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,   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 s, and the world gold standard was abandoned for a fiat currency system after A total of , tonnes of gold exists above ground, as of [update]. 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 [update] , the world's largest gold producer by far was China with tonnes per year. Gold is the most malleable of all metals. It can be drawn into a monoatomic wire, and then stretched about twice before it breaks. Gold leaf can be beaten thin enough to become semi-transparent.
The transmitted light appears greenish blue, because gold strongly reflects yellow and red.
Gold has a density of Whereas most metals are gray or silvery white, gold is slightly reddish-yellow. 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. Colloidal gold , used by electron-microscopists, is red if the particles are small; larger particles of colloidal gold are blue.
Gold has only one stable isotope , 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 to The most stable of these is Au with a half-life of At least 32 nuclear isomers have also been characterized, ranging in atomic mass from to Gold's most stable isomer is m2 Au with a half-life of 2. No other isomer or isotope of gold has three decay paths. 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 by neutron bombardment.
Gold can currently be manufactured in a nuclear reactor by irradiation either of platinum or mercury. Only the mercury isotope Hg, which occurs with a frequency of 0. 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 Hg, which composes 9. 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 Hg.
However, such high-energy neutrons can be produced only by particle accelerators. Although gold is the most noble of the noble metals ,   it still forms many diverse compounds. Au 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. 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. The gold atom centers in Au III complexes, like other d 8 compounds, are typically square planar , with chemical bonds that have both covalent and ionic character. Some free halogens react with gold. Gold does not react with sulfur directly,  but gold III sulfide can be made by passing hydrogen sulfide through a dilute solution of gold III chloride or chlorauric acid.
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. 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 mixture of nitric acid and hydrochloric acid. 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. 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. Caesium auride CsAu , for example, crystallizes in the caesium chloride motif;  rubidium, potassium, and tetramethylammonium aurides are also known. 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 than van der Waals bonding.
The interaction is estimated to be comparable in strength to that of a hydrogen bond. Well-defined cluster compounds are numerous. Medicinal applications of gold and its complexes have a long history dating back thousands of years. Both gold I and gold III compounds have been investigated as possible anti-cancer drugs. Stable complexes can be generated using different types of bi-, tri-, and tetradentate ligand systems, and their efficacy has been demonstrated in vitro and in vivo.
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Gold is thought to have been produced in supernova nucleosynthesis , and from the collision of neutron stars ,  and to have been present in the dust from which the Solar System formed. Therefore, most of the gold that is in the Earth's crust and mantle is thought to have been delivered to Earth later, by asteroid impacts during the Late Heavy Bombardment , about 4 billion years ago.
In , an international group of scientists, established that gold "came to the Earth's surface from the deepest regions of our planet,"  Earth's mantle , evidenced by their findings at Deseado Massif in the Argentinian Patagonia. Traditionally, gold is thought to have formed by the r-process rapid neutron capture in supernova nucleosynthesis ,  but more recently it has been suggested that gold and other elements heavier than iron may also be produced in quantity by the r-process in the collision of neutron stars.
The asteroid that formed Vredefort crater 2. The discovery of the deposit in launched the Witwatersrand Gold Rush. On Earth, gold is found in ores in rock formed from the Precambrian time onward. Electrum's color runs from golden-silvery to silvery, dependent upon the silver content. The more silver, the lower the specific gravity.
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Native gold occurs as very small to microscopic particles embedded in rock, often together with quartz or sulfide minerals such as "Fool's Gold", which is a pyrite. The metal in a native state 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 [ clarification needed ] 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 combined with tellurium as the minerals calaverite , krennerite , nagyagite , petzite and sylvanite see telluride minerals , and as the rare bismuthide maldonite Au 2 Bi and antimonide aurostibite AuSb 2.
Gold also occurs in rare alloys with copper , lead , and mercury : the minerals auricupride Cu 3 Au , novodneprite AuPb 3 and weishanite Au, Ag 3 Hg 2. Recent research suggests that microbes can sometimes play an important role in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits. Another recent study has claimed water in faults vaporizes during an earthquake, depositing gold. When an earthquake strikes, it moves along a fault.