Iridium (Ir)

A thin layer of iridium in 66-million-year-old clay, found worldwide by Luis and Walter Alvarez in 1980, is the evidence that an asteroid killed the dinosaurs: iridium is vanishingly rare in the Earth's crust because it sank into the core, but is comparatively common in meteorites, so a global iridium spike means something very large arrived from space. It is the most corrosion-resistant metal known, unattacked by any acid including aqua regia. Until 2019 the kilogram was defined by a cylinder of 90% platinum and 10% iridium kept near Paris.

Properties

Atomic number

77

Symbol

Ir

Standard atomic weight

192.22 u

Category

Transition metal

Group

9

Period

6

Block

d-block

Electron configuration

[Xe] 6s² 4f¹⁴ 5d⁷

Electrons per shell

2, 8, 18, 32, 15, 2

Melting point

2,719 K — 2445.8 °C — 4434.5 °F

Boiling point

4,701 K — 4427.9 °C — 8002.1 °F

Density

22.42 g/cm³

Electronegativity

2.2 (Pauling)

Oxidation states

+4, +3

Discovered

1803 by Smithson Tennant

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d⁷

Noble gas shorthand

[Xe] 6s² 4f¹⁴ 5d⁷

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 15 · P: 2

What it is used for

• Long-life spark plug electrodes
• Crucibles for growing sapphire and other high-temperature crystals
• Iridium-192 sources for industrial radiography of welds
• Electrodes for producing chlorine and for hydrogen electrolysers
• Deep-space probe casings for radioisotope power sources

Neighbours in the table

Group 9

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