Atomic number 45

Rhodium

Rh
+ phase
− phase

Shells Orbitals 4d 5s

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45 electrons in 5 shells: 2, 8, 18, 16, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Transition metal
Period: 5
Group: 9
Block: d
Standard atomic weight: 102.91(1) Da
Electron configuration: [Kr] 4d⁸ 5s¹
Phase at 25 °C: solid
Melting point: 1964 °C
Boiling point: 3695 °C
Density: 12.4 g/cm³
Electronegativity (Pauling): 2.28
Discovered: 1803, William Hyde Wollaston

Neighbours in the table

Ru Pd

A dark red powder was the first sign of rhodium. William Hyde Wollaston found the element in 1803 in South American platinum ore, soon after palladium, Jefferson Lab notes. With platinum and palladium removed, what remained was sodium rhodium chloride, Na₃RhCl₆·12H₂O. Hydrogen gas then freed the metal. The name comes from the Greek rhodon, rose, for the rose colour of dilute solutions of its salts, IUPAC’s atomic-weights commission (CIAAW) explains.

The metal itself is not rose at all. The NIOSH card describes a white, hard, ductile solid, drawable into wire, with a bluish-grey lustre. Air and water cause no rapid reaction, PubChem’s safety record notes. Heated in air, it slowly takes up oxygen and later gives it back.

Beyond the krypton core come eight 4d electrons and one 5s, [Kr] 4d⁸ 5s¹, making shells of 2, 8, 18, 16 and 1. The ordinary filling order would put two in 5s. OpenStax treats such exceptions as cases where two subshells lie so close in energy that small effects reorder the filling. In positive ions, the textbook adds, transition metals lose outer s electrons first. Jefferson Lab lists +3 as rhodium’s common oxidation state, a bookkeeping charge assuming fully ionic bonds; Los Alamos gives −3 to +6.

Where it is found

Jefferson Lab estimates rhodium at about 1 × 10⁻³ milligrams per kilogram of Earth’s crust, and says it tends to accompany platinum deposits. Los Alamos reports native rhodium mixed with the other platinum metals in the Urals and across the Americas. Most supply comes as a byproduct of platinum mining and refining.

A second route runs through the copper-nickel sulfide ores around Sudbury, Ontario. Los Alamos says the rhodium in them is very small in quantity. The nickel industry works such large tonnages of that ore, though, that recovery pays.

South Africa is the leading producer of mined material containing the platinum-group metals, the US Geological Survey says, and its Bushveld Complex holds the largest resources and reserves. In the United States one Montana company mines them, and copper-nickel mining in Michigan yields small byproduct amounts. The country imported about 15,000 kilograms of rhodium in 2024, against 12,100 kilograms in 2023.

Why it matters

Los Alamos values rhodium as an electrical contact material: its electrical resistance is low, its contact resistance low and stable, and it resists corrosion strongly. Plated rhodium, laid down by electroplating or evaporation, is exceptionally hard. Los Alamos names microscopes and eye-examination equipment among the optical instruments that use it.

Catalytic converters, which cut harmful car exhaust, are the leading American use of the platinum-group metals, the US Geological Survey says, and Jefferson Lab lists them among rhodium’s uses. OpenStax describes the three-way type as carrying a platinum-rhodium catalyst on its surface.

Two radioactive isotopes also serve in medicine, according to IUPAC’s notes in PubChem’s record. Rhodium-105, with a half-life of 35.341 hours in the record’s nuclide table, emits beta particles used in targeted radiotherapy against cancer cells and has treated bone pain. Rhodium-106, with a half-life of only 30.07 seconds in the same table, is used as seeds in ocular brachytherapy, radiation placed against a tumour of the eye.

Properties

Properties of rhodium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	45
Standard atomic weight	102.91(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1964 °C	°C	Los Alamos: 1964 °C
Boiling point	3695 °C	°C	Los Alamos: 3695 °C
Density	12.4 g/cm³	g/cm³
Electronegativity (Pauling)	2.28
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁸
Phase at 25 °C	solid

Safety

Rhodium as plating or a contact is essentially harmless in daily life: in PubChem’s safety record, Haz-Map describes the platinum-group elements as generally nontoxic when metallic. At work, dust or powder of the metal is flammable, and a Japanese government classification warns that breathing it in may bring on allergy or asthma symptoms or difficulty breathing. Haz-Map also records allergic reactions in susceptible people, among them contact urticaria, or hives, in workers. The PubChem Laboratory Chemical Safety Summary and the NIOSH card for rhodium metal carry the detail.

Questions

Why is rhodium used on mirrors and jewellery?

Los Alamos calls the metal highly reflective, hard and durable, and says a plated layer is exceptionally hard. It lists jewellery and decoration among rhodium's uses. The US Geological Survey adds that aluminium and rhodium may take the place of silver, the metal traditionally used for mirrors and other reflecting surfaces.

How much does rhodium cost?

The US Geological Survey gives an annual average of 20,254.10 dollars per troy ounce in 2021, falling to 6,660.58 in 2023 and an estimated 4,600 in 2024, a drop of 31 per cent in a year. It blames weaker demand, investor uncertainty and oversupply, reasons it gives for the platinum-group price falls as a set. Platinum averaged about 950 dollars in 2024 and iridium about 4,800.

What does rhodium do in a catalytic converter?

It shares the work with platinum. OpenStax describes three-way converters whose surface carries a platinum-rhodium catalyst, which turns nitric oxide into nitrogen and oxygen and burns carbon monoxide and fuel such as octane to carbon dioxide and water vapour. In OpenStax's general account of solid catalysts, the gases typically stick to the metal surface, are activated and react there, and the products then leave the surface.

Why is rhodium alloyed with platinum?

To harden it. Los Alamos names hardening platinum and palladium as rhodium's primary use, and notes that rhodium melts higher and weighs less for its volume than platinum. The hardened alloys go into furnace windings, thermocouple elements, bushings for glass-fibre production, spark-plug electrodes for aircraft and laboratory crucibles. Jefferson Lab's note in PubChem's record agrees that alloying platinum and palladium is rhodium's most frequent use.