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 has one naturally occurring isotope, 103Rh, which is stable and accounts for 100% of all naturally found rhodium. No naturally radioactive isotopes of rhodium are known to occur in measurable quantities.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Rhodium-103 | ¹₀³Rh | 45 | 58 | 100% | Stable |
Abundance & Occurrence
Rhodium is one of the rarest elements in Earth's crust, occurring at only about 0.0002 ppm by mass, almost exclusively in platinum-group metal ores and alluvial deposits. The primary commercial source is the Bushveld Igneous Complex in South Africa, where rhodium is recovered as a by-product of nickel and copper refining.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Safety & Handling
- Toxicity: Elemental rhodium metal is considered to have low acute toxicity under normal handling conditions. Rhodium compounds, however, are moderately toxic and should be treated with appropriate caution.
- Skin and Eye Irritation: Rhodium salts and solutions can cause irritation to skin, eyes, and mucous membranes on contact. Gloves and eye protection should be worn when handling rhodium compounds in the laboratory.
- Inhalation Risk: Fine rhodium dust or powder should not be inhaled. Respiratory protection is advisable when machining or handling rhodium in powdered form, as with other platinum-group metals.
- Fire Hazard: Bulk rhodium metal is not considered flammable. However, fine rhodium powder may pose a dust explosion risk in confined spaces at high concentrations, as with many finely divided metals.
- Storage: Rhodium metal and compounds should be stored in sealed containers away from strong acids and oxidising agents. Given its exceptional value, secure storage is also an important practical consideration.
Rhodium in the Real World
Real-World Uses
- Automotive catalytic converters — Rhodium is the key component responsible for reducing nitrogen oxide (NOx) pollutants in vehicle exhaust. Three-way catalytic converters in petrol-engined vehicles use rhodium alongside platinum and palladium to simultaneously oxidise carbon monoxide and hydrocarbons while reducing nitrogen oxides.
- Platinum-rhodium thermocouples — Platinum-rhodium alloy thermocouples (Type B, R, and S) are the international standard for measuring temperatures above 1000 °C in industrial furnaces and glass manufacturing, owing to their stability and accuracy at extreme temperatures.
- Glass fibre manufacturing bushings — Platinum-rhodium alloy bushings are used in the production of glass fibres and optical fibre, where the alloy's high melting point, strength, and corrosion resistance at molten glass temperatures make it uniquely suited.
- Jewelry plating — A thin electroplated layer of rhodium is applied to white gold and sterling silver jewelry to enhance brightness, whiteness, and scratch resistance, and to prevent tarnishing.
- Chemical oxidation catalysts — Rhodium catalysts are employed in the synthesis of acetic acid via the Monsanto process, in hydroformylation reactions to produce aldehydes, and in various pharmaceutical manufacturing steps requiring precise catalytic control.
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 widely used in automotive catalytic converters, where it reduces nitrogen oxide emissions from exhaust gases. It is also used in platinum-rhodium thermocouples, glass fibre manufacturing bushings, jewelry plating for a bright white finish, and as a catalyst in chemical oxidation reactions.
Why is rhodium so expensive?
Rhodium is one of the rarest elements on Earth, with a crustal abundance of only 0.0002 ppm by mass. It is produced almost exclusively as a by-product of platinum and palladium mining, primarily in South Africa. This extreme scarcity, combined with high demand from the automotive industry, makes rhodium one of the most expensive precious metals in the world.
Where is rhodium found in nature?
Rhodium occurs naturally in platinum ores and river sands, typically alongside other platinum-group metals such as palladium, osmium, iridium, and ruthenium. The largest known deposits are found in the Bushveld Igneous Complex in South Africa, which supplies the majority of the world's rhodium.
Is rhodium toxic?
Elemental rhodium metal poses minimal health risk under normal conditions. However, rhodium compounds are considered moderately toxic and may cause skin and eye irritation on contact. Rhodium salts should be handled with appropriate laboratory precautions, including gloves and eye protection.