Why Is Gold Yellow? Relativity and the Colour of Metals

Look around the periodic table’s metals and nearly all of them are the same silvery grey: iron, aluminium, silver, platinum, tin, zinc. Only two elemental metals are distinctly coloured: copper, reddish-orange, and gold, warm yellow. (Cesium is pale gold, too, though you’ll rarely see it outside a sealed ampoule.) Gold’s colour turns out to be one of the clearest everyday effects of Einstein’s theory of relativity.

Why most metals are silver

A metal’s shine comes from its sea of free electrons (see metallic bonding). When light hits the surface, electrons absorb it and re-emit it — they reflect it. For most metals, this happens roughly equally for all visible colours, from red to violet. Reflect every colour equally and you get a neutral, silvery white.

What makes a metal coloured

A metal looks coloured if it absorbs some visible wavelengths more than others. That happens when electrons can jump from one energy band to another using exactly the energy of a visible photon.

In silver, the relevant jump — from the filled 4d band up to the half-filled 5s band — needs about 3.7 eV of energy. That’s in the ultraviolet, beyond what our eyes can see. So silver reflects all visible light and looks silvery.

In gold, the equivalent jump — from the 5d band to the 6s band — needs only about 2.4 eV. That’s in the visible range, in the blue part of the spectrum. Gold absorbs blue light, and reflects mostly the remaining red, orange and yellow. We see gold.

So why is gold’s gap so much smaller than silver’s, when gold sits directly below silver in the same group?

Enter relativity

Gold has 79 protons. That large positive charge pulls its inner electrons so hard that they move at a substantial fraction of the speed of light — for gold’s innermost 1s electrons, more than half of it.

According to special relativity, fast-moving particles behave as though they’re heavier. Heavier electrons orbit closer to the nucleus. This relativistic contraction affects s orbitals most strongly, including gold’s outer 6s orbital, which shrinks and drops in energy. Meanwhile the 5d orbitals, shielded by the contracted s and p electrons, expand and rise in energy.

The result: the 5d and 6s levels move closer together, the gap shrinks into the visible range, and gold absorbs blue light. Calculations that ignore relativity predict that gold would be silvery, like silver. It’s often said that gold’s colour is relativity you can hold in your hand.

Other relativistic surprises

The same effects, strongest in period 6, explain several oddities:

• Mercury is a liquid. Its contracted 6s electrons are held so tightly that they barely take part in metallic bonding. See liquid elements.
• Gold is so unreactive. Its 6s electron is held unusually tightly, so gold resists oxidation — one reason ancient gold artefacts still gleam.
• Gold has a high electron affinity for a metal, and can even form the auride ion, Au⁻. See electron affinity.
• Lead-acid batteries work largely because of relativistic effects on lead — calculations suggest a car battery’s voltage would be much lower without them.

And copper?

Copper, above silver, is also coloured — but not because of relativity. Its 3d-to-4s gap is naturally small enough to absorb some blue and green light, giving it a reddish tint. Copper is the only coloured metal among the lighter elements.

Changing gold’s colour

Jewellers change gold’s colour by alloying it:

• Rose gold: more copper → pinker.
• White gold: palladium or nickel → silvery (often plated with rhodium for extra whiteness).
• Green gold: more silver → slightly greenish yellow.

Very thin gold leaf even looks green when light shines through it, because gold transmits the green-blue light it doesn’t reflect.

Quick answers

Why is gold yellow? Relativistic effects narrow the energy gap between gold’s 5d and 6s electrons, so it absorbs blue light and reflects yellow.

Why is silver not yellow? Silver’s lighter nucleus produces much weaker relativistic effects; its absorption is in the ultraviolet.

Are there other coloured metals? Copper (reddish) and, faintly, cesium (pale gold). Most others are grey or silvery.