Leave a silver spoon out for a few months and it turns dark. Leave iron out in the rain and it rusts. Gold pulled from a tomb after thousands of years still shines as if it had just been polished. Gold is unusual among metals, and the reason lies in how its electrons behave.

What tarnish is

Tarnish is a chemical reaction on the surface of a metal. Silver reacts with traces of sulfur compounds in the air and forms a thin film of black silver sulfide. Copper slowly forms the green patina you see on old roofs and statues. Iron reacts with oxygen and water to form rust, which flakes off and exposes fresh metal underneath.

For any of this to happen, the metal has to bond with something from its surroundings, and gold is very reluctant to do that.

The bond that won’t form

For oxygen to react with a metal, an oxygen molecule first has to stick to the surface and split in two. On most metals this happens easily, because the metal’s electrons can form bonds with the oxygen.

On gold, the same electrons do more harm than good. Its filled 5d electrons sit at an energy where they repel the oxygen more than they attract it. In 1995, the researchers Bjørk Hammer and Jens Nørskov calculated this directly, and found that gold is the only metal where the reaction is unfavourable at every step.

Where relativity comes in

Gold has 79 protons, and their pull is so strong that its innermost electrons move at more than half the speed of light. At that speed, the effects of special relativity become large enough to matter for chemistry. The 6s orbital shrinks and binds its electron unusually tightly, while the 5d orbitals shift in energy.

The same effect gives gold its colour. Most metals reflect all visible light about equally and look silvery. In gold, the energy gap between the 5d and 6s levels has narrowed enough to absorb blue light, so what reflects back to your eye looks yellow.

Not completely untouchable

Gold’s resistance has limits:

• Aqua regia, a mixture of nitric and hydrochloric acid, dissolves it. The nitric acid oxidises a little of the gold, and chloride ions immediately lock the resulting gold ions away, so the reaction keeps going.
• Gold nanoparticles are surprisingly reactive. In the 1980s, Masatake Haruta showed that tiny particles of gold can catalyse reactions even below room temperature, which bulk gold cannot do.

So gold isn’t chemically dead. A piece large enough to wear or to hold simply has nothing in the air that it wants to react with.