Gold is a noble metal: its electrons are held so tightly that it barely reacts with oxygen, water or most acids, which is why it never rusts or tarnishes. Each atom has 79 protons, and relativistic effects on its inner electrons make gold absorb blue light and reflect yellow, which gives it its colour.
What a gold atom looks like, how its crystals are built, why it is yellow and never tarnishes, and how it was made in colliding stars.
Gold is element 79, symbol Au:
- One stable isotope, gold-197, with 79 protons and 118 neutrons.
- Its atoms pack in a face-centred cubic lattice, which makes it soft and malleable.
- Relativity makes it yellow; tightly held electrons stop it tarnishing.
- It was forged in supernovae and neutron star collisions, and reached Earth's crust with asteroid impacts.
- Symbol
- Au, from the Latin aurum
- Atomic number
- 79
- Electron configuration
- [Xe] 4f14 5d10 6s1
- Crystal structure
- Face-centred cubic
- Density
- 19.3 g per cm3
- Melting point
- 1,064 °C
- Boiling point
- 2,970 °C
- Hardness
- 2.5 on the Mohs scale
- Stable isotopes
- One: gold-197
In this article
The gold atom
Gold is element 79: every gold atom has 79 protons in its nucleus and 79 electrons around it. All natural gold is a single isotope, gold-197, with 118 neutrons, which makes gold one of the few elements with only one stable form.
Its outer electron sits alone in the 6s shell, above a full 5d shell. That arrangement, written [Xe] 4f14 5d10 6s1, explains most of what makes gold special: the lone outer electron makes it an excellent conductor, and because that electron is held unusually tightly, gold is reluctant to react.
Gold's inner electrons move so fast that relativity matters. The effect pulls the 6s shell inwards and narrows the gap to the 5d shell, so gold absorbs blue light and reflects yellow. Without relativity, gold would look silvery like its neighbours.
Gold's crystal structure
In solid gold the atoms pack into a face-centred cubic lattice: picture a cube with an atom at each corner and one in the middle of each face. Each tiny cube is about 0.408 nanometres across.
This close-packed arrangement lets layers of atoms slide over one another without breaking, which is why gold is so soft and workable, and why jewellers alloy it with harder metals.
Physical and chemical properties
A single gram of gold can be beaten into a sheet about one square metre in size: gold leaf.
19.3 grams per cubic centimetre, against 11.3 for lead: a small bar is surprisingly heavy.
Gold does not tarnish or rust in air or water. It dissolves in aqua regia, a mix of nitric and hydrochloric acids, and in cyanide solutions used in mining.
It carries electricity well and never corrodes, so it coats connectors in electronics.
Pure gold is only 2.5 on the Mohs scale, about as hard as a fingernail, which is why jewellery is alloyed.
Mixing in silver, copper, palladium or nickel changes both hardness and colour.
How alloying changes the colour is explained in the different types of gold, and how it sets the carat in 9ct vs 18ct vs 24ct.
How gold is formed
Gold is not made in ordinary stars. It forms in the most violent events in the universe, through rapid neutron capture, known as the r-process, when atomic nuclei absorb neutrons faster than they can decay.
- Cosmic collisions
Supernovae, collisions between neutron stars and flares from magnetars forge heavy elements. The neutron star merger detected in 2017, GW170817, produced an estimated 3 to 13 Earth masses of gold.
- Into the solar system
That gold was scattered through space and gathered into the cloud that formed the Sun and planets.
- Down to the core
As the young Earth melted, most of its gold sank with iron into the core, out of reach.
- Back from space
The gold in the crust is thought to have arrived later, delivered by asteroid impacts during the Late Heavy Bombardment about 4 billion years ago.
- Concentrated by water and heat
Hot fluids moving through rock dissolved gold and deposited it in veins; erosion freed it into rivers as nuggets and grains.
Why gold is valuable
Gold's value rests on the same properties. It is rare: all the gold ever mined, around 201,000 tonnes as of 2020, would fit in a cube about 22 metres on each side. It lasts forever, so almost all of that gold still exists. It is beautiful and easy to work.
Those qualities made it money for thousands of years, and they explain why central banks still hold it and why a gold ring is worth its weight in gold long after it goes out of fashion.
What gold is used for
- Jewellery: its colour, softness and resistance to tarnish.
- Investment: bars, coins and the reserves of central banks.
- Electronics: connectors and contacts that must never corrode.
- Space: thin gold films reflect infrared radiation on satellites and spacecraft.
- Medicine and dentistry: crowns, and gold nanoparticles in research and diagnostics.
- Gold is element 79 with a single stable isotope, gold-197.
- A face-centred cubic lattice makes it soft; relativity makes it yellow.
- It formed in cosmic collisions and reached the crust with asteroids.
- Rarity, permanence and beauty are why it has always been valuable.
Questions we are asked
What is the atomic structure of gold?
A gold atom has 79 protons, 118 neutrons in its only stable isotope, gold-197, and 79 electrons arranged as [Xe] 4f14 5d10 6s1, with a single outer electron.
What is the crystal structure of gold?
Face-centred cubic: atoms at each corner of a cube and at the centre of each face, with a cube side of about 0.408 nanometres.
Why is gold yellow?
Relativistic effects on its inner electrons narrow the energy gap between the 5d and 6s shells, so gold absorbs blue light and reflects yellow.
Why does gold not rust or tarnish?
Gold is a noble metal: its electrons are held so tightly that it barely reacts with oxygen, water or most acids.
How is gold formed?
Through rapid neutron capture in supernovae, neutron star collisions and magnetar flares. Earth's accessible gold is thought to have arrived with asteroid impacts about 4 billion years ago.
How much gold has been mined?
Around 201,000 tonnes as of 2020, enough to fill a cube about 22 metres on each side.
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