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Portrait of Gold

Science · Chemistry

Gold

Chemistry — transition metals

Gold is element 79, symbol Au from the Latin aurum, a word connected with the glow of dawn. It is the most malleable and ductile of the metals — a single gram can be beaten into a sheet a metre square — and almost completely unreactive, which is why it survives in nature as the pure metal and why the goldwork of Tutankhamun's tomb emerged untarnished after three thousand years. It is one of the very few coloured metals, an effect traced to relativistic behaviour of its electrons, and it dissolves only in aqua regia and a few other aggressive reagents.

How to spot them

  • ▸Symbol Au, from a Latin word linked to the glow of dawn
  • ▸The most malleable and ductile of all metals
  • ▸One of only a handful of coloured metals, and the only yellow one
  • ▸Dissolved by aqua regia but not by any single common acid
  • ▸Purity measured in carats, with 24 the pure metal

Key facts

Symbol Au, from the Latin aurum, a word linked to the glow of dawn

Related to Aurora, goddess of the dawn

Element 79, the most malleable and ductile metal known

One gram can be hammered into a sheet about a square metre in area, thin enough to see through

Dissolved only by 'royal water', a mixture of nitric and hydrochloric acids

Aqua regia, so named because it attacks this most royal of metals

Rutherford's assistants fired alpha particles at foil of it and found the nucleus (1909)

The Geiger–Marsden experiment; a few particles bounced straight back

Its purity is measured in carats, 24 meaning the pure metal

A carat here is one twenty-fourth by mass; the gemstone carat is a unit of weight instead

Its alchemical symbol was a circle with a central dot, the sign of the Sun

Transmuting base metals into it was the central goal of alchemy

Among the few coloured metals, its hue comes from a relativistic effect on its electrons

The 6s–5d gap is narrowed enough to absorb blue light, so reflected light looks yellow

Key ideas

  • Relativistic contraction of the 6s orbital narrows the 5d–6s energy gap so that blue light is absorbed — hence the yellow colour, and also gold's reluctance to give up electrons
  • Aqua regia works because nitric acid oxidises the metal while chloride ions immediately lock the ions away as tetrachloroaurate, pulling the equilibrium along

Quick-fire facts

  • •Atomic number 79, symbol Au from Latin aurum; melting point 1,064 °C, density 19.3 g/cm³
  • •One of the seven metals of antiquity, and one of the few found in nature in its native metallic state
  • •Alloyed for hardness: 18-carat gold is 75 per cent pure by mass, 9-carat is 37.5 per cent; white gold adds palladium or nickel, rose gold copper
  • •It is dissolved by aqua regia (nitric and hydrochloric acid, typically 1:3) and by cyanide solutions, which is how most ore is now processed
  • •The Geiger–Marsden experiment of 1909, using gold foil, led Rutherford to the nuclear model of the atom
  • •China is the largest producer; South Africa's Witwatersrand basin has yielded more gold than any other field
  • •The gold standard pegged currencies to the metal; Britain left it in 1931 and the United States ended dollar convertibility in 1971
  • •Gold is weighed in troy ounces, about 31.1 grams, not the 28.35-gram avoirdupois ounce
  • •Its inertness and conductivity make it standard for electrical contacts; it also coats spacecraft and astronauts' visors as an infrared reflector
  • •Shakespeare's Merchant of Venice gave English 'All that glisters is not gold', usually misquoted as 'glitters'

Drill it — 10 cards

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Sources