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 and aqua regia . The metal occurs mainly uncombined as nuggets in rocks and 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 .

Notable characteristics

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 sheet 1 meter square 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. It is quite chemically unalterable by heat, moisture, and most corrosive agents and therefore is well suited for its use in coin and jewelry.

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 are called electrum . As the amount of silver increases, the color becomes whiter and the specific gravity lower.

Alloys with copper yield a redder metal, alloys with iron are green, aluminium alloys are purple. 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 and +3.

Applications

Pure gold is too soft for ordinary use and is hardened by alloying with silver and copper. 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. Other uses:

· 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.

· Like silver , gold can form a hard amalgam with mercury , and is sometimes used for dental fillings .

· Colloidal gold (gold nanoparticles) 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 .

History

Gold ( Sanskrit jval , Greek  ( chrysos ), Latin aurum , Anglo-Saxon gold , Chinese  has been known and highly valued since pre-historical times. Egyptian hieroglyphs from 2600 BC describe gold, which the Mesopotamian king Tushratta referred to as being "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 in 700BC.

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 in antiquity was relatively easy to obtain geologically. Primary deposition, the first source of gold, is in igneous rocks. A deposit usually needs some form of secondary enrichment to be economically viable: either chemical or physical changes like erosion or solution, which concentrates the gold in sulfides or quartz. These deposits are termed reef or vein, and are eroded by weathering, with most of the gold being transported into stream beds where, congregating with other heavy minerals, it forms placer deposits. In all these deposits the gold is in its native form, that is, as a shiny yellow metal.

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 ).

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 .

During the 19th century gold rushes occurred whenever large gold deposits were discovered, including the California , Colorado , Otago , Australia , Black Hills , and Klondike gold rushes. The largest gold depository in the world is the U.S. Federal Reserve Bank .

Value

Like other precious metals, gold is measured by troy weight and by grams and when it is alloyed with other metals the term carat is used to indicate the amount of gold present, with 24 carats being pure gold. The purity of a gold bar can also be expressed as a decimal figure, 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 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. But on March 17 , 1968 , economic circumstances caused the collapse of this 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. Since 1968 the price of gold on the open market has ranged widely, with a record high of $850 on 21 January 1980, slumping to a low of $252.90 on 21 June 1999 (London Fixing). Increased demand has led to prices rising to the $400 mark in 2004.

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 , and President Richard Nixon closed the gold window by which foreign countries could exchange American dollars for gold at a fixed rate.

As a tangible investment gold is often held as part of a portfolio because over the long term gold has an extensive history of maintaining its value. However, gold becomes particularly desirable in times of extremely weak confidence and during hyperinflation because gold maintains its value even as fiat money becomes worthless.

Futures contracts based on gold currently trade on the COMEX (Commodity Exchange) which is a subsidiary of the New York Mercantile Exchange . Speculation about the future price of gold and other commodities are carried on here.

Occurrence

Due to its relative chemical inertness, gold often is found as the native metal, occasionally as large nuggets, but usually as minute flakes in some minerals, quartz veins, slate , metamorphic rocks and in alluvial deposits that originated from these sources. Gold is widely distributed, is very often associated with the minerals quartz or pyrite and is combined with tellurium in the minerals petzite , calaverite and sylvanite . Gold is extracted from alluvium by techniques of placer mining .

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 . Mines in South Dakota and Nevada supply two-thirds of gold used in the United States .

Gold is extracted from ores through the use of cyanide , amalgam , and smelting . Refining of the metal is frequently accomplished by electrolysis . This metal occurs in sea water at 0.1 to 2 mg / ton depending on sample location. However, as of 2004 there is no profitable method for recovering gold from sea water.

Although gold is important to industry and the arts, it also retains a unique status among all commodities as a long-term store of value. It has been estimated that all the gold in the world that has ever been refined could form a single cube 20 m (66 ft) on a side.

Compounds

Auric chloride (Au Cl 3 ) and chlorauric acid ( H AuCl 4 ) are the most common compounds of gold. Although gold is a noble metal it can form many compounds

It dissolves in Aqua regia to form the AuCl 4 - ion

Gold halides ( F , Cl , Br , I )

Gold chalcogenides ( O , S , Se , Te )

Gold cluster compounds

Isotopes

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.