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Six metals in the middle of the periodic table travel together through geology, chemistry and industry: ruthenium, rhodium and palladium in period 5, and osmium, iridium and platinum directly below them in period 6. They’re called the platinum group metals (PGMs). They’re found in the same ores, share a remarkable resistance to corrosion, and are some of the rarest and most valuable elements on Earth.
The six
| Element | Symbol | Density (g/cm³) | Melting point (°C) | Notable for |
|---|---|---|---|---|
| Ruthenium | Ru | ~12.1 | ~2,334 | Hard disk layers, catalysts |
| Rhodium | Rh | ~12.4 | ~1,964 | Catalytic converters; often the priciest |
| Palladium | Pd | ~12.0 | ~1,555 | Catalytic converters; absorbs hydrogen |
| Osmium | Os | 22.57 | ~3,033 | Densest element |
| Iridium | Ir | 22.42 | ~2,446 | Most corrosion-resistant metal |
| Platinum | Pt | 21.46 | ~1,768 | Catalysts, jewellery, medicine |
(Values rounded; the heavy trio of osmium, iridium and platinum are the three densest elements known.)
Why they’re so unreactive
Like gold and silver, the PGMs are noble metals: they resist oxidation and attack by most acids. Platinum, iridium and rhodium don’t dissolve even in nitric or hydrochloric acid alone. Platinum needs aqua regia — a mixture of concentrated nitric and hydrochloric acids — and iridium resists even that at room temperature.
This inertness comes from their electron structure: tightly held d electrons and, for the period 6 metals, strong relativistic effects make them reluctant to give up electrons.
How they were discovered
- Platinum was known to Indigenous peoples of South America, who worked it into ornaments. Europeans encountered it in the 1700s in Colombian gold mines, where Spanish miners considered it a nuisance and called it platina, “little silver”.
- Palladium (1802) and rhodium (1804) were discovered by William Hyde Wollaston, who dissolved crude platinum in aqua regia and worked through what was left in solution. Palladium was named after the then newly discovered asteroid Pallas; rhodium from the Greek for “rose”, after the colour of its salts.
- Osmium and iridium were found in 1803 by Smithson Tennant in the black residue that wouldn’t dissolve. Osmium is named for the Greek osmē, “smell”, after the sharp odour of its oxide; iridium for Iris, the rainbow goddess, after its colourful compounds.
- Ruthenium was the last, isolated by Karl Klaus in 1844 and named after Ruthenia (a Latin name for Russia).
Where they come from
PGMs are among the rarest elements in the Earth’s crust. Most of the world’s supply comes from a few deposits, particularly the Bushveld Complex in South Africa and the Norilsk region of Russia, with smaller amounts from Canada, the USA and Zimbabwe. They’re often produced together, and as by-products of nickel mining, so supply of one metal can’t easily be increased on its own. See the most expensive elements.
Why the world needs them
- Catalytic converters are the biggest use. Platinum, palladium and rhodium turn carbon monoxide, unburned fuel and nitrogen oxides in car exhaust into carbon dioxide, water and nitrogen. See reaction rates and catalysts.
- Hydrogen fuel cells use platinum catalysts, and platinum and iridium are important in electrolysers that split water to make green hydrogen.
- Electronics: palladium and ruthenium in capacitors, contacts and hard disk drives.
- Medicine: cisplatin and related platinum drugs are among the most widely used cancer treatments; platinum is used in pacemaker electrodes.
- Chemical industry: platinum-rhodium gauze catalyzes nitric acid production; palladium catalysts are workhorses of pharmaceutical synthesis.
- Jewellery and investment: platinum and palladium.
- Standards: the international prototype kilogram, used to define the kilogram until 2019, was a cylinder of 90% platinum and 10% iridium.
A mark in the rocks
Iridium is very rare in the Earth’s crust but more common in asteroids. In 1980, Luis and Walter Alvarez and colleagues found a thin layer of clay, laid down worldwide 66 million years ago, unusually rich in iridium. They proposed it came from a giant asteroid impact — the event now linked to the extinction of the dinosaurs.
Quick answers
Is gold a platinum group metal? No. Gold and silver are precious and noble metals, but they’re in group 11, not part of the platinum group.
Which platinum group metal is most expensive? Usually rhodium, though prices shift.
Why do they occur together? Their similar chemistry and affinity for iron and sulfur mean geological processes concentrate them in the same ores.
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