Rhodium
Definition and meaning of Rhodium in chemistry.
![Rhodium element card: symbol Rh, atomic number 45, atomic mass 102.91, electron configuration [Kr] 4d⁸ 5s¹, oxidation states +3, Transition metal, group 9, period 5, solid at room temperature.](/assets/images/terms/rhodium-chemical-element.webp)
Rhodium is a rare, silvery-white transition metal with the chemical symbol Rh and an atomic number of 45. It belongs to the platinum group of metals. People recognize rhodium as one of the most valuable and expensive precious metals on Earth.
In more detail
You can find rhodium in group 9 and period 5 of the periodic table. In nature, rhodium occurs alongside other platinum group metals. Miners extract it from river sands and complex nickel-copper sulfide ores.
This noble metal is highly inert, meaning it rarely reacts with other chemicals. Rhodium shows absolutely no reaction to oxygen gas at normal room temperatures. It also completely resists attack by most strong acids, including aqua regia.
The metal possesses exceptional physical hardness and a very high optical reflectance. These physical traits naturally prevent the metal from tarnishing or scratching under normal wear. When rhodium forms chemical compounds, it usually exhibits a +3 oxidation state.
An oxidation state indicates how many electrons an atom has lost or gained. Rhodium acts as an exceptional transition metal catalyst for many industrial chemical reactions. For example, factories use it to synthesize large amounts of acetic acid from methanol.
Rhodium possesses unique and irreplaceable catalytic properties for controlling toxic vehicle emissions. It efficiently reduces harmful nitrogen oxides into safe, harmless nitrogen gas. This specific chemical ability makes rhodium incredibly valuable to the global automotive industry.
Rhodium is also extremely scarce in the Earth's crust compared to other metals. Because of this high demand and low supply, its market price fluctuates wildly. The cost of rhodium frequently exceeds the price of both gold and pure platinum.
Key facts
| Field | General Chemistry |
|---|---|
| Symbol | Rh |
| Atomic number | 45 |
| Atomic mass | 102.91 u |
| Category | Transition-Metal |
| State at room temperature | Solid |
| Melting point | 1964 °C |
| Boiling point | 3695 °C |
| Year discovered | 1803 |
Car manufacturers use the vast majority of global rhodium to build three-way catalytic converters. These devices sit in the exhaust systems of modern gasoline and diesel automobiles. The rhodium crucially helps convert toxic nitrogen oxide emissions into harmless nitrogen gas.
Frequently asked questions
Why is rhodium used to plate jewelry?
Rhodium plating provides a highly reflective, brilliant white finish on the metal. This hard coating effectively resists tarnishing and scratching over long periods of time. Jewelers overwhelmingly use it to protect and enhance white gold and silver pieces.
Why is rhodium so expensive?
Rhodium is exceptionally rare in the Earth's crust, which strictly limits its global supply. At the same time, the automotive industry creates a massive, inelastic demand for emission control systems. This combination of low supply and high demand drives the price up.
Related terms
Sources & references
See our sources and editorial standards.