Gold
The one metal that does not corrode, tarnish or react — which is most of why it became money.
Gold is chemically almost inert: it does not oxidise in air, does not tarnish, and dissolves only in a few specific reagent mixtures. Gold artefacts recovered from ancient burials come out unchanged.
That permanence, combined with scarcity, workability without any furnace technology, and an unmistakable appearance, is why gold became a store of value independently across cultures with no contact between them. Its industrial use — bonding wires, connector plating — exploits the same non-reactivity at a much smaller tonnage.
Why it behaves as it does
Gold is unreactive because of how tightly it holds the electron it would have to give up, and both halves of that are unusual.
Oxidation means losing electrons, and gold's outermost electron is bound far more firmly than the position of the element in the periodic table would suggest. The cause is relativistic: gold's nucleus carries seventy-nine protons, the innermost electrons are moving fast enough for relativistic effects to matter, and the contraction that follows pulls the outer s-orbital in with them. The electron that ought to be loosely held is not.
The same contraction is why gold is yellow rather than silvery. It narrows the energy gap between two of the electron levels enough that the metal absorbs blue light instead of reflecting everything, and what comes back is what is left.
So the resistance to corrosion, the colour, and the fact that gold occurs in nature as metal rather than as an ore of some compound are one property seen three ways. Aqua regia dissolves it only by attacking from both directions at once — nitric acid to oxidise the surface, hydrochloric to carry the ions away before they can settle back.
How to identify it
Gold is heavy, soft enough to mark with a fingernail against a hard surface, and leaves a golden streak. Pyrite, the classic imposter, is brittle, much harder, and streaks greenish black.
Uses
Gold's industrial use is electronic. It does not tarnish, so a gold-plated contact still conducts after years — which is why connectors, bonding wires and printed circuit board contacts use it despite the cost. The quantities are small and the reliability gain is large.
Dentistry uses it for the same reason: it is inert in the mouth. But the great majority of gold is not used at all in the ordinary sense. It is held — as jewellery, as bars, as central bank reserves — and its value rests on that agreement rather than on what it does.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is used as
- Corrosion protection — application · as plating on contacts that must not tarnish
- Jewellery and adornment — application · roughly half of all gold demand in a normal year, alloyed down to 18 or 14 carat because pure gold is too soft to hold a setting
is commonly confused with
- Chalcopyrite — mineral · brassy sulfides have been mistaken for gold for as long as both have been mined
- Pyrite — mineral · the resemblance is superficial — pyrite is hard and brittle where gold is soft and malleable
is produced by
- Cyanidation — process · from ore containing a few grams per tonne, far below what panning could recover
- Electrorefining — process · nobler than copper, so it never dissolves and falls to the bottom of the cell
is sourced from
- Copper — element · a significant share of world gold arrives this way, recovered from copper anode slimes at no mining cost of its own
is a component of
- Amalgam — alloy · in the amalgamation process rather than in a made alloy — the gold is what is being recovered, and the mercury is driven off afterwards
- Electrum — alloy · 20–80% · the variable part, and the reason the alloy needed assaying: natural electrum runs anywhere from a fifth to over half gold, and the difference is not visible
is a source for
- Electrum — alloy · not manufactured but mined: native gold is essentially never pure, and above about a fifth silver it is electrum and has its own name
- Epithermal vein — deposit type · the deposit type most of the gold mined before the twentieth century came out of, and still a major share
is found in
- Seawater — material · at a few parts per trillion. Fritz Haber spent years trying to pay German war reparations with it and found the concentration was a thousand times lower than the figure he had started from
is an alternative to
- Platinum — element · in jewellery, where platinum is denser, whiter, does not need plating to stay white, and is worked by a different trade at a different price
is used in
- Smartphone — object · about thirty milligrams, on contacts and bond wires, because a gold-plated contact still works in ten years — a tonne of phones assays richer than a tonne of ore from any mine on earth
- Printed circuit board — object · a fraction of a micrometre over nickel on the pads, because bare copper oxidises and will not solder after storage
Sources
- Material WorldOur own writing
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
- United States Department of Commerce · US Government work — public information, credit requested