Rhodium (Rh)

Rhodium is the most expensive of the precious metals by a wide margin — it briefly passed $25,000 an ounce in 2021, roughly fourteen times the price of gold — because roughly 80% of it comes from South Africa, annual output is a few tonnes, and emissions rules make it hard to substitute. It is the only practical catalyst for reducing nitrogen oxides in a petrol car's exhaust, which is precisely why catalytic converter theft became an organised crime. White gold jewellery is almost always rhodium plated; when the plating wears through, the yellowish alloy underneath shows.

Properties

Atomic number

45

Symbol

Rh

Standard atomic weight

102.91 u

Category

Transition metal

Group

9

Period

5

Block

d-block

Electron configuration

[Kr] 5s¹ 4d⁸

Electrons per shell

2, 8, 18, 16, 1

Melting point

2,237 K — 1963.8 °C — 3566.9 °F

Boiling point

3,968 K — 3694.8 °C — 6682.7 °F

Density

12.4 g/cm³

Electronegativity

2.28 (Pauling)

Oxidation states

+3

Discovered

1803 by William Hyde Wollaston

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁸

Noble gas shorthand

[Kr] 5s¹ 4d⁸

Shells

K: 2 · L: 8 · M: 18 · N: 16 · O: 1

What it is used for

• Catalytic converters, where it handles the nitrogen oxides
• Rhodium plating on white gold and silver jewellery
• Mirrors for searchlights and for X-ray optics
• Thermocouples for very high temperatures, alloyed with platinum
• Catalysts for making acetic acid and nitric acid

Anomalous electron configuration: [Kr] 5s¹ 4d⁸ rather than the predicted 5s² 4d⁷.

Neighbours in the table

Group 9

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Rhodium sits in group 9 with cobalt, iridium, meitnerium.