Gold

The entire history of gold follows from a single chemical property: it reacts with almost nothing. That is why gold is found as a native metal, why it does not tarnish or rust, why it survives for millennia — and why it became a measure of value long before chemistry existed as a science.

There is very little of it in the Earth's crust: around four milligrams per tonne. All the gold ever mined would fit inside a cube a little over twenty metres on a side.

Chiefly as the native metal, in quartz veins and in the placers formed when those veins erode. Placer gold was recovered by washing: the heavy metal particles settle while the lighter rock is carried away by water.

Appreciable quantities of gold are dissolved in seawater, but the concentration is so low that every attempt to extract it there has lost money.

On current understanding the heavy elements, gold among them, are made when neutron stars merge. The gold in the Earth's crust is matter synthesised in a catastrophe that happened before the Solar System existed.

Gold has been known since the Neolithic and has neither a discoverer nor a date. It was the first metal people worked, for the simple reason that it does not have to be smelted out of an ore.

Alchemy was in large part an attempt to make gold from base metals. The problem turned out to be insoluble by chemical means: turning one element into another affects the nucleus, not the electron shell. Curiously, the task has since been accomplished — gold has been made in nuclear reactions — at a cost incomparably above the market price.

Gold does not react with oxygen, with water, or with most acids. It dissolves in aqua regia, a mixture of concentrated nitric and hydrochloric acids in a ratio of roughly one to three: the nitric acid oxidises the metal while chloride ions tie up the resulting gold ions in a stable complex, and only the two acting together make the reaction possible.

It also dissolves in cyanide solutions in the presence of oxygen, which is the basis of the main modern method of winning it from low-grade ore, and the source of most of the environmental objections to gold mining.

In compounds gold shows oxidation states +1 and +3, but all of them are unstable and readily revert to the metal.

Jewellery and central bank reserves still take most of the output. Pure gold is too soft, so it is alloyed with copper and silver; the fineness states the gold content in parts per thousand.

Technical uses rest on the same chemical indifference: gold-plated contacts in electronics do not oxidise and so do not lose conductivity with age. Thin gold films reflect infrared and are used in spacecraft glazing. In medicine, gold compounds were used against rheumatoid arthritis, and colloidal gold serves as a marker in biological research.