| Name, Symbol, Number
|| Gold, Au, 79
| Chemical series
|| transition metals
| Group, Period, Block
|| 11 (IB), 6, d
| Density, Hardness
|| 19300 kg/m3, 2.5
|| Metallic yellow|
| Atomic properties
| Atomic weight
|| 196.96655 amu
| Atomic radius (calc.)
|| 135 (174) pm
| Covalent radius
|| 144 pm
| van der Waals radius
|| 166 pm
| Electron configuration
4f14 5d10 6s1
| e- 's per energy level
|| 2, 8, 18, 32, 18, 1
| Oxidation states (Oxide)
|| 3, 1 (amphoteric)
| Crystal structure
|| Cubic face centered
| Physical properties
| State of matter
| Melting point
|| 1337.33 K (1947.52 °F)
| Boiling point
|| 3129 K (5173 °F)
| Molar volume
|| 10.21 ×10-6 m3/mol
| Heat of vaporization
|| 334.4 kJ/mol
| Heat of fusion
|| 12.55 kJ/mol
| Vapor pressure
|| 0.000237 Pa at 1337 K
| Speed of sound
|| 1740 m/s at 293.15 K
|| 2.54 (Pauling scale)
| Specific heat capacity
|| 128 J/(kg*K)
| Electrical conductivity
|| 45.2 106/m ohm
| Thermal conductivity
|| 317 W/(m*K)
| 1st ionization potential
|| 890.1 kJ/mol
| 2nd ionization potential
|| 1980 kJ/mol
| Most stable isotopes
| SI units & STP are used except where noted.
Gold is a chemical element in the periodic table that has the symbol Au (L. aurum) and atomic number 79. A soft, shiny, yellow, heavy, 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.
Gold is used as a monetary standard for many nations and is also used in jewelry, dentistry, and in electronics. Its ISO currency code is XAU.
Gold is a metallic element that exhibits a yellow color en masse but can be black, ruby, or purple when finely divided. It is the most malleable and ductile metal known. In fact, 1 gram can be beaten into a 1 meter² sheet, or 1 ounce into 300 feet². A soft metal, gold is often alloyed with other metals to give it more strength.
Gold is also a good conductor of heat and electricity and is not affected by air and most reagents (only silver and copper are better electrical conductors). It is quite chemically unalterable by heat, moisture, oxygen, and most corrosive agents and therefore is well-suited for its use in coin and jewelry. Halogens chemically attack gold and aqua regia dissolves it.
The color of solid gold as well as of the intensely colored, often purple, colloidal solutions that can be made from it is caused by the fact that the plasmon frequency of this element lies in the visible range, which causes red and yellow light to be reflected and blue light to be absorbed. Native gold contains usually eight to ten per cent silver, but often much more. Natural alloys with a high silver content (more than 20%) are called electrum. As the amount of silver increases, the color becomes whiter and the specific gravity lower.
Gold will alloy with many other metals; alloys with copper yield a redder metal, alloys with iron are green, aluminium alloys are purple, alloys with platinum metals produce white, natural bismuth with silver alloys produce black. Jewelry made with combinations of colored gold is sold in the western United States to the tourist trade as Black Hills gold.
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.
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 superior 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 high electrical conductivity and 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, low-resistance 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.
- Disodium aurothiomalate is a treatment for rheumatoid arthritis (administered intramuscularly).
- The gold isotope Au-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.
- 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.
- Gold flake is used on and in some gourmet sweets and drinks. Having no reactivity it adds no taste but is taken as a delicacy.
- 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 color) are more widely used for that purpose.
Gold (Sanskrit jval, Greek χρυσος [khrusos], Latin aurum for "shining dawn", Anglo-Saxon gold, Chinese 金 [jīn]) 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. 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 artifical 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.
Gold ingots like these, from the Bank of Sweden
, form the base of many monetary systems.
Like other precious metals, gold is measured by troy weight and by gram. 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 amount 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 but in 1934 the price of gold was set at $35.00 per troy ounce. 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 reserve asset although the level has generally been declining. The largest gold depository in the world is the U.S. Federal Reserve Bank.
Since 1968 the price of gold on the open market has ranged widely, with a record high of $850 on 21 January 1980, to a low of $252.90 on 21 June 1999 (London Fixing). Prices have risen to the $420 mark in 2004, part of this rise was associated with a depreciation of the US dollar (an inverse relation between the prices exists to a certain extent).
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 confiscated 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.
In the first few years of the 21st century, reports started to circulate that Malaysia was planning a return to the gold standard -- to issue and use gold dinars as currency in international trade. The purported purpose of this move would be to reduce dependence on the United States dollar as a reserve currency, and to establish a non-debt-backed currency in accord with Islamic law against the charging of interest.  Nonetheless, gold dinar currency has not yet emerged.  
As a tangible investment gold is sometimes held as part of a portfolio because over the long term gold has an extensive history of maintaining its value. It has in the last century gained ground in relation to fiat currencies owing to inflation. Speculating in gold, attempting to buy low and sell high, carries large transaction costs. However, gold does become particularly desirable in times of extremely weak confidence and during hyperinflation because gold maintains its value even as fiat money becomes worthless. People who despite the risks enjoy investing in gold are known as goldbugs.
Futures contracts based on gold currently trade on various exchanges around the world. In the US this occurs primarily on COMEX (Commodity Exchange) which is a subsidiary of the New York Mercantile Exchange. Speculation about the future price of gold and other commodities is carried on at COMEX.
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 boundries 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). 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/1000 kg (0.1 - 2 ppb) depending on sample location.
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 will be visible to the naked eye, therefore even in gold mines you will often not see any gold.
Gold is extracted from alluvium ores by techniques of placer mining and from hard rock ores using extractive metallurgy. Refining of the metal is frequently accomplished by chlorination or electrolysis. Gold occurs in sea water at 0.1 to 2 mg/1000 kg (0.1 - 2 ppb) depending on sample location. However, as of 2004 there is no profitable method for recovering gold from sea water.
Since the 1880s South Africa has been the source for about two-thirds of the world's gold supply. The city of Johannesburg was built atop the world's greatest gold finds. Gold fields in the Orange Free State and the Transvaal were deep and require the world's deepest mines. The Boer War of 1899–1901 between the British 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.
Auric chloride (AuCl3) and chlorauric acid (HAuCl4) are the most common compounds of gold. 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, RbAu, and tetramethylammonium auride, (CH3)4N+ Au-.
Although gold is a noble metal it can form many compounds (auric chloride and chloroauric acid are the most common). It also forms:
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.
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.