'''Gold''' () is a nuggets or grains in rocks, underground "veins" and in alluvial deposits. It is one of the coinage metals. Gold is dense, soft, shiny and the most malleable and ductile substance known. Pure gold has a bright yellow color traditionally considered attractive.

Gold formed the basis for the ISO currency code of gold bullion is '''XAU'''.

Modern industrial uses include oxidative corrosion.

Chemically, gold is a mercury, forming amalgam alloys, but does not react with it. Gold is insoluble in nitric acid, which will dissolve silver and base metals, and this is 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.

Characteristics

Gold readily forms alloys with many other metals. These alloys can be produced to increase the hardness or to create exotic colors (see below). Gold is a good conductor of air and most corrosive agents have very little chemical effect on gold, making it well-suited for use in coins and jewelry; conversely, halogens will chemically alter gold, and aqua regia dissolves it via formation of the chloraurate ion.

Common 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.

Recent research undertaken by Sir Frank Reith of the Australian National University shows that microbes play an important role in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.

High quality pure metallic gold is tasteless; in keeping with its resistance to corrosion (it is metal ions which confer taste to metals).

In addition, gold is very dense, a cubic meter weighing 19300 kg. By comparison, the density of lead is 11340 kg/mآ³, and the densest element, iridium, is 22650 kg/mآ³.

Color of gold

Applications

As the metal

=Medium of monetary exchange

In various countries, gold is used as a standard for monetary exchange, in carats (k), pure gold being designated as 24k.

Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called American Buffalo gold bullion coin also at 99.99% purity.

Today, gold has fallen out of favor for use in coins made for general circulation.

=Jewelry

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. Eighteen carat gold containing 25% copper is found in antique and Russian jewellery 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. 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 silver in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe. Alternative white gold alloys are available based on palladium, silver and other white metals (World Gold Council), 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.

thumb|200px|right|The 220 kg gold brick displayed in Chinkuashi Gold Museum, Taiwan.

=Other

* Gold is sometimes used in computer components

• In medieval times, gold was often seen as beneficial for the health (even though it was not), 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. Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions. 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.

• title = The Food Dictionary: Varak | publisher = Barron's Educational Services, Inc | date= 1995 | url = http://www.epicurious.com/cooking/how_to/food_dictionary/entry?id=5061 | accessdate = 2007-05-27}} Gold flake was used by the nobility in [[Medieval Europe as a decoration in foodstuffs 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.

• 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 used in food and has the E Number 175. German: "Goldwater") is a traditional herbal liqueur produced in Gdaإ„sk, Poland, and Schwabach, Germany, and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf. 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.

• ''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.

• 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 publish formulas for several types of gold toners, which use gold as the chloride (Kodak, 2006).

• ''Electronics.'' The concentration of free electrons in gold metal is 5.90أ—10²² cm^-3. Gold is highly conductive to electricity, and has been used for electrical wiring in some high energy applications (silver is even 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, and is unlikely to be replaced in the near future by any other metal.

• Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to ductility and lack of toxicity. Switch contacts are generally subjected to more intense corrosion stress than are sliding contacts.

• 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 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 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.

• Many competitions, and honors, such as the Olympics and the Nobel Prize, award a gold medal to the winner.

• 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.

• The isotope gold-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.Nanoscience and Nanotechnology in Nanomedicine: Hybrid Nanoparticles In Imaging and Therapy of Prostate Cancer - Radiopharmaceutical Sciences Institute, University of Missouri-Columbia

• Automobiles may use gold for heat insulation. McLaren F1 uses gold foil in the engine compartment.Supercars.net. 1994 McLaren F1

  As gold chemical compounds

History

From 6th or 5th century BCE,Chu (state) circulated Ying Yuan,one kind of square gold coin.

The Romans developed new methods for extracting gold on a large scale using Leأ³n (Spain), where seven long aqueducts enabled them to sluice most of a large alluvial deposit. The mines at Roإںia Montanؤƒ in Transylvania were also very large, and until very recently, still mined by opencast methods. They also exploited smaller deposits in Wales, 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 of 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 took over a decade for the economy across North Africa to recover, due to the rapid inflation that it initiated.Mansa Musa - Black History Pages A contemporary Arab historian remarked;

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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, and Colombia.

Although the price of some platinum group metals can be much higher, gold has long been considered the most desirable of currencies (known as the ''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 considered a soft metal according to the thumb|right|250px|This 156 ounce (4,42 kg) nugget was found by an individual prospector in the Southern California Desert using a metal detector.

Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910. 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 mآ³).

One main goal of the alchemists was to produce gold from other substances, such as circle with a point at its center (âک‰), which was also the astrological symbol, the hieroglyph and the ancient Chinese character for the Sun. For modern attempts to produce artificial gold, see gold synthesis.

During the 19th century, Dahlonega. 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.

Occurrence

Ores bearing native gold consist of grains or microscopic particles of metallic gold embedded in rock, often in association with veins of quartz or sulfide minerals like pyrite. These are called "lode" deposits. Native gold is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits (called placer deposits). Such free gold is always richer at the surface of gold-bearing veins owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets.

Gold sometimes occurs in minerals in chemical composition with other elements, especially in association with tellurium. Examples are calaverite, sylvanite, nagyagite, petzite and krennerite. Gold also occurs rarely as a mercury-gold amalgam, and in very low concentrations in seawater.

Production

Since the 1880s, South Africa has been the source for a large proportion of the world’s gold supply, with about 50% of all gold ever produced having come from South Africa. Production in 1970 accounted for 79% of the world supply, producing about 1,000 tonnes. However by 2007 production was just 272 tonnes. This sharp decline was due to the increasing difficulty of extraction, changing economic factors affecting the industry, and tightened safety auditing. In 2007 China (with 276 tonnes) overtook South Africa as the world's largest gold producer, the first time since 1905 that South Africa has not been the largest.China now world's largest gold producer; foreign miners at door - MarketWatch

The city of Johannesburg located in South Africa was founded as a result of the Witwatersrand Gold Rush which resulted in the discovery of some of the largest gold deposits the world has ever seen. Gold fields located within the basin in the Free State and Gauteng provinces are extensive in strike and dip requiring some of the world's deepest mines, with the Savuka and TauTona mines being currently the world's deepest gold mine at 3,777 m. The Second Boer War of 1899–1901 between the British Empire and the Afrikaner Boers was at least partly over the rights of miners and possession of the gold wealth in South Africa.

Other major producers are United States, Australia, China, Russia and Peru. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States. In South America, the controversial project Pascua Lama aims at exploitation of rich fields in the high mountains of Atacama Desert, at the border between Chile and Argentina. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining. – ^{Scholar search}}}

After initial production, gold is often subsequently refined industrially by the Wohlwill process or the Miller process. Other methods of assaying and purifying smaller amounts of gold include parting and inquartation as well as cuppelation, or refining methods based on the dissolution of gold in aqua regia.

The world's oceans hold a vast amount of gold, but in very low concentrations (perhaps 1–2 parts per 10 billion). A number of people have claimed to be able to economically recover gold from sea water, but so far they have all been either mistaken or crooks. Reverend Prescott Jernegan ran a gold-from-seawater swindle in America in the 1890s. A British fraud ran the same scam in England in the early 1900s.Dan Plazak, ''A Hole in the Ground with a Liar at the Top'' (Salt Lake: Univ. of Utah Press, 2006) (contains a chapter on gold-from seawater swindles)

Fritz Haber (the German inventor of the Haber process) attempted commercial extraction of gold from sea water in an effort to help pay Germany's reparations following the First World War. Unfortunately, his assessment of the concentration of gold in sea water was unduly high, probably due to sample contamination. The effort produced little gold and cost the German government far more than the commercial value of the gold recovered. No commercially viable mechanism for performing gold extraction from sea water has yet been identified. Gold synthesis is not economically viable and is unlikely to become so in the foreseeable future.

The average gold mining and extraction costs are $238 per troy ounce but these can vary widely depending on mining type and ore quality. In 2001, global mine production amounted to 2,604 tonnes, or 67% of total gold demand in that year. At the end of 2006, it was estimated that all the gold ever mined totaled 158,000 tonnes.

At current consumption rates, the supply of gold is believed to last 45 years.

Price

thumb|right|200px|LBMA USD morning Gold Fix|price fixings ($US per troy ounce) since 2001 Since 1968 the price of gold on the open market has ranged widely, from a high of $850/oz ($27,300/kg) on url=http://kitco.com/LFgif/au75-pres.gif | title=kitco.com: GOLD - London PM Fix 1975 - present (GIF) | accessdate=2006-07-22}} The 1980 high was not overtaken until 3 January troy ounce was set (a.m. London Gold Fixing).LBMA statistics The current record price was set on March 2008 at $1023.50 (am. London Gold Fixing).[http://www.lbma.org.uk/2008dailygold.htm LBMA statistics

Long term price trends

Compounds

Gold(III) (“auricâ€‌) 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)

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

Isotopes

There is one stable isotope of gold, and 18 radioisotopes with ¹⁹⁵Au being the most stable with a half-life of 186 days.

Gold has been proposed as a "salting" material for gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several days. Such a weapon is not known to have ever been built, tested, or used.

Symbolism

See also

Footnotes

Bibliography

*Faulk W, Taylor G (1979) ''An Immunocolloid Method for the Electron Microscope'' Immunochemistry '''8''', 1081–1083.

• Kodak (2006) Toning black-and-white materials. Technical Data/Reference sheet G-23, May 2006.

• Roth J, Bendayan M, Orci L (1980) ''FITC-Protein A-Gold Complex for Light and Electron Microscopic Immunocytochemistry''. Journal of Histochemistry and Cytochemistry '''28''', 55–57.

• World Gold Council, Jewellery Technology, Jewellery Alloys

• Los Alamos National Laboratory – Gold

  External links

  *''Getting Gold'' 1898 book

• Technical Document on Extraction and Mining of Gold

• Picture in the Element collection from Heinrich Pniok

  Category:Chemical elements