Gold (
/ˈɡoʊld/) is a chemical element with the symbol Au (from Latin: aurum "gold") and an atomic number of 79. Gold is a dense, soft, shiny metal and the most malleable and ductilemetal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a transition metal and agroup 11 element. With exception of the noble gases, gold is the least reactive chemical element known. It has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history.
/ˈɡoʊld/) is a chemical element with the symbol Au (from Latin: aurum "gold") and an atomic number of 79. Gold is a dense, soft, shiny metal and the most malleable and ductilemetal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a transition metal and agroup 11 element. With exception of the noble gases, gold is the least reactive chemical element known. It has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history.Gold resists attacks by individual acids, but it can be dissolved by the aqua regia (nitro-hydrochloric acid), so named because it dissolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, formingamalgam alloys. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items.
The native metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, usually with tellurium. Gold standardshave been the most common basis for monetary policies throughout human history, being widely supplanted by fiat currency only in the late 20th century. Gold has also been frequently linked to a wide variety of symbolisms and ideologies. A total of 165,000 tonnes of gold have been mined in human history, as of 2009.[1] This is roughly equivalent to 5.3 billion troy ouncesor, in terms of volume, about 8500 m3, or a cube 20.4 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.[2]
Besides its widespread monetary and symbolic functions, gold has many practical uses indentistry, electronics, and other fields. Its high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity lead to many uses of gold, including electric wiring, colored glass production and even gold leaf eating.
Characteristics
Gold is the most malleable and ductile of all metals; a single gram can be beaten into a sheet of 1 square meter, or an ounce into 300 square feet. Gold leaf can be beaten thin enough to become translucent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.[3] Such semi-transparent sheets also strongly reflect infrared light, making them useful as infrared (radiant heat) shields in visors of heat-resistant suits, and in sun-visors for spacesuits.[4]
Gold readily creates alloys with many other metals. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors (see below).[5] Gold is a good conductor of heat and electricity and reflects infraredradiation strongly. Chemically, it is unaffected by air, moisture and most corrosive reagents, and is therefore well suited for use in coins and jewelry and as a protective coating on other, more reactive, metals. However, it is not chemically inert.
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 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.
High quality pure metallic gold is tasteless and scentless, in keeping with its resistance to corrosion (it is metal ions which confer taste to metals).[6]
In addition, gold is very dense, a cubic meter weighing 19,300 kg. By comparison, the density of lead is 11,340 kg/m3, and that of the densest element, osmium, is 22,610 kg/m3.
Color
Whereas most other pure metals are gray or silvery white, gold is yellow. This color is determined by the density of loosely bound (valence) electrons; those electrons oscillate as a collective "plasma" medium described in terms of a quasiparticle calledplasmon. The frequency of these oscillations lies in the ultraviolet range for most metals, but it falls into the visible range for gold due to subtle relativistic effects that affect the orbitals around gold atoms.[7][8] Similar effects impart a golden hue to metallic cesium (see relativistic quantum chemistry).
Common colored gold alloys such as rose gold can be created by the addition of various amounts of copper and silver, as indicated in the triangular diagram to the left. Alloys containing palladium or nickel are also important in commercial jewelry as these produce white gold alloys. Less commonly, addition of manganese, aluminium, iron, indium and other elements can produce more unusual colors of gold for various applications.[5]
Isotopes
Main article: Isotopes of gold
Gold has only one stable isotope, 197Au, which is also its only naturally occurring isotope. Thirty six radioisotopes have been synthesized ranging in atomic mass from 169 to 205. The most stable of these is 195Au with a half-life of 186.1 days. The least stable is 171Au, which decays by proton emission with a half-life of 30 µs. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, and β+ decay. The exceptions are 195Au, which decays by electron capture, and 196Au, which decays most often by electron capture (93%) with a minor β- decay path (7%).[9] All of gold's radioisotopes with atomic masses above 197 decay by β- decay.[10]
At least 32 nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range, only 178Au, 180Au,181Au, 182Au, and 188Au do not have isomers. Gold's most stable isomer is 198m2Au with a half-life of 2.27 days. Gold's least stable isomer is 177 m2Au with a half-life of only 7 ns. 184 m1Au has three decay paths: β+ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.[10]
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