Iridium – facts and properties

Position in the periodic table

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Occurrence and extraction

Iridium is among the rarest elements in the Earth's crust. It occurs in ores together with the other platinum metals and sometimes as a natural alloy with osmium. There is a geological reason for its scarcity: iridium dissolves readily in iron. When the young Earth was still molten, most of it sank with the iron into the core, and little remained in the crust. Asteroids, by contrast, contain far more iridium than the rocks at the surface.

This led to one of the best-known hypotheses in Earth history. In 1980 the physicist Luis Alvarez, his son, the geologist Walter Alvarez, and their colleagues reported that a thin layer of clay, found for example near Gubbio in Italy, holds far more iridium than the layers above and below it. The layer marks the boundary between the Cretaceous and the Palaeogene and is about 66 million years old. At that time the dinosaurs, apart from the birds, died out along with many other species. The researchers explained the iridium anomaly by the impact of an asteroid whose dust spread over the whole planet. A suitable crater was later found at Chicxulub in Mexico.

Iridium is obtained as a by-product of platinum and nickel mining, mainly in South Africa and Russia. World output is only a few tonnes a year. The element was found by the English chemist Smithson Tennant in London, in the black residue left when crude platinum is dissolved. Its salts showed many colours, so he named it after the goddess of the rainbow.

Properties and reactions

Iridium is in period 6 and group 9, below rhodium. The electron configuration is [Xe] 4f¹⁴ 5d⁷ 6s², and the electronegativity comes to 2.2. The metal does not melt until 2446 °C, and its boiling point is 4428 °C. A density of 22.42 g/cm³ is roughly twice that of lead.

Compact iridium is silvery white, very hard and so brittle that it can be shaped only at high temperatures. In return it is extremely stable: neither acids nor even hot aqua regia attack it. Only molten salts and, at high temperatures, halogens such as chlorine will do it harm.

In a cation studied in a high vacuum, iridium even reaches oxidation number plus nine, the highest detected so far for any element.

Uses in everyday life and technology

Iridium is needed only in small quantities, but wherever hardness, heat resistance and corrosion resistance count. Spark plugs with a fine iridium tip on the electrode last especially long, because the metal withstands the spark and the heat in the engine. Iridium crucibles are used to grow crystals from melts that are far hotter than a thousand degrees. In PEM electrolysis, which makes hydrogen from water and renewable electricity, iridium oxide serves as the catalyst at the anode because it survives the acidic environment.

An alloy of ninety parts platinum and ten parts iridium is hard, durable and chemically stable. The international prototype metre of 1889 was made of it, and so was the international prototype kilogram, a small cylinder kept near Paris that long defined the unit of mass. The metre now depends on the speed of light, and since 2019 the kilogram has been defined through the Planck constant.

The radioactive isotope iridium-192 is used as a radiation source, for example to inspect welds and in cancer treatment. This is done only in shielded equipment by trained staff.

Quick question

What is the atomic number of iridium (Ir)?

FAQ