Gold

platinum ← gold → mercury Ag ↑ Au ↓ Rg 79Au Periodic table
Appearance
metallic yellow
General properties
Name, symbol, number	gold, Au, 79
Pronunciation	/ˈɡoʊld/
Element category	transition metal
Group, period, block	11, 6, d
Standard atomic weight	196.966569(4)
Electron configuration	[Xe] 4f14 5d10 6s1
Electrons per shell	2, 8, 18, 32, 18, 1 (Image)
Physical properties
Phase	solid
Density (near r.t.)	19.30 g·cm−3
Liquid density at m.p.	17.31 g·cm−3
Melting point	1337.33 K, 1064.18 °C, 1947.52 °F
Boiling point	3129 K, 2856 °C, 5173 °F
Heat of fusion	12.55 kJ·mol−1
Heat of vaporization	324 kJ·mol−1
Molar heat capacity	25.418 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1646 1814 2021 2281 2620 3078
Atomic properties
Oxidation states	-1, 1, 2, 3, 4, 5 (amphoteric oxide)
Electronegativity	2.54 (Pauling scale)
Ionization energies	1st: 890.1 kJ·mol−1
	2nd: 1980 kJ·mol−1
Atomic radius	144 pm
Covalent radius	136±6 pm
Van der Waals radius	166 pm
Miscellanea
Crystal structure	Lattice face centered cubic
Magnetic ordering	diamagnetic[1]
Electrical resistivity	(20 °C) 22.14 nΩ·m
Thermal conductivity	318 W·m−1·K−1
Thermal expansion	(25 °C) 14.2 µm·m−1·K−1
Speed of sound (thin rod)	(r.t.) 2030 m·s−1
Tensile strength	120 MPa
Young's modulus	79 GPa
Shear modulus	27 GPa
Bulk modulus	180 GPa
Poisson ratio	0.44
Mohs hardness	2.5
Vickers hardness	216 MPa
Brinell hardness	25 HB MPa
CAS registry number	7440-57-5
Most stable isotopes
Main article: Isotopes of gold
iso NA half-life DM DE (MeV) DP 195Au syn 186.10 d ε 0.227 195Pt 196Au syn 6.183 d ε 1.506 196Pt β− 0.686 196Hg 197Au 100% 197Au is stable with 118 neutrons 198Au syn 2.69517 d β− 1.372 198Hg 199Au syn 3.169 d β− 0.453 199Hg
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Gold ( /ˈɡoʊld/) is a chemical element with the symbol Au (from Latin: aurum gold) and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive solid chemical elements. The metal therefore occurs often in free elemental (native) form, as nuggets or grains in rocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, usually with tellurium.

Gold resists attacks by individual acids, but it can be dissolved by the aqua regia (nitro-hydrochloric acid), so named because it dissolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, forming amalgam alloys. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items, giving rise to the term the acid test.

Gold has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have been the most common basis for monetary policies throughout human history, being widely supplanted by fiat currency only in the late 20th century. Gold has also been frequently linked to a wide variety of symbolisms and ideologies. A total of 165,000 tonnes of gold have been mined in human history, as of 2009.^[2] This is roughly equivalent to 5.3 billion troy ounces or, in terms of volume, about 8500 m³, or a cube 20.4 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.^[3]

Besides its widespread monetary and symbolic functions, gold has many practical uses in dentistry, electronics, and other fields. Its high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity led to many uses of gold, including electric wiring, colored-glass production and even gold leaf eating.

Characteristics

Gold is the most malleable and ductile of all metals; a single gram can be beaten into a sheet of 1 square meter, or an ounce into 300 square feet. Gold leaf can be beaten thin enough to become translucent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.^[4] 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.^[5]

Gold readily creates alloys with many other metals. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors (see below).^[6] Gold is a good conductor of heat and electricity and reflects infrared radiation strongly. Chemically, it is unaffected by air, moisture and most corrosive reagents, and is therefore well suited for use in coins and jewelry and as a protective coating on other, more reactive, metals. However, it is not chemically inert.

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 out as gold 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.

High quality pure metallic gold is tasteless and scentless, in keeping with its resistance to corrosion (it is metal ions which confer taste to metals).^[7]

In addition, gold is very dense, a cubic meter weighing 19,300 kg. By comparison, the density of lead is 11,340 kg/m³, and that of the densest element, osmium, is 22,610 kg/m³.

Color

Different colors of Ag-Au-Cu alloys

Whereas most other pure metals are gray or silvery white, gold is yellow. This color is determined by the density of loosely bound (valence) electrons; those electrons oscillate as a collective plasma medium described in terms of a quasiparticle called plasmon. The frequency of these oscillations lies in the ultraviolet range for most metals, but it falls into the visible range for gold due to subtle relativistic effects that affect the orbitals around gold atoms.^[8][9] Similar effects impart a golden hue to metallic caesium (see relativistic quantum chemistry).

Common colored gold alloys such as rose gold can be created by the addition of various amounts of copper and silver, as indicated in the triangular diagram to the left. Alloys containing palladium or nickel are also important in commercial jewelry as these produce white gold alloys. Less commonly, addition of manganese, aluminium, iron, indium and other elements can produce more unusual colors of gold for various applications.^[6]

Isotopes

Gold has only one stable isotope, ¹⁹⁷Au, which is also its only naturally occurring isotope. Thirty-six radioisotopes have been synthesized ranging in atomic mass from 169 to 205. The most stable of these is ¹⁹⁵Au with a half-life of 186.1 days. The least stable is ¹⁷¹Au, 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 ¹⁹⁵Au, which decays by electron capture, and ¹⁹⁶Au, which decays most often by electron capture (93%) with a minor β- decay path (7%).^[10] All of gold's radioisotopes with atomic masses above 197 decay by β- decay.^[11]

At least 32 nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range, only ¹⁷⁸Au, ¹⁸⁰Au, ¹⁸¹Au, ¹⁸²Au, and ¹⁸⁸Au do not have isomers. Gold's most stable isomer is ^198m2Au with a half-life of 2.27 days. Gold's least stable isomer is ^{177 m2}Au with a half-life of only 7 ns. ^{184 m1}Au has three decay paths: β+ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.^[11]

Use and applications

Monetary exchange

Gold has been widely used throughout the world as a vehicle for monetary exchange, either by issuance and recognition of gold coins or other bare metal quantities, or through gold-convertible paper instruments by establishing gold standards in which the total value of issued money is represented in a store of gold reserves.

However, production has not grown in relation to the world's economies. Today, gold mining output is declining.^[12] With the sharp growth of economies in the 20th century, and increasing foreign exchange, the world's gold reserves and their trading market have become a small fraction of all markets and fixed exchange rates of currencies to gold were no longer sustained.

At the beginning of World War I the warring nations moved to a fractional gold standard, inflating their currencies to finance the war effort. After World War II gold was replaced by a system of convertible currency following the Bretton Woods system. Gold standards and the direct convertibility of currencies to gold have been abandoned by world governments, being replaced by fiat currency in their stead. Switzerland was the last country to tie its currency to gold; it backed 40% of its value until the Swiss joined the International Monetary Fund in 1999.^[13]

Pure gold is too soft for day-to-day monetary use and is typically hardened by alloying with copper, silver or other base metals. The gold content of alloys is measured in carats (k). Pure gold is designated as 24k. English gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness (American gold coins for circulation after 1837 contained the slightly lower amount of 0.900 fine gold, or 21.6 kt).

Investment

Gold Price per gram between Jan 1971 and Jan 2012. The graph shows nominal price in US Dollars, the price in 1971 and 2011 US Dollars

Many holders of gold store it in form of bullion coins or bars as a hedge against inflation or other economic disruptions. However, some economists do not believe gold serves as a hedge against inflation or currency depreciation.^[14]

The ISO 4217 currency code of gold is XAU.

Modern bullion coins for investment or collector purposes do not require good mechanical wear properties; they are typically fine gold at 24k, although the American Gold Eagle, the British gold sovereign, and the South African Krugerrand continue to be minted in 22k metal in historical tradition. The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin, at 99.999% or 0.99999, while the popular issue Canadian Gold Maple Leaf coin has a purity of 99.99%.

Several other 99.99% pure gold coins are available. In 2006, the United States Mint began production of the American Buffalo gold bullion coin with a purity of 99.99%. The Australian Gold Kangaroos were first coined in 1986 as the Australian Gold Nugget but changed the reverse design in 1989. Other popular modern coins include the Austrian Vienna Philharmonic bullion coin and the Chinese Gold Panda.

Jewelry

Moche gold necklace depicting feline heads. Larco Museum Collection. Lima-Peru

Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower caratage, typically 22k, 18k, 14k or 10k, contain higher percentages of copper, or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color.^[15]

Eighteen-carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen-carat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police and other badges. Blue gold can be made by alloying with iron and purple gold can be made by alloying with aluminium, although rarely done except in specialized jewelry. Blue gold is more brittle and therefore more difficult to work with when making jewelry.^[15]

Fourteen and eighteen carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with palladium or nickel. White 18-carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silvery in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe.^[15]

Alternative white gold alloys are available based on palladium, silver and other white metals,^[15] but the palladium alloys are more expensive than those using nickel. High-carat white gold alloys are far more resistant to corrosion than are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects.

Medicine

In medieval times, gold was often seen as beneficial for the health, in the belief that something so rare and beautiful could not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power.^[16] Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions.^[17] Gold based injections have been explored as a means to help to reduce the pain and swelling of rheumatoid arthritis and tuberculosis.^[17][18] However, only salts and radioisotopes of gold are of pharmacological value, as elemental (metallic) gold is inert to all chemicals it encounters inside the body.

Gold alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges. The gold alloys' slight malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.

Colloidal gold preparations (suspensions of gold nanoparticles) in water are intensely red-colored, and can be made with tightly controlled particle sizes up to a few tens of nanometers across by reduction of gold chloride with citrate or ascorbate ions. Colloidal gold is used in research applications in medicine, biology and materials science. The technique of immunogold labeling exploits the ability of the gold particles to adsorb protein molecules onto their surfaces. Colloidal gold particles coated with specific antibodies can be used as probes for the presence and position of antigens on the surfaces of cells.^[19] In ultrathin sections of tissues viewed by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the antigen.^[20]

Gold, or alloys of gold and palladium, are applied as conductive coating to biological specimens and other non-conducting materials such as plastics and glass to be viewed in a scanning electron microscope. The coating, which is usually applied by sputtering with an argon plasma, has a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earth, and its very high density provides stopping power for electrons in the electron beam, helping to limit the depth to which the electron beam penetrates the specimen. This improves definition of the position and topography of the specimen surface and increases the spatial resolution of the image. Gold also produces a high output of secondary electrons when irradiated by an electron beam, and these low-energy electrons are the most commonly used signal source used in the scanning electron microscope.^[21]

The isotope gold-198, (half-life 2.7 days) is used in some cancer treatments and for treating other diseases.^[22]

Food and drink

• Gold can be used in food and has the E number 175.^[23]
• Gold leaf, flake or dust is used on and in some gourmet foods, notably sweets and drinks as decorative ingredient.^[24] Gold flake was used by the nobility in Medieval Europe as a decoration in food and drinks, in the form of leaf, flakes or dust, either to demonstrate the host's wealth or in the belief that something that valuable and rare must be beneficial for one's health.
• Danziger Goldwasser (German: Gold water of Danzig) or Goldwasser (English: Goldwater) is a traditional German herbal liqueur^[25] produced in what is today Gdańsk, Poland, and Schwabach, Germany, and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf.^[26] However, since metallic gold is inert to all body chemistry, it has no taste, it provides no nutrition, and it leaves the body unaltered.^[27]

Industry

The 220 kg gold brick displayed in Jinguashi Gold Museum, Taiwan, Republic of China.

The world's largest gold bar has a mass of 250 kg. Toi museum, Japan.

A gold nugget of 5 mm in diameter (bottom) can be expanded through hammering into a gold foil of about 0.5 square meter. Toi museum, Japan.

• Gold solder is used for joining the components of gold jewelry by high-temperature hard soldering or brazing. If the work is to be of hallmarking quality, gold solder must match the carat weight of the work, and alloy formulas are manufactured in most industry-standard carat weights to color match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting points, goldsmiths can assemble complex items with several separate soldered joints.
• Gold can be made into thread and used in embroidery.
• Gold produces a deep, intense red color when used as a coloring agent in cranberry glass.
• In photography, gold toners are used to shift the color of silver bromide black-and-white prints towards brown or blue tones, or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak published formulas for several types of gold toners, which use gold as the chloride.^[28]
• As gold is a good reflector of electromagnetic radiation such as infrared and visible light as well as radio waves, it is used for the protective coatings on many artificial satellites, in infrared protective faceplates in thermal protection suits and astronauts' helmets and in electronic warfare planes like the EA-6B Prowler.
• Gold is used as the reflective layer on some high-end CDs.
• Automobiles may use gold for heat dissipation. McLaren uses gold foil in the engine compartment of its F1 model.^[29]
• Gold can be manufactured so thin that it appears transparent. It is used in some aircraft cockpit windows for de-icing or anti-icing by passing electricity through it. The heat produced by the resistance of the gold is enough to deter ice from forming.^[30]

Electronics

The concentration of free electrons in gold metal is 5.90×10²² cm⁻³. Gold is highly conductive to electricity, and has been used for electrical wiring in some high-energy applications (only silver and copper are more conductive per volume, but gold has the advantage of corrosion resistance). For example, gold electrical wires were used during some of the Manhattan Project's atomic experiments, but large high current silver wires were used in the calutron isotope separator magnets in the project.

Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a thin layer coating electrical connectors of all kinds, thereby ensuring good connection. For example, gold is used in the connectors of the more expensive electronics cables, such as audio, video and USB cables. The benefit of using gold over other connector metals such as tin in these applications is highly debated. Gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy. However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and in use for contacts with a very high failure cost (certain computers, communications equipment, spacecraft, jet aircraft engines) remains very common.^[31]

Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion, electrical conductivity, ductility and lack of toxicity.^[32] Switch contacts are generally subjected to more intense corrosion stress than are sliding contacts. Fine gold wires are used to connect semiconductor devices to their packages through a process known as wire bonding.

Commercial chemistry

Gold is attacked by and dissolves in alkaline solutions of potassium or sodium cyanide, to form the salt gold cyanide—a technique that has been used in extracting metallic gold from ores in the cyanide process. Gold cyanide is the electrolyte used in commercial electroplating of gold onto base metals and electroforming.

Gold chloride (chloroauric acid) solutions are used to make colloidal gold by reduction with citrate or ascorbate ions. Gold chloride and gold oxide are used to make highly valued cranberry or red-colored glass, which, like colloidal gold suspensions, contains evenly sized spherical gold nanoparticles.^[33]

History

The Turin Papyrus Map

Funerary mask of Tutankhamun

Jason returns with the golden fleece on an Apulian red-figure calyx krater, ca. 340–330 BC.

Gold has been known and used by artisans since the Chalcolithic. Gold artifacts in the Balkans appear from the 4th millennium BC, such as those found in the Varna Necropolis, Bulgaria. Gold artifacts such as the golden hats and the Nebra disk appeared in Central Europe from the 2nd millennium BC Bronze Age.

Egyptian hieroglyphs from as early as 2600 BC describe gold, which king Tushratta of the Mitanni claimed was more plentiful than dirt in Egypt.^[34] Egypt and especially Nubia had the resources to make them major gold-producing areas for much of history. The earliest known map is known as the Turin Papyrus Map and shows the plan of a gold mine in Nubia together with indications of the local geology. The primitive working methods are described by both Strabo and Diodorus Siculus, and included fire-setting. Large mines were also present across the Red Sea in what is now Saudi Arabia.

The legend of the golden fleece may refer to the use of fleeces to trap gold dust from placer deposits in the ancient world. Gold is mentioned frequently in the Old Testament, starting with Genesis 2:11 (at Havilah) and is included with the gifts of the magi in the first chapters of Matthew New Testament. The Book of Revelation 21:21 describes the city of New Jerusalem as having streets made of pure gold, clear as crystal. The south-east corner of the Black Sea was famed for its gold. Exploitation is said to date from the time of Midas, and this gold was important in the establishment of what is probably the world's earliest coinage in Lydia around 610 BC.^[35] From the 6th or 5th century BC, the
strawberry silver and gold investment