Gold's atomic number

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.^[19] From 6th or 5th century BC, Chu (state) circulated Ying Yuan, one kind of square gold coin.

The Romans developed new methods for extracting gold on a large scale using hydraulic mining methods, especially in Spain from 25 BC onwards and in Romania from 150 AD onwards. One of their largest mines was at Las Medulas in León (Spain), where seven long aqueducts enabled them to sluice most of a large alluvial deposit. The mines at Roşia Montană in Transylvania were also very large, and until very recently, still mined by opencast methods. They also exploited smaller deposits in Britain, such as placer and hard-rock deposits at Dolaucothi. The various methods they used are well described by Pliny the Elder in his encyclopedia Naturalis Historia written towards the end of the first century AD.

The Mali Empire in Africa was famed throughout the old world for its large amounts of gold. Mansa Musa, ruler of the empire (1312–1337) became famous throughout the old world for his great hajj to Mecca in 1324. When he passed through Cairo in July 1324, he was reportedly accompanied by a camel train that included thousands of people and nearly a hundred camels. He gave away so much gold that it depressed the price in Egypt for over a decade.^[20] A contemporary Arab historian remarked:

“	Gold was at a high price in Egypt until they came in that year. The mithqal did not go below 25 dirhams and was generally above, but from that time its value fell and it cheapened in price and has remained cheap till now. The mithqal does not exceed 22 dirhams or less. This has been the state of affairs for about twelve years until this day by reason of the large amount of gold which they brought into Egypt and spent there [...]	”
—Chihab Al-Umari[21]

The European exploration of the Americas was fueled in no small part by reports of the gold ornaments displayed in great profusion by Native American peoples, especially in Central America, Peru, Ecuador and Colombia.

Although the price of some platinum group metals can be much higher, gold has long been considered the most desirable of precious metals, and its value has been used as the standard for many currencies (known as the gold standard) in history. Gold has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties. Gold as a sign of wealth and prestige was made fun of by Thomas More in his treatise Utopia. On that imaginary island, gold is so abundant that it is used to make chains for slaves, tableware and lavatory-seats. When ambassadors from other countries arrive, dressed in ostentatious gold jewels and badges, the Utopians mistake them for menial servants, paying homage instead to the most modestly-dressed of their party.

There is an age-old tradition of biting gold in order to test its authenticity. Although this is certainly not a professional way of examining gold, the bite test should score the gold because gold is a soft metal, as indicated by its score on the Mohs' scale of mineral hardness. The purer the gold the easier it should be to mark it. Painted lead can cheat this test because lead is softer than gold (and may invite a small risk of lead poisoning if sufficient lead is absorbed by the biting).

Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910.^[22] It has been estimated that all the gold in the world that has ever been refined would form a single cube 20 m (66 ft) on a side (equivalent to 8000 m³).^[22]

One main goal of the alchemists was to produce gold from other substances, such as lead — presumably by the interaction with a mythical substance called the philosopher's stone. Although they never succeeded in this attempt, the alchemists promoted an interest in what can be done with substances, and this laid a foundation for today's chemistry. Their symbol for gold was the circle with a point at its center (☉), which was also the astrological symbol, and the ancient Chinese character, for the Sun. For modern creation of artificial gold by neutron capture, see gold synthesis.

During the 19th century, gold rushes occurred whenever large gold deposits were discovered. The first documented discovery of gold in the United States was at the Reed Gold Mine near Georgeville, North Carolina in 1803.^[23] The first major gold strike in the United States occurred in a small north Georgia town called Dahlonega.^[24] Further gold rushes occurred in California, Colorado, Otago, Australia, Witwatersrand, Black Hills, and Klondike.

Because of its historically high value, much of the gold mined throughout history is still in circulation in one form or another.

Price records

Since 1968 the price of gold on the open market has ranged widely, from a high of $850/oz ($27,300/kg) on January 21, 1980, to a low of $252.90/oz ($8,131/kg) on June 21, 1999 (London Gold Fixing).^[35] The 1980 high was not overtaken until January 3, 2008 when a new maximum of $865.35 per troy ounce was set (a.m. London Gold Fixing).^[36] The current record price was set on March 17, 2008 at $1023.50/oz ($32,900/kg)(am. London Gold Fixing).^[36]

Long term price trends

Since April 2001 the gold price has more than tripled in value against the US dollar,^[37] prompting speculation that this long secular bear market (or the Great Commodities Depression) has ended and a bull market has returned.^[38] In March 2008, the gold price increased above $1000,^[39] which in real terms is still well below the $850/oz. peak on January 21, 1980. Indexed for inflation, the 1980 high would equate to a price of around $2400 in 2007 US dollars.

In the last century, major economic crises (such as the Great Depression, World War II, the first and second oil crisis) lowered the Dow/Gold ratio (which is inherently inflation adjusted) substantially, in most cases to a value well below 4.^[40] During these difficult times, investors tried to preserve their assets by investing in precious metals, most notably gold and silver.

Compounds

Although gold is a noble metal, it forms many and diverse compounds. The oxidation state of gold in its compound ranges from −1 to +5 but Au(I) and Au(III) dominate. Gold(I), referred to as the aurous ion, is the most common oxidation state with “soft” ligands such as thioethers, thiolates, and tertiary phosphines. Au(I) compounds are typically linear. A good example is Au(CN)₂⁻, which is the soluble form of gold encountered in mining. Curiously, aurous complexes of water are rare. The binary gold halides, such as AuCl, form zig-zag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.^[41]

Gold(III) (“auric”) is a common oxidation state and is illustrated by gold(III) chloride, AuCl₃. Its derivative is chloroauric acid, HAuCl₄, which forms when Au dissolves in aqua regia. Au(III) complexes, like other d⁸ compounds, are typically square planar.

Less common oxidation states: Au(-I), Au(II), and Au(V)

Compounds containing the Au⁻ anion are called aurides. Caesium auride, CsAu which crystallizes in the caesium chloride motif.^[42] Other aurides include those of Rb⁺, K⁺, and tetramethylammonium (CH₃)₄N⁺.^[43] Gold(II) compounds are usually diamagnetic with Au-Au bonds such as [Au(CH₂)₂P(C₆H₅)₂]₂Cl₂. A noteworthy, legitimate Au(II) complex is the tetraxenonogold(II) cation, which contains xenon as a ligand, [AuXe₄](Sb₂F₁₁)₂.^[44] Gold pentafluoride is the sole example of Au(V), the highest verified oxidation state.^[45]

Some gold compounds exhibit aurophilic bonding, which describes the tendency of gold ions to interact at distances that are too long to be a conventional Au-Au bond but shorter that van der Waals bonding. The interaction is estimated to be comparable in strength to that of a hydrogen bond.

Mixed valence compounds

Well-defined cluster compounds are numerous.^[43] In such cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C₆H₅)₃)}₆²⁺. Gold chalcogenides, e.g. "AuS" feature equal amounts of Au(I) and Au(III).

gold

Gold (pronounced /ˈɡoʊld/) is a chemical element with the symbol Au (Latin: aurum) and an atomic number of 79. It has been a highly sought-after precious metal in jewelry, in sculpture, and for ornamentation since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is dense, soft, shiny and the most malleable and ductile pure metal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. It is one of the coinage metals and formed the basis for the gold standard used before the collapse of the Bretton Woods system in 1971.

At the end of 2006, it was estimated that all the gold ever mined totaled 158,000 tonnes. ^[1] This can be represented by a cube with an edge length of just 20.2 meters. Modern industrial uses include dentistry and electronics, where gold has traditionally found use because of its good resistance to oxidative corrosion and excellent quality as a conductor of electricity. Chemically, gold is a transition metal and can form trivalent and univalent cations upon solvation. At STP it is attacked by aqua regia (a mixture of acids), forming chloroauric acid and by alkaline solutions of cyanide but not by single acids such as hydrochloric, nitric or sulfuric acids. Gold dissolves in mercury, forming amalgam alloys, but does not react with it. Since gold is insoluble in nitric acid which will dissolve silver and base metals, this is exploited as the basis of the gold refining technique known as "inquartation and parting". Nitric acid has long been used to confirm the presence of gold in items, and this is the origin of the colloquial term "acid test", referring to a gold standard test for genuine value.

• In medieval times, gold was often seen as beneficial for the health, in the belief that something that rare and beautiful could not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power.^[7] Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions. ^[8] 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.
• In modern times injectable gold has been proven to help to reduce the pain and swelling of rheumatoid arthritis and tuberculosis.^[8][9]

• Dentistry. 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 (colloidal sols of gold nanoparticles) in water are intensely red-colored, and can be made with tightly-controlled particle sizes up to a few tens of nm 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 (Faulk and Taylor 1979). In ultrathin sections of tissues viewed by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the antigen (Roth et al. 1980). Colloidal gold is also the form of gold used as gold paint on ceramics prior to firing.

• 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 SEM's 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.

• The isotope gold-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.^[10]

Food and drink

• Gold can be used in food and has the E Number 175.^[11]
• Gold leaf, flake or dust is used on and in some gourmet foods, notably sweets and drinks as decorative ingredient.^[12] 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.
• Goldwasser (English: Goldwater) is a traditional herbal liqueur produced in Gdańsk, Poland, and Schwabach, Germany, and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf^[13]. However, since metallic gold is inert to all body chemistry, it adds no taste nor has it any other nutritional effect and leaves the body unaltered.^[14]

• 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 is ductile and malleable, meaning it can be drawn into very thin wire and can be beaten into very thin sheets known as gold leaf.
• 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 (Kodak, 2006).
• 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 insulation. McLaren uses gold foil in the engine compartment of its F1 model.^[15]
• 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.^[16]