Group 8: the iron group

Eight electrons and many oxidation states

Iron is [Ar] 3d⁶ 4s² and osmium [Xe] 4f¹⁴ 5d⁶ 6s², while ruthenium shifts one electron to give [Kr] 4d⁷ 5s¹. That makes eight electrons beyond the core. Iron rarely uses more than three of them: its common ions are Fe²⁺ and Fe³⁺, and rust is a hydrated iron(III) oxide.

The heavier members can go much further. Ruthenium and osmium can involve all eight electrons in bonding, forming the volatile tetroxides RuO₄ and OsO₄, while their everyday compounds are mostly +3 and +4.

Trends down the group

Iron melts at 1811 K, ruthenium at 2607 K and osmium at 3306 K. The density change is even bigger: 7.874 g/cm³ for iron, 12.45 g/cm³ for ruthenium and 22.59 g/cm³ for osmium, the densest element of all. A lump of osmium weighs nearly three times as much as a lump of iron the same size. Electronegativity goes from 1.83 for iron to 2.2 for both ruthenium and osmium.

Where you meet group 8

Iron is everywhere: steel in buildings, cars and cans, cast iron in pans, and the iron in hemoglobin that picks up oxygen in your lungs. By mass, iron is also the most abundant element in Earth as a whole, because most of the planet’s core is iron. Ruthenium hardens platinum and palladium alloys and is used in electrical contacts, osmium alloys once tipped fountain pen nibs, and osmium tetroxide, though toxic, is used to stain samples for electron microscopes.

Hassium and the old group VIII

Hassium (108), named after the German state of Hesse, has been made only a few atoms at a time. Even so, chemists turned some of those atoms into hassium tetroxide, HsO₄, which behaved just as expected for a heavier relative of osmium. In older periodic tables, groups 8, 9 and 10 were lumped together as a single group VIII, and iron, cobalt and nickel were known as the iron triad.