From Wikipedia, the free encyclopedia Jump to: navigation, search This article is about the metal. For the color, see Gold (color). For other uses, see Gold (disambiguation). platinum ← gold → mercury Ag ↑ Au ↓ Rg 79Au Periodic table Appearance metallic yellow General properties Name, symbol, number gold, Au, 79 Pronunciation /ˈɡoʊld/ Element category transition metal Group, period, block 11, 6, d Standard atomic weight 196.966569g·mol−1 Electron configuration [Xe] 4f14 5d10 6s1 Electrons per shell 2, 8, 18, 32, 18, 1 (Image) Physical properties Phase solid Density (near r.t.) 19.30 g·cm−3 Liquid density at m.p. 17.31 g·cm−3 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−1 Heat of vaporization 324 kJ·mol−1 Specific heat capacity (25 °C) 25.418 J·mol−1·K−1 Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1646 1814 2021 2281 2620 3078 Atomic properties Oxidation states -1, 1, 2, 3, 4, 5 (amphoteric oxide) Electronegativity 2.54 (Pauling scale) Ionization energies 1st: 890.1 kJ·mol−1 2nd: 1980 kJ·mol−1 Atomic radius 144 pm Covalent radius 136±6 pm Van der Waals radius 166 pm Miscellanea Crystal structure Lattice face centered cubic Magnetic ordering diamagnetic Electrical resistivity (20 °C) 22.14 nΩ·m Thermal conductivity (300 K) 318 W·m−1·K−1 Thermal expansion (25 °C) 14.2 µm·m−1·K−1 Speed of sound (thin rod) (r.t.) 2030 m·s−1 Tensile strength 120 MPa Young's modulus 79 GPa Shear modulus 27 GPa Bulk modulus 180 GPa Poisson ratio 0.44 Mohs hardness 2.5 Vickers hardness 216 MPa Brinell hardness 25 HB MPa CAS registry number 7440-57-5 Most stable isotopes Main article: Isotopes of gold iso NA half-life DM DE (MeV) DP 195Au syn 186.10 d ε 0.227 195Pt 196Au syn 6.183 d ε 1.506 196Pt β− 0.686 196Hg 197Au 100% 197Au is stable with 118 neutrons 198Au syn 2.69517 d β− 1.372 198Hg 199Au syn 3.169 d β− 0.453 199Hg v • d • e Gold ( /ˈɡoʊld/) is a chemical element with the symbol Au (from Latin: aurum "gold", originally "shining dawn") and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. 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 metal is dense, soft, shiny and the most malleable and ductile pure metal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Gold is one of the coinage metals and has served as a symbol of wealth and a store of value throughout history. Gold standards have provided a basis for monetary policies. It also has been linked to a 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 ounces or, in terms of volume, about 8,500 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. Although primarily used as a store of value, gold has many modern industrial uses including dentistry and electronics. Gold has traditionally found use because of its good resistance to oxidative corrosion and excellent quality as a conductor of electricity. Chemically, gold is a transition metal. Compared with other metals, pure gold is chemically least reactive, resisting individual acids but being attacked by the acid mixture aqua regia, so named because it desolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, forming amalgam 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, and this is the origin of the colloquial term "acid test", referring to a gold standard test for genuine value. Contents [hide] 1 Characteristics 1.1 Color 1.2 Isotopes 2 Use and applications 2.1 Monetary exchange 2.2 Investment 2.3 Jewelry 2.4 Medicine 2.5 Food and drink 2.6 Industry 2.7 Electronics 2.8 Commercial chemistry 3 History 4 Occurrence 4.1 Gallery of specimens of crystalline native gold 5 Production 6 Consumption 7 Chemistry 7.1 Less common oxidation states 7.2 Mixed valence compounds 8 Toxicity 9 Price 10 Symbolism 11 State emblem 12 See also 13 References 14 External links Characteristics Native gold nuggets Native gold + gold on quartz. 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.[2] 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.[3] 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).[4] Gold is a good conductor of heat and electricity and reflects infrared radiation 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).[5] 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 Different colors of Ag-Au-Cu alloys 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 called plasmon. 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.[6][7] 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.[4] 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%).[8] All of gold's radioisotopes with atomic masses above 197 decay by β- decay.[9] 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 198 m2Au 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.[9] Use and applications Monetary exchange Gold has been widely used throughout the world as a vehicle for monetary exchange, either by issuance and recognition of gold coins or other bare metal quantities, or through gold-convertible paper instruments by establishing gold standards in which the total value of issued money is represented in a store of gold reserves. However, production has not grown in relation to the world's economies. Today, gold mining output is declining.[10] With the sharp growth of economies in the 20th century, and increasing foreign exchange, the world's gold reserves and their trading market have become a small fraction of all markets and fixed exchange rates of currencies to gold were no longer sustained. At the beginning of World War I the warring nations moved to a fractional gold standard, inflating their currencies to finance the war effort. After World War II gold was replaced by a system of convertible currency following the Bretton Woods system. Gold standards and the direct convertibility of currencies to gold have been abandoned by world governments, being replaced by fiat currency in their stead. Switzerland was the last country to tie its currency to gold; it backed 40% of its value until the Swiss joined the International Monetary Fund in 1999.[11] Pure gold is too soft for day-to-day monetary use and is typically hardened by alloying with copper, silver or other base metals. The gold content of alloys is measured in carats (k). Pure gold is designated as 24k. English gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness (American gold coins for circulation after 1837 contained the slightly lower amount of 0.900 fine gold, or 21.6 kt). Investment Main article: Gold as an investment Many holders of gold store it in form of bullion coins or bars as a hedge against inflation or other economic disruptions. However, some economists do not believe gold serves as a hedge against inflation or currency depreciation.[12] The ISO 4217 currency code of gold is XAU. Modern bullion coins for investment or collector purposes do not require good mechanical wear properties; they are typically fine gold at 24k, although the American Gold Eagle, the British gold sovereign, and the South African Krugerrand continue to be minted in 22k metal in historical tradition. The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin, at 99.999% or 0.99999, while the popular issue Canadian Gold Maple Leaf coin has a purity of 99.99%. Several other 99.99% pure gold coins are available. In 2006, the United States Mint began production of the American Buffalo gold bullion coin with a purity of 99.99%. The Australian Gold Kangaroos were first coined in 1986 as the Australian Gold Nugget but changed the reverse design in 1989. Other popular modern coins include the Austrian Vienna Philharmonic bullion coin and the Chinese Gold Panda. Jewelry Main article: Jewellery Moche gold necklace depicting feline heads. Larco Museum Collection. Lima-Peru Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower caratage, typically 22k, 18k, 14k or 10k, contain higher percentages of copper, or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color. Eighteen-carat gold containing 25% copper is found in antique and Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold. Fourteen-carat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police and other badges. Blue gold can be made by alloying with iron and purple gold can be made by alloying with aluminium, although rarely done except in specialized jewelry. Blue gold is more brittle and therefore more difficult to work with when making jewelry. Fourteen and eighteen carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with palladium or nickel. White 18-carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silvery in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe. Alternative white gold alloys are available based on palladium, silver and other white metals,[13] but the palladium alloys are more expensive than those using nickel. High-carat white gold alloys are far more resistant to corrosion than are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects. Medicine In medieval times, gold was often seen as beneficial for the health, in the belief that something that rare and beautiful could not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power.[14] Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions.[15] However, only salts and radioisotopes of gold are of pharmacological value, as elemental (metallic) gold is inert to all chemicals it encounters inside the body. In modern times, injectable gold has been proven to help to reduce the pain and swelling of rheumatoid arthritis and tuberculosis.[15][16] Gold alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges. The gold alloys' slight malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others. Colloidal gold preparations (suspensions of gold nanoparticles) in water are intensely red-colored, and can be made with tightly controlled particle sizes up to a few tens of nanometers across by reduction of gold chloride with citrate or ascorbate ions. Colloidal gold is used in research applications in medicine, biology and materials science. The technique of immunogold labeling exploits the ability of the gold particles to adsorb protein molecules onto their surfaces. Colloidal gold particles coated with specific antibodies can be used as probes for the presence and position of antigens on the surfaces of cells.[17] In ultrathin sections of tissues viewed by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the antigen.[18] Colloidal gold is also the form of gold used as gold paint on ceramics prior to firing. Gold, or alloys of gold and palladium, are applied as conductive coating to biological specimens and other non-conducting materials such as plastics and glass to be viewed in a scanning electron microscope. The coating, which is usually applied by sputtering with an argon plasma, has a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earth, and its very high density provides stopping power for electrons in the electron beam, helping to limit the depth to which the electron beam penetrates the specimen. This improves definition of the position and topography of the specimen surface and increases the spatial resolution of the image. Gold also produces a high output of secondary electrons when irradiated by an electron beam, and these low-energy electrons are the most commonly used signal source used in the scanning electron microscope.[19] The isotope gold-198, (half-life 2.7 days) is used in some cancer treatments and for treating other diseases.[20] Food and drink Gold can be used in food and has the E number 175.[21] Gold leaf, flake or dust is used on and in some gourmet foods, notably sweets and drinks as decorative ingredient.[22] Gold flake was used by the nobility in Medieval Europe as a decoration in food and drinks, in the form of leaf, flakes or dust, either to demonstrate the host's wealth or in the belief that something that valuable and rare must be beneficial for one's health. Gold foil along with silver is sometimes used in South Asian sweets such as barfi.[23] Danziger Goldwasser (German: Gold water of Danzig) or Goldwasser (English: Goldwater) is a traditional German herbal liqueur[24] produced in what is today Gdańsk, Poland, and Schwabach, Germany, and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf.[25] However, since metallic gold is inert to all body chemistry, it adds no taste nor has it any other nutritional effect and leaves the body unaltered.[26] Industry The 220 kg gold brick displayed in Jinguashi Gold Museum, Taiwan, Republic of China The world's largest gold bar weighs 250 kg. Toi museum, Japan. A gold nugget of 5 mm in diameter (bottom) can be expanded through hammering into a gold foil of about 0.5 square meter. Toi museum, Japan. Gold solder is used for joining the components of gold jewelry by high-temperature hard soldering or brazing. If the work is to be of hallmarking quality, gold solder must match the carat weight of the work, and alloy formulas are manufactured in most industry-standard carat weights to color match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting points, goldsmiths can assemble complex items with several separate soldered joints. Gold can be made into thread and used in embroidery. Gold is ductile and malleable, meaning it can be drawn into very thin wire and can be beaten into very thin sheets known as gold leaf. Gold produces a deep, intense red color when used as a coloring agent in cranberry glass. In photography, gold toners are used to shift the color of silver bromide black and white prints towards brown or blue tones, or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak published formulas for several types of gold toners, which use gold as the chloride.[27] As gold is a good reflector of electromagnetic radiation such as infrared and visible light as well as radio waves, it is used for the protective coatings on many artificial satellites, in infrared protective faceplates in thermal protection suits and astronauts' helmets and in electronic warfare planes like the EA-6B Prowler. Gold is used as the reflective layer on some high-end CDs. Automobiles may use gold for heat dissipation. McLaren uses gold foil in the engine compartment of its F1 model.[28] Gold can be manufactured so thin that it appears transparent. It is used in some aircraft cockpit windows for de-icing or anti-icing by passing electricity through it. The heat produced by the resistance of the gold is enough to deter ice from forming.[29]