Gold

Gold, ₇₉Au
Gold
Appearance	Metallic yellow
Standard atomic weight Ar°(Au)
	196.966570±0.000004; 196.97±0.01 (abridged);
Gold in the periodic table
	platinum ← gold → mercury
Atomic number (Z)	79
Group	group 11
Period	period 6
Block	d-block
Electron configuration	[Xe] 4f14 5d10 6s1
Electrons per shell	2, 8, 18, 32, 18, 1
Physical properties
Phase at STP	solid
Melting point	1337.33 K (1064.18 °C, 1947.52 °F)
Boiling point	3243 K (2970 °C, 5378 °F)
Density (at 20° C)	19.283 g/cm3
when liquid (at m.p.)	17.31 g/cm3
Heat of fusion	12.55 kJ/mol
Heat of vaporization	342 kJ/mol
Molar heat capacity	25.418 J/(mol·K)
Atomic properties
Oxidation states	−3, −2, −1, 0, +1, +2, +3, +5 (an amphoteric oxide)
Electronegativity	Pauling scale: 2.54
Ionization energies	1st: 890.1 kJ/mol ; 2nd: 1980 kJ/mol ; ;
Atomic radius	empirical: 144 pm
Covalent radius	136±6 pm
Van der Waals radius	166 pm
	Spectral lines of gold
Other properties
Natural occurrence	primordial
Crystal structure	face-centered cubic (fcc) (cF4)
Lattice constant	a = 407.86 pm (at 20 °C)
Thermal expansion	14.13×10−6/K (at 20 °C)
Thermal conductivity	318 W/(m⋅K)
Electrical resistivity	22.14 nΩ⋅m (at 20 °C)
Magnetic ordering	diamagnetic
Molar magnetic susceptibility	−28.0×10−6 cm3/mol (at 296 K)
Tensile strength	120 MPa
Young's modulus	79 GPa
Shear modulus	27 GPa
Bulk modulus	180 GPa
Speed of sound thin rod	2030 m/s (at r.t.)
Poisson ratio	0.4
Mohs hardness	2.5
Vickers hardness	188–216 MPa
Brinell hardness	188–245 MPa
CAS Number	7440-57-5
History
Naming	from Latin aurum 'gold'
Discovery	In the Middle East (before 6000 BCE)
Symbol	"Au": from Latin aurum
Isotopes of gold v; e;
β−	196Hg
	Category: Gold; view; talk; edit; \| references

Gold is a chemical element; it has symbol Au (from Latin aurum) and atomic number 79. In its pure form, it is a bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal, a group 11 element, and one of the noble metals. It is one of the least reactive chemical elements, being the second-lowest in the reactivity series. It is solid under standard conditions.

Quick Facts Appearance, Standard atomic weight Ar°(Au) ...

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Gold often occurs in free elemental (native state), as nuggets or grains, in rocks, veins, and alluvial deposits. It occurs in a solid solution series with the native element silver (as in electrum), naturally alloyed with other metals like copper and palladium, and mineral inclusions such as within pyrite. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides).

Gold is resistant to most acids, though it does dissolve in aqua regia (a mixture of nitric acid and hydrochloric acid), forming a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid alone, which dissolves silver and base metals, a property long used to refine gold and confirm the presence of gold in metallic substances, giving rise to the term 'acid test'. Gold dissolves in alkaline solutions of cyanide, which are used in mining and electroplating. Gold also dissolves in mercury, forming amalgam alloys, and as the gold acts simply as a solute, this is not a chemical reaction.

A relatively rare element,^[9]^[10] gold is a precious metal that has been used for coinage, jewelry, and other works of art throughout recorded history. In the past, a gold standard was often implemented as a monetary policy. Gold coins ceased to be minted as a circulating currency in the 1930s, and the world gold standard was abandoned for a fiat currency system after the Nixon shock measures of 1971.

In 2020, the world's largest gold producer was China, followed by Russia and Australia.^[11] As of 2020^[update], a total of around 201,296 tonnes of gold exist above ground.^[12] This is equal to a cube, with each side measuring roughly 21.7 meters (71 ft). The world's consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.^[13] Gold's high malleability, ductility, resistance to corrosion and most other chemical reactions, as well as conductivity of electricity have led to its continued use in corrosion-resistant electrical connectors in all types of computerized devices (its chief industrial use). Gold is also used in infrared shielding, the production of colored glass, gold leafing, and tooth restoration. Certain gold salts are still used as anti-inflammatory agents in medicine.

Gold is the most malleable of all metals. It can be drawn into a wire of single-atom width, and then stretched considerably before it breaks.^[14] Such nanowires distort via the formation, reorientation, and migration of dislocations and crystal twins without noticeable hardening.^[15] A single gram of gold can be beaten into a sheet of 1 square metre (11 sq ft), and an avoirdupois ounce into 28 square metres (300 sq ft). Gold leaf can be beaten thin enough to become semi-transparent. The transmitted light appears greenish-blue because gold strongly reflects yellow and red.^[16] Such semi-transparent sheets also strongly reflect infrared light, making them useful as infrared (radiant heat) shields in the visors of heat-resistant suits and in sun visors for spacesuits.^[17] Gold is a good conductor of heat and electricity.

Gold has a density of 19.3 g/cm³, almost identical to that of tungsten at 19.25 g/cm³; as such, tungsten has been used in the counterfeiting of gold bars, such as by plating a tungsten bar with gold.^[18]^[19]^[20]^[21] By comparison, the density of lead is 11.34 g/cm³, and that of the densest element, osmium, is 22.588±0.015 g/cm³.^[22]

Color

Whereas most metals are gray or silvery white, gold is slightly reddish-yellow.^[23] This color is determined by the frequency of plasma oscillations among the metal's valence electrons, in the ultraviolet range for most metals but in the visible range for gold due to relativistic effects affecting the orbitals around gold atoms.^[24]^[25] Similar effects impart a golden hue to metallic caesium.

Common colored gold alloys include the distinctive eighteen-karat rose gold created by the addition of copper. Alloys containing palladium or nickel are also important in commercial jewelry as these produce white gold alloys. Fourteen-karat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police and other badges. Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. Blue gold can be made by alloying with iron, and purple gold can be made by alloying with aluminium. Less commonly, addition of manganese, indium, and other elements can produce more unusual colors of gold for various applications.^[26]

Colloidal gold, used by electron-microscopists, is red if the particles are small; larger particles of colloidal gold are blue.^[27]

Isotopes

Gold has only one stable isotope, ¹⁹⁷
Au, which is also its only naturally occurring isotope, so gold is both a mononuclidic and monoisotopic element. Thirty-six radioisotopes have been synthesized, ranging in atomic mass from 169 to 205. The most stable of these is ¹⁹⁵
Au with a half-life of 186.1 days. The least stable is ¹⁷¹
Au, which decays by proton emission with a half-life of 30 μs. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, and β⁺ decay. The exceptions are ¹⁹⁵
Au, which decays by electron capture, and ¹⁹⁶
Au, which decays most often by electron capture (93%) with a minor β⁻ decay path (7%).^[28] All of gold's radioisotopes with atomic masses above 197 decay by β⁻ decay.^[29]

At least 32 nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range, only ¹⁷⁸
Au, ¹⁸⁰
Au, ¹⁸¹
Au, ¹⁸²
Au, and ¹⁸⁸
Au do not have isomers. Gold's most stable isomer is ^198m2
Au with a half-life of 2.27 days. Gold's least stable isomer is ^177m2
Au with a half-life of only 7 ns. ^184m1
Au has three decay paths: β⁺ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.^[29]

Synthesis

The possible production of gold from a more common element, such as lead, has long been a subject of human inquiry, and the ancient and medieval discipline of alchemy often focused on it; however, the transmutation of the chemical elements did not become possible until the understanding of nuclear physics in the 20th century. The first synthesis of gold was conducted by Japanese physicist Hantaro Nagaoka, who synthesized gold from mercury in 1924 by neutron bombardment.^[30] An American team, working without knowledge of Nagaoka's prior study, conducted the same experiment in 1941, achieving the same result and showing that the isotopes of gold produced by it were all radioactive.^[31] In 1980, Glenn Seaborg transmuted several thousand atoms of bismuth into gold at the Lawrence Berkeley Laboratory.^[32]^[33] Gold can be manufactured in a nuclear reactor, but doing so is highly impractical and would cost far more than the value of the gold that is produced.^[34]

Although gold is the most noble of the noble metals,^[35]^[36] it still forms many diverse compounds. The oxidation state of gold in its compounds ranges from −1 to +5, but Au(I) and Au(III) dominate its chemistry. Au(I), referred to as the aurous ion, is the most common oxidation state with soft ligands such as thioethers, thiolates, and organophosphines. Au(I) compounds are typically linear. A good example is Au(CN)−2, which is the soluble form of gold encountered in mining. The binary gold halides, such as AuCl, form zigzag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.^[37]

Au(III) (referred to as auric) is a common oxidation state, and is illustrated by gold(III) chloride, Au₂Cl₆. The gold atom centers in Au(III) complexes, like other d⁸ compounds, are typically square planar, with chemical bonds that have both covalent and ionic character. Gold(I,III) chloride is also known, an example of a mixed-valence complex.

Gold does not react with oxygen at any temperature^[38] and, up to 100 °C, is resistant to attack from ozone:^[39] ${\ce {Au + O2 ->}}({\text{no reaction}})$ ${\ce {Au{}+O3->[{} \atop {t<100^{\circ }{\text{C}}}]}}({\text{no reaction}})$

Some free halogens react to form the corresponding gold halides.^[40] Gold is strongly attacked by fluorine at dull-red heat^[41] to form gold(III) fluoride AuF₃. Powdered gold reacts with chlorine at 180 °C to form gold(III) chloride AuCl₃.^[42] Gold reacts with bromine at 140 °C to form a combination of gold(III) bromide AuBr₃ and gold(I) bromide AuBr, but reacts very slowly with iodine to form gold(I) iodide AuI: ${\ce {2Au{}+3F2->[{} \atop \Delta ]2AuF3}}$ ${\ce {2Au{}+3Cl2->[{} \atop \Delta ]2AuCl3}}$ ${\ce {2Au{}+2Br2->[{} \atop \Delta ]AuBr3{}+AuBr}}$ ${\ce {2Au{}+I2->[{} \atop \Delta ]2AuI}}$

Gold does not react with sulfur directly,^[43] but gold(III) sulfide can be made by passing hydrogen sulfide through a dilute solution of gold(III) chloride or chlorauric acid.

Unlike sulfur, phosphorus reacts directly with gold at elevated temperatures to produce gold phosphide (Au₂P₃).^[44]

Gold readily dissolves in mercury at room temperature to form an amalgam, and forms alloys with many other metals at higher temperatures. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors.^[26]

Gold is unaffected by most acids. It does not react with hydrofluoric, hydrochloric, hydrobromic, hydriodic, sulfuric, or nitric acid. It does react with selenic acid, and is dissolved by aqua regia, a 1:3 mixture of nitric acid and hydrochloric acid. Nitric acid oxidizes the metal to +3 ions, but only in minute amounts, typically undetectable in the pure acid because of the chemical equilibrium of the reaction. However, the ions are removed from the equilibrium by hydrochloric acid, forming AuCl−4 ions, or chloroauric acid, thereby enabling further oxidation: ${\ce {2Au{}+6H2SeO4->[{} \atop {200^{\circ }{\text{C}}}]Au2(SeO4)3{}+3H2SeO3{}+3H2O}}$ ${\ce {Au{}+4HCl{}+HNO3->HAuCl4{}+NO\uparrow +2H2O}}$

Gold is similarly unaffected by most bases. It does not react with aqueous, solid, or molten sodium or potassium hydroxide. It does however, react with sodium or potassium cyanide under alkaline conditions when oxygen is present to form soluble complexes.^[43]

Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds). Gold ions in solution are readily reduced and precipitated as metal by adding any other metal as the reducing agent. The added metal is oxidized and dissolves, allowing the gold to be displaced from solution and be recovered as a solid precipitate.

Rare oxidation states

Less common oxidation states of gold include −1, +2, and +5.

The −1 oxidation state occurs in aurides, compounds containing the Au⁻ anion. Caesium auride (CsAu), for example, crystallizes in the caesium chloride motif;^[45] rubidium, potassium, and tetramethylammonium aurides are also known.^[46] Gold has the highest electron affinity of any metal, at 222.8 kJ/mol, making Au⁻ a stable species,^[47] analogous to the halides.

Gold also has a –1 oxidation state in covalent complexes with the group 4 transition metals, such as in titanium tetraauride and the analogous zirconium and hafnium compounds. These chemicals are expected to form gold-bridged dimers in a manner similar to titanium(IV) hydride.^[48]

Gold(II) compounds are usually diamagnetic with Au–Au bonds such as [Au(CH₂)₂P(C₆H₅)₂]₂Cl₂. The evaporation of a solution of Au(OH)₃ in concentrated H₂SO₄ produces red crystals of gold(II) sulfate, Au

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