Group 9: the cobalt group

Nine electrons beyond the core

Cobalt is [Ar] 3d⁷ 4s² and iridium [Xe] 4f¹⁴ 5d⁷ 6s², while rhodium moves one electron into the d subshell to give [Kr] 4d⁸ 5s¹. Although nine electrons sit beyond the core, these metals use only a few of them in everyday chemistry. Cobalt forms Co²⁺ and Co³⁺, rhodium is mostly +3, and iridium is +3 or +4.

Cobalt compounds show off the colors typical of transition metals: anhydrous cobalt(II) chloride is blue and turns pink when it absorbs water, which is why it is used in humidity indicator paper.

Trends down the group

Melting point rises from 1768 K for cobalt to 2237 K for rhodium and 2719 K for iridium. Density more than doubles, from 8.9 g/cm³ to 12.41 g/cm³ and then 22.56 g/cm³, which makes iridium the second densest element, just behind osmium at 22.59 g/cm³. Electronegativity is 1.88 for cobalt, 2.28 for rhodium and 2.2 for iridium.

The heavier members are among the least reactive metals known. Iridium is often called the most corrosion-resistant metal, shrugging off even aqua regia, the acid mixture that dissolves gold.

Where you meet group 9

Cobalt sits at the center of vitamin B12, goes into many lithium-ion battery cathodes and strong magnets, and has colored glass and pottery deep blue for centuries. Its name comes from kobold, a goblin of German folklore that miners blamed for troublesome ores. Rhodium helps catalytic converters break down nitrogen oxides and gives white gold jewelry its bright finish. Iridium tips long-life spark plugs.

A clue to the dinosaurs

Iridium is rare in Earth’s crust but more common in many asteroids. In 1980 scientists reported a thin, iridium-rich clay layer about 66 million years old and proposed that a giant asteroid impact ended the age of the dinosaurs; the layer has since been found worldwide. Meitnerium (109), named after the physicist Lise Meitner, has been made only a few atoms at a time, so its properties are predicted, not measured.