Gold

79 platinum ← gold → mercury Ag ↑ Au ↓ Rg periodic table
General
Name, Symbol, Number	gold, Au, 79
Chemical series	transition metals
Group, Period, Block	11, 6, d
Appearance	metallic yellow
Atomic mass	196.966569 (4) g/mol
Electron configuration	[ Xe] 4f14 5d10 6s1
Electrons per shell	2, 8, 18, 32, 18, 1
Physical properties
Phase	solid
Density (near r.t.)	19.3 g/cm³
Liquid density at m.p.	17.31 g/cm³
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
Heat of vaporization	324 kJ/mol
Heat capacity	(25 °C) 25.418 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 1646 1814 2021 2281 2620 3078
Atomic properties
Crystal structure	cubic face centered
Oxidation states	3, 1 ( amphoteric oxide)
Electronegativity	2.54 ( Pauling scale)
Ionization energies	1st: 890.1 kJ/mol
	2nd: 1980 kJ/mol
Atomic radius	135 pm
Atomic radius (calc.)	174 pm
Covalent radius	144 pm
Van der Waals radius	166 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 22.14 nΩ·m
Thermal conductivity	(300 K) 318 W/(m·K)
Thermal expansion	(25 °C) 14.2 µm/(m·K)
Speed of sound (thin rod)	( r.t.) (hard-drawn) 2030 m/s
Young's modulus	78 GPa
Shear modulus	27 GPa
Bulk modulus	220 GPa
Poisson ratio	0.44
Mohs hardness	2.5
Vickers hardness	216 MPa
Brinell hardness	2450 MPa
CAS registry number	7440-57-5
Notable isotopes
Main article: Isotopes of gold iso NA half-life DM DE ( MeV) DP 195Au syn 186.10 d ε 0.227 195 Pt 196Au syn 6.183 d ε 1.506 196 Pt β- 0.686 196 Hg 197Au 100% Au is stable with 118 neutrons 198Au syn 2.69517 d β- 1.372 198 Hg 199Au syn 3.169 d β- 0.453 199 Hg
References

Gold is a chemical element in the periodic table that has the symbol Au (from the Latin aurum) and atomic number 79. A soft, shiny, yellow, dense, malleable, ductile (trivalent and univalent) transition metal, gold does not react with most chemicals but is attacked by chlorine, fluorine and aqua regia. The metal occurs as nuggets or grains in rocks and in alluvial deposits and is one of the coinage metals.

For millennia, gold has served as money and is also used in jewelry, dentistry, and in electronics. Gold forms the basis for a monetary standard used by the International Monetary Fund (IMF) and the Bank for International Settlements (BIS). Its ISO currency code is XAU.

Notable characteristics

Gold is a metallic element with a characteristic yellow colour, but can also be black or ruby when finely divided, while colloidal solutions are intensely colored and often purple. These colors are the result of gold's plasmon frequency lying in the visible range, which causes red and yellow light to be reflected, and blue light to be absorbed. Only silver colloids exhibit the same interactions with light, albeit at a shorter frequency, making silver colloids yellow in colour.

It is the most malleable and ductile metal known; a single gram can be beaten into a sheet of one square metre, or an ounce into 300 square feet. A soft metal, gold will readily form alloys with many other metals. This can be done to increase its strength, or create several exotic colors, sold for instance in the western United States to the tourist trade as " Black Hills" gold. Adding copper yields a redder metal, iron blue, Silver produces green, aluminium purple, platinum metals white, and natural bismuth together with silver alloys produce black. Native gold contains usually eight to ten per cent silver, but often much more — alloys with a silver content over 20% are called electrum. As the amount of silver increases, the colour becomes whiter and the specific gravity reduces.

Gold is a good conductor of heat and electricity, and is not affected by air and most reagents. Heat, moisture, oxygen, 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.

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 the addition of virtually 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 Frank Reith of the Australian National University shows that microbes play an important role in the formation of gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits. [1]

Applications

Pure gold is too soft for ordinary use and is hardened by alloying with silver, copper, and other metals. Gold and its many alloys are most often used in jewelry, coinage and as a standard for monetary exchange in many countries. Because of its high electrical conductivity and resistance to corrosion and other desirable combinations of physical and chemical properties, gold also emerged in the late 20th century as an essential industrial metal.

• Gold can be made into thread and used in embroidery.
• Gold performs critical functions in computers, communications equipment, spacecraft, jet aircraft engines, and a host of other products.
• The resistance to oxidation of gold has led to its widespread use as thin layers electroplated on the surface of electrical connectors to ensure a good connection.
• Gold is used in restorative dentistry especially in tooth restorations such as crowns and bridges.
• Colloidal gold (a gold nanoparticle) is an intensely colored solution that is currently studied in many labs for medical, biological and other applications. It is also the form used as gold paint on ceramics prior to firing.
• Chlorauric acid is used in photography for toning the silver image.
• Gold(III) chloride is used as a catalyst in organic chemistry. It is also the usual starting point for making other gold compounds.
• Gold is used as a coating enabling biological material to be viewed under a scanning electron microscope.
• Many competitions and honors, such as the Olympics and the Nobel Prize, award a gold medal to the winner (with silver to the second-place finisher, and bronze to the third.)
• Since it is a good reflector of both infrared and visible light, it is used for the protective coatings on many artificial satellites and on the astronauts helmets to prevent blindness from the sun.
• Disodium aurothiomalate is a treatment for rheumatoid arthritis (administered intramuscularly). It inhibits lymphocyte proliferation, lysosomal enzyme release, the release of reactive oxygen species from macrophages, and IL-1 production. However, it can also cause photosensitive rashes, gastrointestinal disturbance, and kidney damage.
• The gold isotope Au-198, ( half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.
• Gold flake is used on and in some gourmet sweets and drinks. Called varak or (varaq) in India. Having no reactivity it adds no taste but is taken as a delicacy. Some use it as an excuse to create super-expensive delicacies ($1,000 cocktails). For similar reasons, it also used as the basis for some superstitious, over the top, health claims. Only the salts and radioisotopes (mentioned above) have any evidence of medicinal value.
• White gold (an alloy of gold with platinum, palladium, nickel, and/or zinc) serves as a substitute for platinum.
• Green gold (a gold/ silver alloy) is used in specialized jewelry while gold alloys with copper (reddish colour) are more widely used for that purpose ( rose gold)*.

History

Gold ( Sanskrit jval, Tamil thangAm Greek χρυσóς [khrisós], Latin aurum for "shining dawn", Anglo-Saxon gold, Chinese 金 [jīn], Japanese 金 [kin] ) has been known and highly valued since prehistoric times. It may have been the first metal used by humans and was valued for ornamentation and rituals. Egyptian hieroglyphs from as early as 2600 BC describe gold, which king Tushratta of the Mitanni claimed was as "common as dust" in Egypt. Egypt and Nubia had the resources to make them major gold-producing areas for much of history. Gold is also mentioned several times in the Old Testament. 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 between 643 and 630 BC.

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.

Gold has long been considered one of the most 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 (see gold album).

Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910. [2] 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) a side.

The primary 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, the Egyptian hieroglyph and the ancient Chinese character for the Sun (now 日). For modern attempts to produce artificial gold, see gold synthesis.

During the 19th century, gold rushes occurred whenever large gold deposits were discovered, including the California, Colorado, Otago, Australia, Witwatersrand, Black Hills, and Klondike gold rushes.

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

Occurrence

Due to its relative chemical inertness gold is usually found as the native metal or alloy. Occasionally large accumulations of native gold (also known as nuggets) occur but usually gold occurs as minute grains. These grains occur between mineral grain boundaries or as inclusions within minerals. Common gold associations are quartz often as veins and sulfide minerals. The most common sulfide associations are pyrite, chalcopyrite, galena, sphalerite, arsenopyrite, stibnite and pyrrhotite. Rarer mineral associations are petzite, calaverite, sylvanite, muthmannite, nagyagite and krennerite.

Gold is widely distributed in the Earth's crust at a background level of 0.03 g/1000 kg (0.03 ppm by weight). Hydrothermal ore deposits of gold occur in metamorphic rocks and igneous rocks; alluvial deposits and placer deposits originate from these sources.

The primary source of gold is usually igneous rocks or surface concentrations. A deposit usually needs some form of secondary enrichment to form an economically viable ore deposit: either chemical or physical processes like erosion or solution or more generally metamorphism, which concentrates the gold in sulfide minerals or quartz. There are several primary deposit types, common ones are termed reef or vein. Primary deposits can be weathered and eroded, with most of the gold being transported into stream beds where it congregates with other heavy minerals to form placer deposits. In all these deposits the gold is in its native form. Another important ore type is in sedimentary black shale and limestone deposits containing finely disseminated gold and other platinum group metals.

Gold occurs in sea water at 0.1 to 2 mg/ t (0.1 to 2 ppb by weight) depending on sample location.

Production

Economic gold extraction can be achieved from ore grades as little as 0.5 g/1000 kg (0.5 ppm) on average in large easily mined deposits, typical ore grades in open-pit mines are 1–5 g/1000 kg (1-5 ppm), ore grades in underground or hard rock mines are usually at least 3 g/1000 kg (3 ppm) on average. Ore grades of 30 g/1000 kg (30 ppm) are usually needed before gold is visible to the naked eye, therefore in most gold mines you will not see any gold. It is claimed, that all the gold that has been mined throughout the history of mankind could be incorporated in a solid ball with a diameter of 27 metres.

Since the 1880s South Africa has been the source for a large proportion of the world's gold supply. Production in 1970 accounted for 79% of the world supply, producing about 1,000 tonnes, however production in 2004 was 342 tonnes. This decline was due to the increasing difficulty of extraction and changing economic factors affecting the industry in South Africa.

The city of Johannesburg was built atop the world's greatest gold finds. Gold fields in the Orange Free State and the Transvaal are deep and require the world's deepest mines. The Second Boer War of 1899– 1901 between the British Empire and the white Boers was at least partly over the rights of miners and possession of the gold wealth in South Africa.

Other major producers are Canada, United States and Western Australia. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States. Siberian regions of the USSR also used to be significant in the global gold mining industry. Kolar Gold Fields in India is another example of a city being built on the greatest gold deposits in India. In South America, the controversial project Pascua Lama aims at exploitation of rich fields in the high mountains of Atacama, at the border between Chile and Argentina.

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 idea of producing gold out of lesser metals or other cheap substances has fascinated people throughout the centuries. Scientists, kings and charlatans obsessed with the secret art of alchemy accidentally invented practically useful materials (e.g. porcelain), while searching in vain for the philosopher's stone, which was supposed to turn mercury into gold. Modern science has since proven the impossibility of making gold from other elements via chemical reactions.

However, it is possible to obtain infinitesimally small amounts of gold by artificial nuclear transformations in particle accelerators The gold isotopes produced would likely be radioactive. No economically feasible method to manufacture gold artificially has been found and published yet. The possibility of cheap man-made gold would have unforeseen economic and political consequences.

The world's oceans hold a vast amount of gold, but in very low concentrations (perhaps 1-2 parts per billion). 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.

Typical gold mining costs are $238 per troy ounce on average but these can vary widely depending on mining type and ore quality. At the end of 2001, it was estimated that all the gold ever mined totalled 145,000 tonnes. In 2001, global mine production amounted to 2,604 tonnes, or 67% of total gold demand in that year [3].

Price

Like other precious metals, gold is measured by troy weight and by grams. When it is alloyed with other metals the term carat or karat is used to indicate the amount of gold present, with 24 carats being pure gold and lower ratings proportionally less. The purity of a gold bar can also be expressed as a decimal figure ranging from 0 to 1, known as the millesimal fineness, such as 0.995.

The price of gold is determined on the open market, but a procedure known as the Gold Fixing in London, originating in 1919, provides a twice-daily benchmark figure to the industry.

Historically gold was used to back currency in an economic system known as the gold standard in which one unit of currency was equivalent to a certain weight of gold. As part of this system, governments and central banks attempted to control the price of gold by setting values at which they would exchange it for currency. For a long period the United States government set the price of gold at $20.67 per troy ounce ($664.56/kg) but in 1934 the price of gold was set at $35.00 per troy ounce ($1125.27/kg). By 1961 it was becoming hard to maintain this price, and a pool of US and European banks began to act together to defend the price against market forces.

On March 17, 1968, economic circumstances caused the collapse of the gold pool, and a two-tiered pricing scheme was established whereby gold was still used to settle international accounts at the old $35.00 per troy ounce ($1.13/g) but the price of gold on the private market was allowed to fluctuate; this two-tiered pricing system was abandoned in 1975 when the price of gold was left to find its free-market level. Central banks still hold historical gold reserves as a store of value although the level has generally been declining. The largest gold depository in the world is that of the U.S. Federal Reserve Bank, in New York.

Since 1968 the price of gold on the open market has ranged widely, with a record high of $850/oz ($27,300/kg) on 21 January 1980, to a low of $252.90/oz ($8,131/kg) on 21 June 1999 (London Fixing). Prices have risen to the $570/oz ($18,300/kg) mark in early 2006 [4].

Restrictions on gold ownership

Because of its use as a reserve store of value, the possession of gold is sometimes restricted or banned. Within the United States, the private possession of gold except as jewelry and coin collecting was banned between 1933 and 1975. President Franklin D. Roosevelt expropriated gold by Executive Order 6102, and President Richard Nixon closed the gold window by which foreign countries could exchange American dollars for gold at a fixed rate.

Compounds/isotopes

Although gold is a noble metal, it can form many compounds, auric chloride (AuCl₃) and chlorauric acid (HAuCl₄) being the most common. Gold compounds can be aurous (univalent, +1) or auric (trivalent, +3). Gold also can under extreme conditions form a +5 state with fluorine (gold pentafluoride, AuF₅), as well as (unusually for a metal), a -1 state. Such compounds containing the Au^- anion are called aurides and include caesium auride, CsAu, rubidium auride, RbAu, and tetramethylammonium auride, (CH₃)₄N⁺ Au^-.

Gold also forms:

• The AuCl₄^- ion after dissolving in aqua regia
• Gold halides ( F,Cl, Br, I)
• Gold chalcogenides (O, S, Se, Te)
• Gold cluster compounds
• Gold hydrazide: an olive-green powder, AuN₂H₃, one of several explosive compounds known archaically as aurum fulminans

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

Precautions

The human body does not absorb gold very well, thus compounds of gold are not normally very toxic. Liver and kidney damage has, however, been reported for up to 50% of arthritis patients treated with gold-containing drugs. Gold used in dentistry is widely regarded as the safest form of restorative material, as well as the most successful.

Symbolism

Gold has been associated with the extremities of utmost evil and great sanctity throughout history. The Golden Calf is a widely-recognised symbol of idolatry and revolt against God. In Communist propaganda, the golden pocket watch and its fastening golden chain were the characteristic accessories of the class enemy, the bourgeois and the industrial tycoons.

On the other hand, eminent orators such as John Chrysostom were said to have a mouth of gold with a silver tongue. Gold is associated with notable anniversaries, particularly in a 50 year cycle, such as a golden wedding anniversary, golden jubilee, etc.

Great human achievements are frequently rewarded with gold, in the form of medals and decorations. Winners of races and prizes are usually awarded the gold medal (such as the Olympic Games and the Nobel Prize), while many award statues are depicted in gold (such as the Academy Awards, the Emmy Awards and the British Academy Film Awards).

Medieval kings were inaugurated under the signs of sacred oil and a golden crown, the latter symbolizing the eternal shining light of heaven and thus a Christian king's divinely inspired authority. Wedding rings are traditionally made of gold; since it is long-lasting and unaffected by the passage of time, it is considered a suitable material for everyday wear as well as a metaphor for the relationship. In Orthodox Christianity, the wedded couple is adorned with a golden crown during the ceremony, an amalgamation of symbolic rites.

The symbolic value of gold varies wildly around the world, even within geographic regions. For example, gold is quite common in Turkey but considered a most valuable gift in Sicily.