Rhodium – facts and properties
- Symbol
- Rh
- Atomic number
- 45
- Atomic mass
- 102.906 u
- Period
- 5
- Group
- 9
- Category
- transition metal
- Electron configuration
- [Kr] 4d⁸ 5s¹
- Electronegativity
- 2.28
- Atomic radius
- 173 pm
- Valence electrons
- 9
- State at standard conditions
- solid
- Melting point
- 1964 °C
- Boiling point
- 3695 °C
- Density
- 12.4 g/cm³
- Discovered
- 1803
Rhodium is a silvery-white, hard and shiny metal with atomic number 45. It belongs to the platinum metals and is one of the rarest and most expensive metals of all. Its name comes from the Greek rhodon, meaning rose, because many of its salts are pink to red. Rhodium has two major uses: it cleans exhaust gas in cars, and as a thin coating it covers jewellery so that it shines and does not tarnish.
Position in the periodic table
Occurrence and extraction
Rhodium is very scarce in the Earth's crust, scarcer even than gold. It occurs native in small amounts in platinum ores and also in nickel-copper ores. By far the largest share of the world supply comes from South Africa, where the ores lie in the Bushveld complex, with Russia and North America adding more. Rhodium is a by-product of platinum production: a tonne of ore usually holds less than one gram of rhodium. That explains the high price, which at times has been many times the price of gold and fluctuates strongly because supply and demand are so tightly linked.
The element was discovered in 1803 by the English chemist William Hyde Wollaston, soon after his find of palladium. He dissolved crude platinum ore in aqua regia and removed the platinum and palladium from the solution. What remained was a dark red salt, from which he obtained the new metal. Because of the colour he chose the name rhodium.
In nature only a single isotope occurs, rhodium-103. Elements with only one stable isotope are called monoisotopic.
Properties and reactions
Rhodium is in period 5 and group 9, below cobalt. Its electron configuration is [Kr] 4d⁸ 5s¹ and its electronegativity is 2.28. The metal has a density of 12.4 g/cm³, melts at 1964 °C and boils at 3695 °C. It is hard but can be worked well at high temperature, and it has a very high reflectivity for light.
Chemically rhodium is exceptionally inert. Acids, even aqua regia, hardly attack the compact metal, and it does not tarnish in air. Finely divided powder dissolves slowly in hot concentrated sulfuric acid and in aqua regia. It oxidises to rhodium oxide only at high temperatures. The most common oxidation number in compounds is plus three, and solutions of rhodium salts are pink to red.
Rhodium is particularly good as a catalyst. On its surface and in its complexes, gases such as carbon monoxide, hydrogen and nitrogen oxides are activated and converted, at fairly low temperatures and with great selectivity.
Uses in everyday life and technology
Most rhodium ends up in the car catalytic converter. There it reduces the toxic nitrogen oxides formed in combustion to harmless nitrogen, with carbon monoxide serving as the reducing agent and becoming carbon dioxide. The so-called three-way catalyst at the same time removes unburnt hydrocarbons and carbon monoxide, which is done mainly by platinum and palladium. Because strict emission limits apply, demand for rhodium is high and the metal is expensive.
Jewellery is often rhodium-plated. A wafer-thin layer of rhodium is applied electrically to white gold, silver or platinum. It makes the surface especially white, hard and scratch-resistant and protects against tarnishing. Over time it wears off and has to be renewed. Because of its shine and durability rhodium is also used for reflectors in optical instruments.
In industry rhodium drives important processes: rhodium catalysts are used to make acetic acid and aldehydes, the starting materials for plasticisers and detergents. Wilkinson's catalyst, a rhodium complex for hydrogenating double bonds, is named after Geoffrey Wilkinson, whose 1973 Nobel Prize, however, honoured his work on sandwich compounds such as ferrocene. In addition, thermocouples for very high temperatures contain an alloy of platinum and rhodium, and in the glass-fibre industry the nozzles are made of such alloys.
Quick question
What is the atomic number of rhodium (Rh)?
FAQ
Why is rhodium so expensive?
It is extremely rare and arises only as a by-product of platinum mining. At the same time it is needed in large quantities for car catalytic converters. Because mining cannot be expanded quickly, the price fluctuates strongly.
What is rhodium used for in a car?
In the three-way catalytic converter it turns nitrogen oxides in the exhaust into harmless nitrogen. The metal speeds up this reaction without being used up. Platinum and palladium also ensure that carbon monoxide and unburnt hydrocarbons are oxidised.
What does rhodium plating mean?
Rhodium plating means covering jewellery with a very thin layer of rhodium. It makes white gold and silver shine brighter, makes the surface harder and protects it from tarnishing. The layer wears off over time and is then renewed.