Rhodium · element 45
The most expensive metal you can buy, riding in every petrol car's exhaust to strip out nitrogen oxides
Key facts
In 2021 rhodium briefly cost more than ten times as much as gold, ounce for ounce. Almost nobody owns any, yet nearly everyone who drives a petrol car carries a gram or so of it under the floor. In the catalytic converter, rhodium turns harmful nitrogen oxides back into ordinary nitrogen, and stricter emission rules turned it from a footnote into one of the most sought-after materials on Earth.
Rhodium is a silvery-white platinum group metal in group 9, beneath cobalt and above iridium. It is exceptionally hard, does not tarnish, and reflects light brilliantly, which is why jewellers plate white gold with it. Its name comes from the Greek rhodon, a rose, for the pink-red colour of its salts.
Where it is found
Rhodium is one of the rarest stable elements in the crust, at around a part per billion or less. It never forms ores of its own and is recovered in small amounts from platinum and nickel ores. South Africa's Bushveld Complex yields roughly four fifths of newly mined rhodium, with Russia, Zimbabwe and Canada supplying most of the rest, and total output is only a few dozen tonnes a year.
Because so much rhodium has been installed in vehicles, scrap catalytic converters are now a major secondary source, and recycling supplies a significant fraction of annual demand. Natural rhodium consists of a single isotope, rhodium-103, one of the few elements with only one stable form.
How it was discovered
Around 1800 the English chemist William Hyde Wollaston was developing a secret process in London to make platinum malleable. He dissolved crude South American platinum in aqua regia and precipitated the platinum out. From what remained he discovered palladium in 1802 and, in 1803, a second new metal.
He obtained a rose-red salt, sodium rhodium chloride, and reduced it with hydrogen to a grey powder. The salt's colour gave the element its name. Wollaston's platinum business made him wealthy, and he kept his methods secret until near the end of his life.
What we use it for
Around four fifths of rhodium goes into three-way catalytic converters for petrol engines. Platinum and palladium oxidise unburnt fuel and carbon monoxide, but only rhodium can reduce nitrogen oxides to nitrogen and oxygen. Tighter limits on those gases in China and Europe from the late 2010s drove the price surge. Rhodium also sits in the gauze that oxidises ammonia in nitric acid plants, the first step towards most fertiliser.
The glass industry uses platinum-rhodium alloys for the bushings through which molten glass is drawn into fibres and for making LCD glass, since rhodium hardens platinum. Rhodium catalysts make aldehydes and acetic acid in bulk. Thin rhodium plating gives white gold its bright finish, protects silverware from tarnish and coats mirrors and searchlight reflectors. Platinum-rhodium thermocouples measure furnace temperatures above 1500 °C, and rhodium wires detect neutrons inside reactor cores.
Reading the numbers
Rhodium melts at about 1964 °C and boils at 3695 °C, high enough to handle molten glass and hot exhaust for years. Its density of 12.41 grams per cubic centimetre is almost identical to ruthenium's and greater than lead's, yet it is little more than half as dense as platinum, so rhodium alloys are relatively light for precious metals.
The configuration [Kr] 4d8 5s1 puts nine electrons outside the krypton core, with a single s electron as in neighbouring ruthenium, a pattern common in period 5 where the 4d and 5s levels sit close together. Rhodium usually settles for the +3 state, as cobalt above it does, and adopts +1 in many of its catalysts. The three measured ionisation energies, 720, 1740 and 2997 kJ/mol, rise smoothly along the period from ruthenium. Its electronegativity of 2.28 is among the highest of any metal, a numerical way of saying it is noble: bulk rhodium will not dissolve even in aqua regia. Its group 9 position beside the other platinum metals explains why it is always found with them and so hard to separate.
Safety and the environment
Metallic rhodium is inert, non-toxic and safe against the skin, which is why it is plated onto rings and earrings. Its soluble compounds, such as rhodium chloride, are irritants and possible sensitisers, though so little rhodium is used that toxicity data remain thin.
Its environmental story is mixed. Rhodium in catalytic converters has removed vast quantities of smog-forming gases from city air, but mining platinum metals uses much energy and water, converters shed minute metal particles onto roadsides, and rhodium's price has fuelled a wave of converter thefts. Efficient recycling of converters is the best answer to all three.
Did you know?
- Rhodium is usually the most expensive metal traded on world markets, and in 2021 its price rose above ten times that of gold.
- In 1979 the Guinness Book of Records presented Paul McCartney with a rhodium-plated disc as the most successful songwriter of all time.
- William Hyde Wollaston discovered rhodium and palladium within about a year of each other from the same batch of crude platinum.
- All natural rhodium is a single isotope, rhodium-103, making it one of the few monoisotopic elements.
- Thefts of catalytic converters surged worldwide when rhodium and palladium prices spiked, because a single converter can hold a gram or more of rhodium.
- White gold jewellery is bright white because of a thin rhodium plating; the alloy underneath is often slightly yellow or grey.
More in the transition metals
Column-mates come first, since elements in one column have outer electrons arranged in a similar way and tend to behave alike; the rest are the nearest neighbours in the family.
See all 38 transition metals on one page, or browse all 118 elements.
Frequently asked questions
Why is rhodium so expensive?
Rhodium is extremely rare, mined almost entirely as a by-product in South Africa and Russia, and cannot be produced quickly when demand rises. Around four fifths of it goes into catalytic converters, so when emission laws tightened the market ran short. That combination of tiny supply and inflexible demand makes its price both high and volatile.
Is rhodium plating on jewellery safe?
Yes. Rhodium is inert and free of nickel, so rhodium-plated jewellery is often recommended for people with metal allergies. The plating is very thin and wears away over a few years, exposing the alloy underneath, which may then need replating.
Does rhodium tarnish?
No. Rhodium does not react with oxygen, sulfur or moisture at normal temperatures, so it stays bright indefinitely. That is why it is plated onto silver and white gold, which do tarnish or dull. Only the plating wearing thin lets tarnish appear.
Where does rhodium come from?
Most new rhodium comes from the Bushveld Complex in South Africa, where it is separated from platinum ores, with smaller amounts from Russia, Zimbabwe and Canada. A large and growing share is recycled from used catalytic converters. There are no mines that produce rhodium on its own.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Rhodium reference article. Spotted an error? Tell us.