Gold
From Wikipedia, the free encyclopedia
Atomic Number: 79
Atomic Symbol: AU
Appearance: metallic yellow
Pronunciation
/ˈe¡oÊŠld/
Element category
transition metal
Group, period, block
11, 6, d
Standard atomic weight
196.966569(5)
Electron configuration
[Xe] 4f14 5d10 6s1
2, 8, 18, 32, 18, 1
History
Naming
aurum in Latin, meaning glow of sunrise
Discovery
Middle East (before 6000 BC)
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
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
(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
face centered cubic
Magnetic ordering
diamagnetic[1]
Electrical resistivity
(20 °C) 22.14 nΩ·m
Thermal conductivity
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[citation needed] GPa
Poisson ratio
0.44
Mohs hardness
2.5
Vickers hardness
216 MPa
Brinell hardness
25 HB = ? MPa
CAS registry number
7440-57-5
Gold is a chemical element with the symbol Au and atomic number 79. It is a dense, soft, malleable, and ductile metal with an attractive, bright yellow color and luster that is maintained without tarnishing in air or water. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements, 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. Less commonly, it occurs in minerals as gold compounds, such as with tellurium as calaverite, sylvanite and krennerite.
Gold resists attacks by individual acids, but it can be dissolved by 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. It dissolves in mercury, forming amalgam alloys; 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. Gold standards have sometimes been monetary policies, but were widely supplanted by fiat currency starting in the 1930s. The last gold certificate and gold coin currencies were issued in the U.S. in 1932. In Europe, most countries left the gold standard with the start of World War I in 1914 and, with huge war debts, did not return to gold as a medium of exchange.
A total of 174,100 tons of gold have been mined in human history, according to GFMS as of 2012. This is roughly equivalent to 5.6 billion troy ounces or, in terms of volume, about 9261 m3, or a cube 21.0 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.
Besides its widespread monetary and symbolic functions, gold has many practical uses in dentistry, electronics, and other fields. Its high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity has led to many uses, including electric wiring, colored-glass production, and gold leafing.
Most of the Earth's gold probably lies at its core, the metal's high density having made it sink there in the planet's youth. Virtually all discovered gold is considered to have been deposited later by meteorites that contained the element, with the asteroid that formed Vredefort crater having been implicated in the formation of the largest gold mining region on earth, Witwatersrand basin.
Etymology
"Gold" is cognate with similar words in many Germanic languages, deriving via Proto-Germanic *gulþÄ. from Proto-Indo-European *gÊ°el- ("yellow/green").
The symbol Au is from the Latin: aurum, according to some sources meaning "shining dawn", from Sabine ausum "glowing dawn" although according to definitions within Latin dictionaries the meaning of the word aurum is the same as today's use of gold in reference to the metal. The disagreement between definitions is possibly due to the accumulation of evidence from archaeology of the original anciency of the metal in civilization; in reference to "the dawn of civilization", and in this respect has become the adopted modern meaning, disassociated from the original etymological Latin.
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. 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.
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. 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 weighing 19300 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.
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. Similar effects impart a golden hue to metallic cesium.
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, aluminum, iron, indium and other elements can produce more unusual colors of gold for various applications.
Isotopes
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%). All of gold's radioisotopes with atomic masses above 197 decay by β- - ' decay.
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 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.
Monetary exchange
Gold is commonly formed into bars for use in monetary exchange.
Gold has been widely used throughout the world as money, for efficient indirect exchange (versus barter), and to store wealth in hoards. For exchange purposes, mints produce standardized gold bullion coins, bars and other units of fixed weight and purity.
The first coins containing gold were struck in Lydia, Asia Minor, around 600 BC. The talent coin of gold in use during the periods of Grecian history both before and during the time of the life of Homer weighed between 8.42 and 8.75 grams. From an earlier preference in using silver, European economies re-established the minting of gold as coinage during the thirteenth and fourteenth centuries.
Bills (that mature into gold coin) and gold certificates (convertible into gold coin at the issuing bank) added to the circulating stock of gold standard money in most 19th century industrial economies. In preparation for World War I the warring nations moved to fractional gold standards, inflating their currencies to finance the war effort. Post-war, the victorious countries, most notably Britain, gradually restored gold-convertibility, but international flows of gold via bills of exchange remained embargoed; international shipments were made exclusively for bilateral trades or to pay war reparations.
After World War II gold was replaced by a system of nominally convertible currencies related by fixed exchange rates following the Bretton Woods system. Gold standards and the direct convertibility of currencies to gold have been abandoned by world governments, led in 1971 by the United States' refusal to redeem its dollars in gold. Fiat currency now fills most monetary roles. 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.
Central banks continue to keep a portion of their liquid reserves as gold in some form, and metals exchanges such as the London Bullion Market Association still clear transactions denominated in gold, including future delivery contracts. Today, gold mining output is declining.[32] 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 have been replaced by floating prices for gold and gold future contract. Though the gold stock grows by only 1 or 2% per year, very little metal is irretrievably consumed. Inventory above ground would satisfy many decades of industrial and even artisan uses at current prices.
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).
Although the prices of some platinum group metals can be much higher, gold has long been considered the most desirable of precious metals, and its value has been used as the standard for many currencies. Gold has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties. Gold as a sign of wealth and prestige was ridiculed by Thomas More in his treatise Utopia. On that imaginary island, gold is so abundant that it is used to make chains for slaves, tableware, and lavatory seats. When ambassadors from other countries arrive, dressed in ostentatious gold jewels and badges, the Utopians mistake them for menial servants, paying homage instead to the most modestly dressed of their party.
Edited by Moe145
12/02/2013 09:43 am
First Page
Posted 11/30/2013 09:47 am
Those are amazing! 
