Gold (Au)

Gold (symbol Au) is the chemical element with atomic number 79: a transition metal, solid at 0 °C, with an atomic mass of 196.97 u. Known since antiquity.

Gold is the most malleable metal and one of the most inert: it never rusts or tarnishes. Its unique color comes from the effects of Einstein's relativity on its electrons. Melt down all the gold ever mined, about 216,000 tonnes, and you'd get a cube barely 22 meters on a side.

Key facts

Atomic number: 79
Symbol: Au
French name: Or
Atomic mass: 196.97 u
Molar mass: 196.97 g/mol
Family: Transition metals
Position: period 6, group 11, d-block
State at 0 °C: Solid
Melting point: 1,064.2 °C
Boiling point: 2,969.9 °C
Density: 19.3 g/cm³
Electronegativity (Pauling): 2.54
First ionization energy: 890 kJ/mol
Atomic radius: 135 pm
Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s¹
Abundance in Earth's crust: 0.004 ppm (75th most abundant element)
Indicative price: 75,000 $/kg (order of magnitude)
Radioactive: no
Discovery: Antiquity

Origin of the name

An Old English word; the symbol Au comes from the Latin aurum.

What does gold look like?

A bright yellow, very dense (19.3 g/cm³) metal, so soft when pure that a fingernail can scratch it.

Where is gold found?

About 4 parts per billion of Earth's crust. It occurs mostly in native form, as nuggets, flakes or microscopic grains in quartz, and sometimes combined with tellurium. The oceans hold thousands of tonnes of it, but far too diluted to recover.

Natural sources

Native gold
Electrum
Calaverite
Sylvanite

How is gold produced?

Crushed ore is usually treated with cyanide, which dissolves the gold; the gold is then recovered on activated carbon and refined. Artisanal gold mining still uses mercury, with severe health consequences.

Top producers

China
Russia
Australia
Canada
United States

What is gold used for?

Jewelry (about half of demand).
Central bank reserves and investment.
Connectors and contacts in electronics.
Nanoparticles in rapid tests (the red line on pregnancy or Covid tests).
Dentistry and decorative gold leaf.

In everyday life

A gold ring or chain
The contacts of a SIM card or motherboard
A pregnancy or rapid antigen test
Gold leaf on a pastry or a picture frame

Role in living things

No biological role. Gold salts (such as auranofin) have been used to treat rheumatoid arthritis.

Hazards of gold

Metallic gold is harmless, so much so that it is approved as a food additive (E175). The danger lies in mining it: cyanide and mercury from gold panning pollute rivers and poison communities.

Cosmic origin

Neutron stars: Created when two neutron stars collide — the “forge” of gold.

History of gold

Known since prehistory, gold was one of the first metals ever worked, because it is found pure in nature. The Egyptians mined it on a large scale more than 4,000 years ago. Its symbol Au comes from the Latin aurum.

Did you know?

In 2017, the observation of two colliding neutron stars (GW170817) confirmed that these cataclysms make gold.
Gold leaf can be beaten to less than a ten-thousandth of a millimeter thick, so thin that it lets greenish light through.
In the 1920s, chemist Fritz Haber tried to extract gold from seawater to pay Germany's war reparations; he failed, as the concentration was far lower than expected.
Without relativistic effects, gold would be the color of silver.

Isotopes of gold

Gold has only one stable isotope, gold-197: all the gold in the world is made of identical atoms, forged mostly in collisions between neutron stars.

gold-195: radioactive, half-life 186 days
gold-197: 100% of natural gold, stable
gold-198: radioactive, half-life 2.69 days. A former treatment for some cancers, still used as a radioactive tracer.
gold-199: radioactive, half-life 3.14 days

Related elements

Same family (transition metals): Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Yttrium (Y), Zirconium (Zr), Niobium (Nb), Molybdenum (Mo), Technetium (Tc), Ruthenium (Ru), Rhodium (Rh), Palladium (Pd), Silver (Ag), Cadmium (Cd), Hafnium (Hf), Tantalum (Ta), Tungsten (W), Rhenium (Re), Osmium (Os), Iridium (Ir), Platinum (Pt), Mercury (Hg), Rutherfordium (Rf), Dubnium (Db), Seaborgium (Sg), Bohrium (Bh), Hassium (Hs).

Same column (group 11): Copper (Cu), Silver (Ag), Roentgenium (Rg).