ChemSphere
45Rh102.906

Rhodium

Transition metal · Period 5 · Group 9 · d-block

The rare, rose-named metal that cleans car exhaust

Atomic mass
102.906 u
State at 25 °C
Solid
Electronegativity
2.28
Melting point
1,963.9 °C
Boiling point
3,694.9 °C
Configuration
[Kr] 4d8 5s1
Building atom…
45 protons 58 neutrons 45 electrons

Nucleus drawn at reduced count for clarity. Drag to rotate.

Rhodium, explained

Video lesson in production

Bhavya’s explainer for this element will play here.

Overview

Rhodium is a hard, silvery-white platinum-group metal in group 9, below cobalt and above iridium. Its configuration, [Kr] 4d8 5s1, gives it a strong preference for the +3 oxidation state, and its compounds are often red or pink, as in the rose-coloured chlorides that gave it its name. Rhodium is one of the most reflective metals and one of the least reactive: it does not oxidize in air at room temperature, is untouched by most acids, and even aqua regia attacks the solid metal only slowly.

Its value lies in catalysis. A rhodium surface binds nitrogen monoxide strongly enough to break the N–O bond but releases the products easily, which makes it the best metal for reducing nitrogen oxides in a car's three-way catalytic converter; this use takes about 80% of world supply. In industry, soluble rhodium complexes catalyse the Monsanto process for acetic acid and hydroformylation, which converts alkenes into aldehydes for plastics and detergents. Rhodium is also electroplated onto white gold and silver to give them a bright, tarnish-free finish. Annual production is only about 30 tonnes, so prices swing wildly.

Discovery

William Hyde Wollaston discovered rhodium in 1803, shortly after palladium, while purifying platinum ore from South America. After dissolving the ore in aqua regia and removing platinum and palladium, he was left with a rose-red salt, sodium chlororhodate, from which he obtained the metal by reduction with hydrogen. He named the element from the Greek rhodon, meaning 'rose', for the colour of its salts.

Where it’s found
Rhodium is recovered as a by-product of platinum and nickel mining, with about 80% coming from South Africa's Bushveld Complex.
What it’s used for
  • Catalytic converters
  • Acetic acid production
  • White gold plating
  • Thermocouples
  • Glass-fibre spinning bushings
In living things
Rhodium has no biological role. Its compounds are rarely encountered, and some are considered toxic and possibly carcinogenic.
Safety
Rhodium metal is inert and non-toxic in bulk, but finely divided powder is flammable and soluble rhodium salts should be handled as toxic.

How it compares

Where Rhodium sits among all 118 elements on the trends that shape the table.

  • Electronegativity2.28Pauling

    #14 of 100 elements with data

  • Ionization energy720kJ/mol

    #44 of 104 elements with data

  • Atomic radius135pm

    #59 of 95 elements with data

  • Electron affinity110kJ/mol

    #25 of 109 elements with data

  • Density12.4g/cm³

    #31 of 114 elements with data

  • Melting point2,240K

    #14 of 110 elements with data

Density
12.4 g/cm³
Atomic radius
135 pm
Covalent radius
125 pm
Molar heat
24.98 J/(mol·K)
Appearance
silvery white metallic
Discovered by
William Hyde Wollaston

Inside the atom

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

Orbital diagram

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d8 · shorthand [Kr] 4d8 5s1

  1. 1s2
  2. 2s2
  3. 2p6
  4. 3s2
  5. 3p6
  6. 4s2
  7. 3d10
  8. 4p6
  9. 5s1
  10. 4d8

Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Rhodium bonds.

Ionization energies

Energy to remove each electron in turn, on a log scale. Each electron is harder to remove than the last, because the remaining ones feel more of the nucleus.

Signature reactions

The chemistry Rhodium is known for.

  1. Redox · Heterogeneous catalysis

    Reducing nitrogen oxides

    On the rhodium surface, nitrogen monoxide splits apart and its oxygen atoms are taken up by carbon monoxide. Two pollutants become harmless nitrogen and carbon dioxide before the exhaust leaves the tailpipe.

    2NO(g)+2CO(g)N2(g)+2CO2(g)

    Conditions: rhodium catalyst, hot exhaust (300–800 °C)

  2. Catalysis · Carbonylation

    Monsanto acetic acid process

    A rhodium iodide complex inserts carbon monoxide into methanol with over 99% selectivity. Introduced in 1970, it transformed the large-scale manufacture of acetic acid for vinyl acetate and other chemicals.

    CH3OH(l)+CO(g)CH3COOH(l)

    Conditions: [Rh(CO)2I2]^- catalyst, iodide promoter, 150–200 °C, 30–60 atm

  3. Redox · Synthesis

    Chlorination

    Hot chlorine converts rhodium into red-brown rhodium(III) chloride. The hydrated form of this chloride is the usual starting material for making rhodium catalysts.

    2Rh(s)+3Cl2(g)2RhCl3(s)

    Conditions: heated, about 400 °C

Where you’ll meet it

  • White gold

    White gold

    Most white gold jewellery is electroplated with a thin, bright layer of rhodium.

    Photo: Wikimedia Commons
  • Sudbury Basin

    Sudbury Basin

    Ores in Ontario's Sudbury Basin, an ancient impact structure, yield rhodium alongside nickel and copper.

    Photo: Wikimedia Commons
  • Bushveld Igneous Complex

    Bushveld Igneous Complex

    South Africa's Bushveld Complex produces most of the world's rhodium.

    Photo: Wikimedia Commons
  • Thermocouple

    Thermocouple

    Platinum-rhodium thermocouples measure temperatures up to about 1,600 °C in furnaces and kilns.

    Photo: Wikimedia Commons

Research

No research on Rhodium yet

When Bhavya publishes a project that studies Rhodium, it will be linked here.

See all research