table Ag ↑ Au ↓ Rg platinum ← gold → mercury Atomic number 79 Standard atomic weight 196.966569(4) Element category transition metal Group, block group 11, d-block Period period 6 Electron configuration [Xe] 4f14 5d10 6s1 per shell 2, 8, 18, 32, 18, 1 Physical properties Phase solid Melting point 1337.33 K (1064.18 °C, 1947.52 °F) Boiling point 3243 K (2970 °C, 5378 °F) Density (near r.t.) 19.30 g·cm−3 (at 0 °C, 101.325 kPa) Liquid density at m.p.: 17.31 g·cm−3 Heat of fusion 12.55 kJ·mol−1 Heat of vaporization 342 kJ·mol−1 Molar heat capacity 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 5, 4, 3, 2, 1, −1 (an amphoteric oxide) Electronegativity 2.54 (Pauling scale) Ionization energies 1st: 890.1 kJ·mol−1 2nd: 1980 kJ·mol−1 Atomic radius empirical: 144 pm Covalent radius 136±6 pm Van der Waals radius 166 pm Miscellanea Crystal structure face-centered cubic (fcc) Speed of sound thin rod: 2030 m·s−1 (at r.t.) Thermal expansion 14.2 µm·m−1·K−1 (at 25 °C) Thermal conductivity 318 W·m−1·K−1 Electrical resistivity at 20 °C: 22.14 nΩ·m Magnetic ordering diamagnetic[1] Tensile strength 120 MPa Youngs modulus 79 GPa Shear modulus 27 GPa Bulk modulus 180 GPa[citation needed] Poisson ratio 0.44 Mohs hardness 2.5 Vickers hardness 216 MPa Brinell hardness 25 MPa (HB=?) CAS number 7440-57-5 History Naming from Latin aurum, meaning glow of sunrise Discovery In the Middle East (before 6000 BCE) 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% – (α) 0.9545 193Ir 198Au syn 2.69517 d β− 1.372 198Hg 199Au syn 3.169 d β− 0.453 199Hg Decay modes in parentheses are predicted, but have not yet been observed view talk edit · references Gold is a chemical element with symbol Au and atomic number 79. It is a bright yellow dense, soft, malleable and ductile metal. The properties remain when exposed to air or water. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements, and is solid under standard conditions. The metal therefore occurs often in free elemental (native) form, as nuggets or grains, in rocks, in veins and in alluvial deposits. It occurs in a solid solution series with the native element silver (as electrum) and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides). Golds atomic number of 79 makes it one of the higher atomic number elements that occur naturally in the universe, and is traditionally thought to have been produced in supernova nucleosynthesis to seed the dust from which the Solar System formed. Because the Earth was molten when it was just formed, almost all of the gold present in the Earth sank into the planetary core. Therefore most of the gold that is present today in the Earths crust and mantle is thought to have been delivered to Earth later, by asteroid impacts during the late heavy bombardment, about 4 billion years ago. Gold resists attacks by individual acids, but it can be dissolved by aqua regia (nitro-hydrochloric acid), so named because it dissolves gold into a soluble gold tetrachloride cation. Gold compounds also dissolve in alkaline solutions of cyanide, which have been used in mining. It dissolves in mercury, forming amalgam alloys; it 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, giving rise to the term acid test. This metal has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. In the past, a gold standard was often implemented as a monetary policy within and between nations, but gold coins ceased to be minted as a circulating currency in the 1930’s, and the world gold standard (see article for details) was finally abandoned for a fiat currency system after 1976. The historical value of gold was rooted in its medium rarity, easy handling and minting, easy smelting, non-corrosiveness, distinct color, and non-reactivity to other elements. A total of 174,100 tonnes of gold have been mined in human history, according to GFMS as of 2012.[2] This is roughly equivalent to 5.6 billion troy ounces or, in terms of volume, about 9200 m3, or a cube 21 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.[3] Gold’s high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity have led to its continued use in corrosion resistant electrical connectors in all types of computerized devices (its chief industrial use). Gold is also used in infrared shielding, colored-glass production, and gold leafing. Certain gold salts are still used as anti-inflammatories in medicine. Etymology Gold is cognate with similar words in many Germanic languages, deriving via Proto-Germanic *gulþą from Proto-Indo-European *ǵʰelh₃- (to shine, to gleam; to be yellow or green).[4][5] The symbol Au is from the Latin: aurum, the Latin word for gold.[6] The Proto-Indo-European ancestor of aurum was *h₂é-h₂us-o-, meaning glow. This word is derived from the same root (Proto-Indo-European *h₂u̯es- to dawn) as *h₂éu̯sōs, the ancestor of the Latin word Aurora, dawn.[7] This etymological relationship is presumably behind the frequent claim in scientific publications that aurum meant shining dawn.[8][9] Characteristics Gold is the most malleable 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 transparent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.[10] 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.[11] Gold readily dissolves in mercury at room temperature to form an amalgam, and forms alloys with many other metals at higher temperatures. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors.[12] 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. Gold is almost insoluble, but can be dissolved in aqua regia or solutions of sodium or potassium cyanide, for example. 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 as 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. In addition, gold is very dense, a cubic meter has a mass of 19,300 kg. By comparison, the density of lead is 11,340 kg/m3, and that of the densest element, osmium, is 22,588 ± 15 kg/m3.[13] 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.[14][15] Similar effects impart a golden hue to metallic caesium. 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.[12] 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 golds 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%).[16] All of golds radioisotopes with atomic masses above 197 decay by β− decay.[17] 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. Golds most stable isomer is 198m2Au with a half-life of 2.27 days. Golds least stable isomer is 177m2Au with a half-life of only 7 ns. 184m1Au has three decay paths: β+ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.[17] Modern applications The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.[3] Jewelry Main article: Jewelry 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 carat rating, 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.[18] 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.[18] 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.[18] Alternative white gold alloys are available based on palladium, silver and other white metals,[18] 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. By 2014 the gold jewelry industry was escalating despite a dip in gold prices. Demand in the first quarter of 2014 pushed turnover to $23.7 billion according to a World Gold Council report. Investment Gold prices (US$ per troy ounce), in nominal US$ and inflation adjusted US$. 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, economist Martin Feldstein does not believe gold serves as a hedge against inflation or currency depreciation.[19] 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 and the British gold sovereign continue to be minted in 22k (0.92) metal in historical tradition, and the South African Krugerand, first released in 1967, is also 22k (0.92).[20] 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 producing 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 modern coins include the Austrian Vienna Philharmonic bullion coin and the Chinese Gold Panda. Electronics connectors Only 10% of the world consumption of new gold produced goes to industry,[3] but by far the most important industrial use for new gold is in fabrication of corrosion-free electrical connectorss in computers and other electrical devices. For example, according to the World Gold council, a typical cell phone may contain 50 mg of gold, worth about 50 cents. But since nearly one billion cell phones are produced each year, a gold value of 50 cents in each phone adds to $500 million dollars in gold from just this application.[21] Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a thin layer coating electrical connectors, thereby ensuring good connection. For example, gold is used in the connectors of the more expensive electronics cables, such as audio, video and USB cables. The benefit of using gold over other connector metals such as tin in these applications has been debated; gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy. However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and in use for contacts with a very high failure cost (certain computers, communications equipment, spacecraft, jet aircraft engines) remains very common.[22] Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion, electrical conductivity, ductility
Posted on: Sat, 24 Jan 2015 06:31:39 +0000
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