Gold – facts and properties
- Symbol
- Au
- Atomic number
- 79
- Atomic mass
- 196.967 u
- Period
- 6
- Group
- 11
- Category
- transition metal
- Electron configuration
- [Xe] 4f¹⁴ 5d¹⁰ 6s¹
- Electronegativity
- 2.54
- Atomic radius
- 179 pm
- Valence electrons
- 11
- State at standard conditions
- solid
- Melting point
- 1064.18 °C
- Boiling point
- 2856 °C
- Density
- 19.282 g/cm³
- Discovered
- known since antiquity
Gold is a soft, bright yellow precious metal with atomic number 79. It has been known since antiquity and was long used as jewellery, treasure and money. Chemically it is exceptionally unreactive, so ancient gold still looks as it did on the first day. The symbol Au comes from the Latin aurum. Besides jewellery and investment, gold is needed in electronics, in dentistry and in space technology.
Position in the periodic table
Occurrence and extraction
Gold often occurs native, as the pure metal, in grains and flakes in quartz veins or as nuggets and gold dust in river deposits known as placers. Today the biggest producers are China, Russia, Australia and Canada. The Witwatersrand Basin in South Africa supplied a large part of the world's gold in the twentieth century. All the gold ever mined would fill a cube only a little over twenty metres on each side.
Rock often holds only tiny amounts of gold. Large mines dissolve the fine gold with a cyanide solution, because in the presence of oxygen gold forms soluble cyanide complexes. Cyanides are highly toxic, so this requires strict safety measures. In small-scale mining, for example in parts of South America, Africa and Asia, mercury is widely used, which combines with gold to form an amalgam. The mercury is afterwards evaporated and ends up in the air and in water. This small-scale mining is regarded as one of the largest sources of mercury in the environment.
Properties and reactions
Gold is in period 6 and group 11, directly below silver. Its electron configuration, [Xe] 4f¹⁴ 5d¹⁰ 6s¹, has a single 6s electron outside a full 5d shell. With an electronegativity of 2.54 it comes first among the metals. Gold has a density of 19.282 g/cm³ and melts at 1064.18 °C, while it boils at 2856 °C. Gold is the most malleable of all metals: a single gram can be drawn into a wire several kilometres long, and gold leaf is only about one ten-thousandth of a millimetre thick.
Why is gold yellow while silver is white? The reason is relativity. The nuclei of heavy gold atoms carry a large charge, and the s electrons close to them move so fast that their mass grows noticeably. As a result the 6s orbital contracts and drops in energy, while the 5d electrons are held less tightly. The energy gap between the two shrinks until gold absorbs blue light. In silver the same absorption lies in the ultraviolet. What is left is yellow and red, which we see as golden.
The tighter hold on its electrons also makes gold noble. Oxygen and most acids do not attack it. It dissolves in aqua regia: the nitric acid oxidises the gold, and the chloride ions of the hydrochloric acid keep the resulting ions in solution as a stable complex, [AuCl₄]⁻.
Uses in everyday life and technology
Most gold is made into jewellery or held as an investment in bars and coins. Central banks keep gold reserves because gold is scarce, durable and accepted everywhere. Pure gold is too soft for jewellery, so it is alloyed, mostly with copper and silver. The gold content is given in carats: 24 carats is pure gold, 18 carats is 750 parts in a thousand, and 14 carats is 585 parts in a thousand.
Gold leaf decorates domes, picture frames and lettering. The metal itself is non-toxic and is approved for decoration as the food colour E 175. In electronics gold is found in contacts, connectors and the fine bonding wires inside chips, because it does not corrode and conducts well. The mirrors of the James Webb Space Telescope carry an extremely thin coat of gold, since gold reflects infrared light very well. Dentists use gold alloys for crowns and inlays, and gold nanoparticles colour the test line of rapid tests red.
Physics owes gold a great deal too. In the experiment of Geiger and Marsden, alpha particles were fired at an extremely thin gold foil. Some were deflected sharply, and in 1911 Ernest Rutherford concluded that almost all of an atom's mass sits in a tiny nucleus.
Molecules containing this element
- Gold(I) tert-dodecanethiolate C12H25SAu
Quick question
Which metal reacts more vigorously with water?
FAQ
Why does gold not rust?
Rust forms when iron reacts with oxygen and water. Gold holds on to its electrons so firmly that it does not react with air, water or most acids.
Why is gold yellow?
The electrons in heavy gold atoms move so fast that relativistic effects come into play. Gold therefore absorbs blue light, and the light it reflects looks yellow.
What does 18 carat mean?
It means that 750 parts in a thousand of the alloy are gold, three quarters. The rest is other metals, which make the gold harder and change its colour.