuable in specialized electrical contacts and laboratory equipment.
History
Rhodium was discovered in 1803 by English chemist William Hyde Wollaston, as part of the same investigation into platinum ore residues that also led him to discover palladium. After dissolving crude platinum in acid and carefully separating out the other metals present, Wollaston was left with a solution containing a rose-red compound he hadn’t seen before. He named the new element rhodium, from the Greek word rhodon, meaning rose, directly describing the distinctive color of its salts. Because it occurs alongside platinum in ore and is exceptionally scarce, rhodium remained a rare and largely unfamiliar metal for well over a century after its discovery.
Fun facts
- Rhodium plating gives white gold jewelry its bright, reflective white shine and helps protect it from scratches and tarnish.
- It's among the rarest metals mined commercially, and its market price has historically swung far more dramatically than gold's.
- The great majority of the world's rhodium supply is used inside catalytic converters to reduce harmful emissions from vehicle exhaust.
Frequently asked questions
Why is rhodium so expensive?
It's extremely rare and isn't mined on its own — it's recovered as a byproduct of platinum and nickel mining, so supply is limited and tightly tied to those other industries. Strong, steady demand from the automotive industry for catalytic converters adds further pressure, which is part of why its price has historically been so volatile.
How was rhodium discovered?
English chemist William Hyde Wollaston discovered rhodium in 1803 while investigating leftover residue from platinum ore, the same broad research that also led him to discover palladium around the same time. He named it after the Greek word rhodon, meaning rose, because of the rose-red color of its salts.
If rhodium is so rare and expensive, why is it used in ordinary jewelry?
Jewelry typically uses only an extremely thin plated layer of rhodium over white gold or silver, just enough to give a bright white finish and added scratch resistance, so the small amount of rhodium involved keeps the cost manageable relative to the value it adds.
Compounds
4 notable compounds containing Rh
- RhCl3salt
Rhodium(III) chloride
A dark red salt that is the principal commercial entry point for producing rhodium catalysts and other rhodium compounds.
Used for: Precursor for industrial rhodium catalysts, including Wilkinson's catalyst
- Rh(NO3)3salt
Rhodium(III) nitrate
A red, water-soluble salt used as a soluble source of rhodium ion in catalyst preparation and electroplating solutions.
Used for: Rhodium source in electroplating baths and catalyst synthesis
- Rh2O3oxide
Rhodium(III) oxide
A gray-black oxide formed when rhodium metal is heated in air or oxygen, used mainly as an intermediate in refining and recovering rhodium.
Used for: Intermediate in rhodium refining and catalyst manufacturing
- RhCl(PPh3)3reagent
Wilkinson's catalyst
A landmark rhodium complex bearing three triphenylphosphine ligands that catalyzes the hydrogenation of alkenes under mild conditions, a discovery that helped found modern homogeneous catalysis.
Used for: Homogeneous catalyst for hydrogenating alkenes in organic synthesis
Isotopes
40 known isotopes
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| Isotope | Natural abundance | Half-life | Decay mode | Mass excess | Binding energy / nucleon | Decay width |
|---|---|---|---|---|---|---|
| Rh-103 stable | 100% | Stable | — | -88,031.71 keV | 8,584.19 keV | — |
Show all 40 isotopes
| Rh-89 | — | 120 ns | Proton emission, Electron capture / beta-plus decay, ECP | -45,651 keV | 8,189 keV | 3.80 × 10-9 eV |
| Rh-90 | — | 29 ms | Electron capture / beta-plus decay (100%), ECP (0.7%) | -51,634 keV | 8,254 keV | 1.57 × 10-14 eV |
| Rh-91 | — | 1.47 s | Electron capture / beta-plus decay (100%), Beta-plus, proton emission (1.3%) | -58,570 keV | 8,328 keV | 3.10 × 10-16 eV |
| Rh-92 | — | 4.66 s | Electron capture / beta-plus decay (100%), Beta-plus, proton emission (1.9%) | -62,999.1 keV | 8,373.42 keV | 9.79 × 10-17 eV |
| Rh-93 | — | 12.2 s | Electron capture / beta-plus decay (100%) | -69,011.81 keV | 8,434.83 keV | 3.74 × 10-17 eV |
| Rh-94 | — | 70.6 s | Electron capture / beta-plus decay (100%), ECP (1.8%) | -72,907.62 keV | 8,472.4 keV | 6.46 × 10-18 eV |
| Rh-95 | — | 5.02 m | Electron capture / beta-plus decay (100%) | -78,340.62 keV | 8,525.37 keV | 1.51 × 10-18 eV |
| Rh-96 | — | 9.90 m | Electron capture / beta-plus decay (100%) | -79,687.73 keV | 8,534.67 keV | 7.68 × 10-19 eV |
| Rh-97 | — | 30.7 m | Electron capture / beta-plus decay (100%) | -82,597.56 keV | 8,559.89 keV | 2.48 × 10-19 eV |
| Rh-98 | — | 8.72 m | Electron capture / beta-plus decay (100%) | -83,175.22 keV | 8,560.8 keV | 8.72 × 10-19 eV |
| Rh-99 | — | 16.1 d | Electron capture / beta-plus decay (100%) | -85,584.52 keV | 8,580.2 keV | 3.28 × 10-22 eV |
| Rh-100 | — | 20.5 h | Electron capture / beta-plus decay (100%) | -85,591.13 keV | 8,575.17 keV | 6.18 × 10-21 eV |
| Rh-101 | — | 3.3 Y | Electron capture (100%) | -87,412.43 keV | 8,588.22 keV | 4.38 × 10-24 eV |
| Rh-102 | — | 207.3 d | Electron capture / beta-plus decay (78%), Beta-minus decay (22%) | -86,783.32 keV | 8,576.98 keV | 2.55 × 10-23 eV |
| Rh-104 | — | 42.3 s | Beta-minus decay (99.55%), Electron capture / beta-plus decay (0.45%) | -86,959.34 keV | 8,568.95 keV | 1.08 × 10-17 eV |
| Rh-105 | — | 35.341 h | Beta-minus decay (100%) | -87,851.27 keV | 8,572.71 keV | 3.59 × 10-21 eV |
| Rh-106 | — | 30.07 s | Beta-minus decay (100%) | -86,362.66 keV | 8,553.93 keV | 1.52 × 10-17 eV |
| Rh-107 | — | 21.7 m | Beta-minus decay (100%) | -86,863.71 keV | 8,554.1 keV | 3.50 × 10-19 eV |
| Rh-108 | — | 16.8 s | Beta-minus decay (100%) | -85,031.16 keV | 8,532.67 keV | 2.72 × 10-17 eV |
| Rh-109 | — | 80.8 s | Beta-minus decay (100%) | -84,999.25 keV | 8,528.14 keV | 5.65 × 10-18 eV |
| Rh-110 | — | 3.35 s | Beta-minus decay (100%) | -82,828.69 keV | 8,504.26 keV | 1.36 × 10-16 eV |
| Rh-111 | — | 11 s | Beta-minus decay (100%) | -82,303.87 keV | 8,495.63 keV | 4.15 × 10-17 eV |
| Rh-112 | — | 3.6 s | Beta-minus decay (100%) | -79,731.05 keV | 8,468.87 keV | 1.27 × 10-16 eV |
| Rh-113 | — | 2.80 s | Beta-minus decay (100%) | -78,766.94 keV | 8,456.82 keV | 1.63 × 10-16 eV |
| Rh-114 | — | 1.85 s | Beta-minus decay (100%) | -75,710.28 keV | 8,426.62 keV | 2.47 × 10-16 eV |
| Rh-115 | — | 0.99 s | Beta-minus decay (100%) | -74,229.23 keV | 8,410.65 keV | 4.61 × 10-16 eV |
| Rh-116 | — | 0.68 s | Beta-minus decay (100%) | -70,735.74 keV | 8,377.61 keV | 6.71 × 10-16 eV |
| Rh-117 | — | 0.44 s | Beta-minus decay (100%) | -68,896.76 keV | 8,359.28 keV | 1.04 × 10-15 eV |
| Rh-118 | — | 286 ms | Beta-minus decay (100%), Beta-minus, neutron emission (2.1%) | -64,886.84 keV | 8,322.85 keV | 1.60 × 10-15 eV |
| Rh-119 | — | 171 ms | Beta-minus decay (100%), Beta-minus, neutron emission (6.4%) | -62,822.8 keV | 8,303.4 keV | 2.67 × 10-15 eV |
| Rh-120 | — | 132 ms | Beta-minus decay (100%), Beta-minus, neutron emission (5.4%), B-2N | -58,620 keV | 8,266 keV | 3.46 × 10-15 eV |
| Rh-121 | — | 151 ms | Beta-minus decay (100%), Beta-minus, neutron emission | -56,250.13 keV | 8,245.24 keV | 3.02 × 10-15 eV |
| Rh-122 | — | 300 ns | Beta-minus decay, Beta-minus, neutron emission, B-2N | -51,880 keV | 8,208 keV | 1.52 × 10-9 eV |
| Rh-123 | — | 42.2 ms | Beta-minus decay (100%), Beta-minus, neutron emission (24.2%), B-2N | -49,190 keV | 8,185 keV | 1.08 × 10-14 eV |
| Rh-124 | — | 30 ms | Beta-minus decay (100%), Beta-minus, neutron emission, B-2N | -44,710 keV | 8,148 keV | 1.52 × 10-14 eV |
| Rh-125 | — | 26.5 ms | Beta-minus decay (100%), Beta-minus, neutron emission, B-2N | -41,830 keV | 8,124 keV | 1.72 × 10-14 eV |
| Rh-126 | — | 19 ms | Beta-minus decay (100%), Beta-minus, neutron emission, B-2N | -37,200 keV | 8,087 keV | 2.40 × 10-14 eV |
| Rh-127 | — | 20 ms | Beta-minus decay (100%), Beta-minus, neutron emission, B-2N | -33,730 keV | 8,060 keV | 2.28 × 10-14 eV |
| Rh-128 | — | — | Beta-minus decay (100%), Beta-minus, neutron emission, B-2N | -27,340 keV | 8,010 keV | — |