Gold

Gold,  79Au
Gold-crystals.jpg
General properties
Appearancemetallic yellow
Standard atomic weight (Ar, standard)196.966569(5)[1]
Gold in the periodic table
HydrogenHelium
LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
SodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorineArgon
PotassiumCalciumScandiumTitaniumVanadiumChromiumManganeseIronCobaltNickelCopperZincGalliumGermaniumArsenicSeleniumBromineKrypton
RubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumTinAntimonyTelluriumIodineXenon
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon
FranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganesson
Ag

Au

Rg
platinumgoldmercury
Atomic number (Z)79
Groupgroup 11
Periodperiod 6
Element category  transition metal
Blockd-block
Electron configuration[Xe] 4f14 5d10 6s1
Electrons per shell
2, 8, 18, 32, 18, 1
Physical properties
Phase at STPsolid
Melting point1337.33 K ​(1064.18 °C, ​1947.52 °F)
Boiling point3243 K ​(2970 °C, ​5378 °F)
Density (near r.t.)19.30 g/cm3
when liquid (at m.p.)17.31 g/cm3
Heat of fusion12.55 kJ/mol
Heat of vaporization342 kJ/mol
Molar heat capacity25.418 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)164618142021228126203078
Atomic properties
Oxidation states5, 3, 2, 1, −1, −2, −3 ​(an amphoteric oxide)
ElectronegativityPauling scale: 2.54
Ionization energies
  • 1st: 890.1 kJ/mol
  • 2nd: 1980 kJ/mol
Atomic radiusempirical: 144 pm
Covalent radius136±6 pm
Van der Waals radius166 pm
Color lines in a spectral range
Miscellanea
Crystal structureface-centered cubic (fcc)
Face centered cubic crystal structure for gold
Speed of sound thin rod2030 m/s (at r.t.)
Thermal expansion14.2 µm/(m·K) (at 25 °C)
Thermal conductivity318 W/(m·K)
Electrical resistivity22.14 nΩ·m (at 20 °C)
Magnetic orderingdiamagnetic[2]
Magnetic susceptibility−28.0·10−6 cm3/mol (at 296 K)[3]
Tensile strength120 MPa
Young's modulus79 GPa
Shear modulus27 GPa
Bulk modulus180 GPa[4]
Poisson ratio0.4
Mohs hardness2.5
Vickers hardness188–216 MPa
Brinell hardness188–245 MPa
CAS Number7440-57-5
History
Namingfrom Latin aurum, meaning gold
DiscoveryIn the Middle East (before 6000 BCE)
Main isotopes of gold
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
195Ausyn186.10 dε195Pt
196Ausyn6.183 dε196Pt
β196Hg
197Au100%stable
198Ausyn2.69517 dβ198Hg
199Ausyn3.169 dβ199Hg
| references | in Wikidata

Gold is a chemical element with symbol Au (from Latin: aurum) and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions. Gold often occurs in free elemental (native) form, as nuggets or grains, in rocks, in veins, and in alluvial deposits. It occurs in a solid solution series with the native element silver (as electrum) and also naturally alloyed with copper and palladium. 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, which forms a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to refine gold and to confirm the presence of gold in metallic objects, giving rise to the term acid test. Gold also dissolves in alkaline solutions of cyanide, which are used in mining and electroplating. Gold dissolves in mercury, forming amalgam alloys, but this is not a chemical reaction.

A relatively rare element,[5][6] gold is a precious metal that has been used for coinage, jewelry, and other arts throughout recorded history. In the past, a gold standard was often implemented as a monetary policy, but 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 1971.

A total of 186,700 tonnes of gold exists above ground, as of 2015.[7] The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.[8] Gold's high malleability, ductility, resistance to corrosion and most other chemical reactions, and 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, colored-glass production, gold leafing, and tooth restoration. Certain gold salts are still used as anti-inflammatories in medicine. As of 2016, the world's largest gold producer by far was China with 450 tonnes per year.[9]

Characteristics

A gold nugget of 5 millimetres (0.20 in) in diameter (bottom) can be expanded through hammering into a gold foil of about 0.5 square metres (5.4 sq ft). Toi museum, Japan.

Gold is the most malleable of all metals; a single gram can be beaten into a sheet of 1 square meter, and an avoirdupois ounce into 300 square feet. Gold leaf can be beaten thin enough to become semi-transparent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.[10] Such semi-transparent sheets also strongly reflect infrared light, making them useful as infrared (radiant heat) shields in visors of heat-resistant suits, and in sun-visors for spacesuits.[11] 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 counterfeiting of gold bars, such as by plating a tungsten bar with gold,[12][13][14][15] or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.[16] 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.[17]

Color

Different colors of Ag–Au–Cu alloys

Whereas most metals are gray or silvery white, gold is slightly reddish-yellow.[18] 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.[19][20] 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. White gold alloys can be made with palladium or nickel. 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, aluminium, indium and other elements can produce more unusual colors of gold for various applications.[21]

Colloidal gold, used by electron-microscopists, is red if the particles are small; larger particles of colloidal gold are blue.[22]

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%).[23] All of gold's radioisotopes with atomic masses above 197 decay by β decay.[24]

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.[24]

Synthesis

The 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.[25] 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.[26]

Gold can currently be manufactured in a nuclear reactor by irradiation either of platinum or mercury.

Only the mercury isotope 196Hg, which occurs with a frequency of 0.15% in natural mercury, can be converted to gold by neutron capture, and following electron capture-decay into 197Au with slow neutrons. Other mercury isotopes are converted when irradiated with slow neutrons into one another, or formed mercury isotopes which beta decay into thallium.

Using fast neutrons, the mercury isotope 198Hg, which composes 9.97% of natural mercury, can be converted by splitting off a neutron and becoming 197Hg, which then disintegrates to stable gold. This reaction, however, possesses a smaller activation cross-section and is feasible only with un-moderated reactors.

It is also possible to eject several neutrons with very high energy into the other mercury isotopes in order to form 197Hg. However, such high-energy neutrons can be produced only by particle accelerators.[clarification needed]

Other Languages
Acèh: Meuih
Afrikaans: Goud
Alemannisch: Gold
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Ænglisc: Gold
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dansk: Guld
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кырык мары: Шӧртньӹ
лакку: Муси
лезги: Къизил
ລາວ: ທອງຄຳ
Latina: Aurum
latviešu: Zelts
Lëtzebuergesch: Gold
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Simple English: Gold
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