Cobalt, rhodium and iridium, situated in group 9 of the periodic table, are shiny, silvery-white metals of considerable hardness, although rhodium and iridium are harder than cobalt.
Cobalt is more reactive than the other two group members, although the difference in reactivity is less pronounced than that which exists between iron, and ruthenium and osmium. Cobalt oxidizes only at high temperatures, but dissolves in dilute acids, and reacts on heating with most of the non-metallic elements.
Rhodium and iridium also combine with many of the non-metals when heated, but like their neighbours, ruthenium and osmium, they are extremely resistant to attack from acids, including from aqua regia.
All three elements have high melting and boiling points, the points rising as we descend the group.
Iridium is the densest of all the chemical elements, and many of its fellow members in the third transition series - including tungsten, rhenium, osmium, platinum and gold - are also especially dense. Cobalt, like the metals which flank it in the first transition series - iron and nickel - is ferromagnetic.
Like most transition metals, the group 9 elements form coloured compounds. Cobalt's, famously, are blue, and are used as pigments; rhodium's tend to be a rose-colour; and iridium's are variously coloured.
