Transition metals

What makes a metal a transition metal

The transition metals occupy groups 3 to 12, the wide central block of the periodic table. Moving across a row, each element adds an electron to an inner d subshell rather than to the outer shell. That inner filling is what gives the block its name: it marks the transition between the reactive s-block metals on the left and the p-block on the right.

Because the d electrons are close in energy to the outer ones, many transition metals can lose different numbers of electrons. Iron forms both 2+ and 3+ ions; manganese has stable states from +2 to +7. This flexibility drives much of their chemistry.

Why their compounds are so colourful

When other atoms bond to a transition metal ion, they split the energies of its d orbitals. Electrons can then jump between them by absorbing particular colours of visible light, and we see the colours that are left. That is why copper sulfate is blue, potassium permanganate deep purple and chromium compounds yellow, orange or green. The colours of rubies, emeralds and sapphires all come from traces of transition metals.

Catalysts that run industry

Their ability to change oxidation state and bind other molecules on their surfaces makes transition metals superb catalysts. They speed up reactions without being used up.

• Iron: the Haber process, which makes ammonia for fertiliser and feeds about half the world
• Platinum, palladium and rhodium: catalytic converters that clean car exhaust
• Nickel: hydrogenating vegetable oils
• Vanadium: the contact process for sulfuric acid

Record holders

The transition metals include the extremes of the metallic world. Tungsten has the highest melting point of any metal, 3,422 °C. Osmium is the densest naturally occurring element, closely followed by iridium. Silver conducts electricity and heat better than any other element. Mercury, at the other end, is the only metal that is liquid at room temperature.

The metals of civilisation

Gold, silver and copper were among the first metals people used, and the Bronze and Iron Ages are named after transition metal technologies. Steel, an alloy of iron with small amounts of carbon and often chromium, nickel or vanadium, remains the backbone of buildings, bridges and machines. Titanium makes aircraft and medical implants light and strong, and zinc protects steel from rust by galvanising.