Rhodium (Rh)

Rhodium (symbol Rh) is the chemical element with atomic number 45: a transition metal, solid at 0 °C, with an atomic mass of 102.91 u. Discovered in 1803 (William Hyde Wollaston).

Rhodium is a precious metal even rarer than gold and platinum. About 80% of output goes into catalytic converters, where it turns polluting nitrogen oxides into harmless nitrogen. In 2021 its price topped $29,000 an ounce, more than fifteen times the price of gold at the time.

Key facts

Atomic number: 45
Symbol: Rh
French name: Rhodium
Atomic mass: 102.91 u
Molar mass: 102.91 g/mol
Family: Transition metals
Position: period 5, group 9, d-block
State at 0 °C: Solid
Melting point: 1,963.9 °C
Boiling point: 3,694.9 °C
Density: 12.41 g/cm³
Electronegativity (Pauling): 2.28
First ionization energy: 720 kJ/mol
Atomic radius: 135 pm
Electron configuration: [Kr] 4d⁸ 5s¹
Abundance in Earth's crust: 0.001 ppm (78th most abundant element)
Indicative price: 150,000 $/kg (order of magnitude)
Radioactive: no
Discovery: 1803, William Hyde Wollaston

Origin of the name

From the Greek rhodon, rose, for the color of its salts.

What does rhodium look like?

A very hard, silvery-white, extraordinarily reflective metal that never tarnishes in air and resists almost all acids.

Where is rhodium found?

One of the rarest stable elements in Earth's crust, at about 0.001 ppm. It is almost entirely concentrated in the platinum deposits of South Africa and Russia.

Natural sources

Platinum ores (Merensky Reef)
Bowieite
Rhodplumsite

How is rhodium produced?

It is recovered in tiny amounts during platinum and nickel refining: world output is only 25 to 30 tonnes a year, more than 80% of it in South Africa. Recycling of catalytic converters supplies a growing share.

Top producers

South Africa
Russia
Zimbabwe

What is rhodium used for?

Car catalytic converters (reducing nitrogen oxides).
Plating white-gold jewelry for a brilliant white finish.
Platinum-rhodium gauzes for making nitric acid.
Thermocouples for measuring very high temperatures.
Bushings for producing glass fiber.

In everyday life

Car catalytic converter
White-gold ring (rhodium finish)
Nitrogen fertilizers (made using rhodium gauzes)

Role in living things

No known biological role; it is rarely encountered in an absorbable form.

Hazards of rhodium

The metal is inert. Some of its salts are irritants and sensitizers, and a few compounds are suspected carcinogens.

Cosmic origin

Neutron stars: Created when two neutron stars collide — the “forge” of gold.
Dying stars: Made slowly in red giants (stars like the Sun at the end of their lives).

History of rhodium

In 1803–1804, Englishman William Hyde Wollaston, trying to purify platinum, isolated two new metals: first palladium, then rhodium. He named the latter after the rose-pink color of its salts (rhodon means “rose” in Greek).

Did you know?

In 1979, Guinness World Records presented Paul McCartney with a rhodium disc to honor him as the best-selling musician in history.
A catalytic converter contains only a few grams of precious metals, which explains why catalytic converters get stolen.
Its price is one of the most volatile: it rose more than twentyfold between 2016 and 2021.

Isotopes of rhodium

A single stable isotope, rhodium-103. Its radioactive forms arise mostly from the decay of ruthenium and palladium.

rhodium-103: 100% of natural rhodium, stable
rhodium-103m: radioactive, half-life 56.1 min
rhodium-105: radioactive, half-life 1.47 days
rhodium-106: radioactive, half-life 30.1 s. Born from ruthenium-106, it is what actually emits the energetic electrons from eye plaques.

Related elements

Same family (transition metals): Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Yttrium (Y), Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Technetium (Tc), Ruthenium (Ru), Palladium (Pd), Silver (Ag), Cadmium (Cd), Hafnium (Hf), Tantalum (Ta), Tungsten (W), Rhenium (Re), Osmium (Os), Iridium (Ir), Platinum (Pt), Gold (Au), Mercury (Hg), Rutherfordium (Rf), Dubnium (Db), Seaborgium (Sg), Bohrium (Bh), Hassium (Hs).

Same column (group 9): Cobalt (Co), Iridium (Ir), Meitnerium (Mt).