Rhodium
Atomic Data
| Atomic Number | 45 |
| Symbol | Rh |
| Atomic Weight | 102.91 u |
| Density (STP) | 12.41 g/cm³ |
| Melting Point | 1963.85 °C (2237 K) |
| Boiling Point | 3694.85 °C (3968 K) |
| Electronegativity | 2.28 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d8 5s1 |
| Oxidation States | +3 |
| Phase at STP | Solid |
| Category | Transition Metal |
| Period / Group | 5 / 9 |
| CAS Number | 7440-16-6 |
Electron Configuration
[Kr] 4d8 5s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 8 | 44 |
| O | 5 | 5s | 1 | 45 |
| Total | 45 | 45 | ||
Isotopes of Rhodium
Rhodium is monoisotopic: ¹⁰³Rh is its only naturally occurring stable isotope, accounting for 100% of all natural Rhodium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Rhodium-103 | ¹⁰³Rh | 45 | 58 | 100 | Stable |
Abundance & Occurrence
Rhodium is present in Earth's crust at approximately 0.0002 ppm by mass and at approximately 0.6 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Rhodium fumes and dust: Rhodium metal dust and fumes are respiratory and skin irritants; animal studies suggest rhodium compounds have moderate toxicity, though human data is limited: use engineering controls to minimise exposure.
- Sensitisation: Some rhodium compounds have caused allergic sensitisation in exposed workers; monitor for signs of occupational asthma or contact dermatitis.
- Catalytic converter dust: Recycling or processing spent automotive catalysts containing rhodium generates fine dust; respirators are required to avoid inhalation of mixed platinum group metal dust.
- General precautions: Handle rhodium salts with gloves and eye protection; avoid generating aerosols of rhodium compound solutions, which may be more readily absorbed than the metal.
Rhodium in the Real World
Real-World Uses
- Automotive catalytic converters: Rhodium is the critical catalyst for reducing nitrogen oxides (NO and NO₂) to nitrogen in three-way catalytic converters; it is irreplaceable for meeting exhaust emission standards and drives the high price of catalytic converters.
- Jewellery electroplating: A thin rhodium layer electroplated onto white gold, silver, and platinum jewellery produces an extremely bright, white, scratch-resistant, and tarnish-proof finish that maintains appearance over years of wear.
- Optical mirrors and reflectors: Rhodium-coated mirrors provide high reflectance across the UV and visible spectrum with exceptional hardness and chemical resistance, used in scientific instruments and projection systems.
- Industrial catalysis: Rhodium catalysts are used in carbonylation reactions to produce acetic acid (the Monsanto and Cativa processes) and in hydroformylation (the oxo process) to manufacture aldehydes from olefins and syngas.
- Laboratory crucibles and equipment: Platinum-rhodium alloys retain strength and resist oxidation at temperatures up to 1800 °C, used in crucibles for growing single crystals, thermocouple sheaths, and glass-spinning bushings for optical fibre manufacture.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is rhodium used for?
Rhodium is most familiar as a catalytic component in the three-way catalytic converter in vehicles, where it converts nitrogen oxides (NOx) into harmless nitrogen and oxygen. It is also used as a bright, hard electroplated coating on jewellery and silverware to prevent tarnishing. Rhodium-platinum thermocouples are used to measure very high temperatures, and rhodium is used in industrial catalysts for acetic acid production.
Is rhodium the most expensive element?
Rhodium is among the most expensive naturally occurring elements. Its price has fluctuated dramatically: reaching over $29,000 per troy ounce in 2021: driven by demand from catalytic converters and limited global supply (most comes as a byproduct of platinum and palladium mining in South Africa and Russia). Its extreme rarity and industrial essentiality make it price-volatile.
How was rhodium discovered?
Rhodium was discovered in 1803 by British chemist William Hyde Wollaston while working with platinum ore dissolved in aqua regia. After removing platinum and palladium, he isolated a red-coloured salt of the new metal and named it rhodium after the Greek 'rhodon', meaning rose, for the colour of its dilute solution salts.
How does rhodium in a catalytic converter reduce pollution?
In a three-way catalytic converter, rhodium acts as a reduction catalyst for nitrogen oxides. The hot exhaust gases flow over a rhodium-coated ceramic honeycomb, where NOx molecules (NO and NO2) are reduced to harmless N2 and O2. Simultaneously, platinum and palladium oxidise carbon monoxide to CO2 and unburned hydrocarbons to CO2 and water. The converter converts over 90% of these three pollutants under normal operating conditions.