   #copyright

Gold

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                79                platinum ← gold → mercury
                Ag
                ↑
                Au
                ↓
                Rg

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                         Name, Symbol, Number gold, Au, 79
                                         Chemical series transition metals
                                             Group, Period, Block 11, 6, d
                                                Appearance metallic yellow
                                          Atomic mass 196.966569 (4) g/mol
                              Electron configuration [Xe] 4f^14 5d^10 6s^1
                                   Electrons per shell 2, 8, 18, 32, 18, 1
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 19.3 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
                         Heat capacity (25 °C) 25.418 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 1646 1814 2021 2281 2620 3078

                                                         Atomic properties
                                     Crystal structure cubic face centered
                                                     Oxidation states 3, 1
                                                       ( amphoteric oxide)
                                    Electronegativity 2.54 (Pauling scale)
                                     Ionization energies 1st: 890.1 kJ/mol
                                                          2nd: 1980 kJ/mol
                                                      Atomic radius 135 pm
                                              Atomic radius (calc.) 174 pm
                                                    Covalent radius 144 pm
                                               Van der Waals radius 166 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                              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.) (hard-drawn)
                                                          2030   m·s^−1
                                                    Young's modulus 78 GPa
                                                      Shear modulus 27 GPa
                                                      Bulk modulus 220 GPa
                                                        Poisson ratio 0.44
                                                         Mohs hardness 2.5
                                                  Vickers hardness 216 MPa
                                              Brinell hardness  ? 2450 MPa
                                             CAS registry number 7440-57-5
                                                         Selected isotopes

                   CAPTION: 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% Au is stable with 118 neutrons
                                ^198Au syn  2.69517 d β^- 1.372     ^198Hg
                                ^199Au syn  3.169 d   β^- 0.453     ^199Hg

                                                                References

   Gold ( IPA: /ˈgəʊld/) is a highly sought-after precious metal that for
   many centuries has been used as money, a store of value and in jewelry.
   The metal occurs as nuggets or grains in rocks and in alluvial deposits
   and is one of the coinage metals. It is a soft, shiny, yellow, dense,
   malleable, and ductile (trivalent and univalent) transition metal.
   Modern industrial uses include dentistry and electronics. Gold forms
   the basis for a monetary standard used by the International Monetary
   Fund (IMF) and the Bank for International Settlements (BIS). Its ISO
   currency code is XAU.

   Gold is a chemical element in the periodic table with the symbol Au
   (from the Latin aurum) and atomic number 79. The adjective auric refers
   to something made of gold. Gold does not react with most chemicals but
   is attacked by chlorine, fluorine, aqua regia and cyanide. Gold
   dissolves in Mercury. In particular, gold is insoluble in nitric acid,
   which will dissolve most other metals. Nitric acid has long been used
   to confirm the presence of gold in items.

Notable characteristics

   Gold is a metallic element with a characteristic yellow colour, but can
   also be black or ruby when finely divided, while colloidal solutions
   are intensely colored and often purple. These colors are the result of
   gold's plasmon frequency lying in the visible range, which causes red
   and yellow light to be reflected, and blue light to be absorbed. Only
   silver colloids exhibit the same interactions with light, albeit at a
   shorter frequency, making silver colloids yellow in colour.

   It is the most malleable and ductile metal known; a single gram can be
   beaten into a sheet of one square meter, or an ounce into 300 square
   feet. Gold readily forms alloys with many other metals. These alloys
   can be produced to increase the hardness or to create exotic colors.
   Adding copper yields a redder metal, iron blue, aluminium purple,
   platinum metals white, and natural bismuth together with silver alloys
   produce black. Native gold contains usually eight to ten per cent
   silver, but often much more — alloys with a silver content over 20% are
   called electrum. As the amount of silver increases, the colour becomes
   whiter and the specific gravity becomes lower.

   Gold is a good conductor of heat and electricity, and is not affected
   by air and most reagents. Heat, moisture, oxygen, and most corrosive
   agents have very little chemical effect on gold, making it well-suited
   for use in coins and jewelry; conversely, halogens will chemically
   alter gold, and aqua regia dissolves it.

   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 the addition
   of virtually 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.

   Recent research undertaken by Frank Reith of the Australian National
   University shows that microbes play an important role in the formation
   of gold deposits, transporting and precipitating gold to form grains
   and nuggets that collect in alluvial deposits.

   The concentration of free electrons in gold metal is 5.90×10^22 cm^-3.

Applications

   Pure gold is too soft for ordinary use and is hardened by alloying with
   silver, copper, and other metals. Gold and its many alloys are most
   often used in jewelry, coinage and as a standard for monetary exchange
   in various countries. When selling it in the form of jewelry, gold is
   measured in karats (k), with pure gold being 24k. However, it is more
   commonly sold in lower measurements of 22k, 18k, and 14k. A lower "k"
   indicates a higher percent of copper or silver mixed into the alloy,
   with copper being the more commonly used metal between the two.
   Fourteen karat gold-copper alloy will be nearly identical in colour to
   certain bronze alloys, and both may be used to produce police and other
   badges. Eighteen karat gold with a high copper content is found in some
   antique jewelry and will have a distinct, though not dominant copper
   cast, giving an attractively warm color. A similar karat weight when
   alloyed with silvery metals will appear less warm in colour, and some
   low karat white metal alloys may be sold as "white gold", silvery in
   appearance with a sightly yellow cast but far more resistant to
   corrosion than silver or sterling silver. Karat weights of twenty and
   higher are more common in modern jewelry. Because of its high
   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, particularly as
   a thin plating on printed circuit board contacts and electrical
   connectors.
     * Gold can be made into thread and used in embroidery.
     * Gold is ductile and malleable meaning it is able to be drawn into
       very thin wire and can be beaten into very thin sheets known as
       Gold leaf.
     * Gold produces a deep, intense red colour when used as a coloring
       agent in glass.
     * Gold performs critical functions in computers, communications
       equipment, spacecraft, jet aircraft engines, and a host of other
       products.
     * The 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 connection.
     * Gold alloys are used in restorative dentistry especially in tooth
       restorations such as crowns and permanent bridges as their slight
       malleability makes a superior molar mating surface with other
       teeth, unlike a harder ceramic crown. Use of gold crowns in more
       prominent teeth such as incisors is favored in some cultures and
       discouraged in others.
     * Colloidal gold (a gold nanoparticle) 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.
     * Gold(III) chloride is used as a catalyst in organic chemistry. It
       is also the usual starting point for making other gold compounds.
     * Gold is used as a coating enabling biological material to be viewed
       under a scanning electron microscope.
     * Many competitions and honours, 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
       and on astronauts' helmets to prevent blindness from the sun.
     * Disodium aurothiomalate is a treatment for rheumatoid arthritis
       (administered intramuscularly). It inhibits lymphocyte
       proliferation, lysosomal enzyme release, the release of reactive
       oxygen species from macrophages, and IL-1 production. However, it
       can also cause photosensitive rashes, gastrointestinal disturbance,
       and kidney damage.
     * The isotope gold-198, ( half-life: 2.7 days) is used in some cancer
       treatments and for treating other diseases.
     * Gold flake is used on and in some gourmet sweets and drinks. Called
       varak or (varaq) in India. Having no reactivity it adds no taste
       but is taken as a delicacy. Some use it as an excuse to create
       super-expensive delicacies ($1,000 cocktails). For similar reasons,
       it is also used as the basis for some superstitious, over the top,
       health claims. Only the salts and radioisotopes (mentioned above)
       have any evidence of medicinal value.
     * White gold (an alloy of gold with platinum, palladium, nickel,
       and/or zinc) serves as a substitute for platinum.
     * Green gold (a gold/silver alloy) is used in specialized jewelry
       while gold alloys with copper (reddish colour) are more widely used
       for that purpose ( rose gold).

History

   Funerary mask of Tutankhamun
   Enlarge
   Funerary mask of Tutankhamun

   Gold has been known and highly valued since prehistoric times. It may
   have been the first metal used by humans and was valued for
   ornamentation and rituals. Egyptian hieroglyphs from as early as 2600
   BCE describe gold, which king Tushratta of the Mitanni claimed was as
   "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 between 643 and
   630 BCE.

   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 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). Gold as a sign of wealth and prestige was made fun of
   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.

   There is an age-old tradition of biting gold in order to test its
   authenticity. Although this is certainly not a professional way of
   examining gold, the bite test should score the gold because gold is
   considered a soft metal according to the Mohs' scale of mineral
   hardness. The purer the gold the easier it should be to mark it.
   Painted lead can cheat this test because lead is softer than gold.

   Gold in antiquity was relatively easy to obtain geologically; however,
   75% of all gold ever produced has been extracted since 1910. It has
   been estimated that all the gold in the world that has ever been
   refined would form a single cube 20 m (66 ft) on a side (8000 m³).

   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
   centre (☉), which was also the astrological symbol, the Egyptian
   hieroglyph and the ancient Chinese character for the Sun (now 日). For
   modern attempts to produce artificial gold, see gold synthesis.

   During the 19th century, gold rushes occurred whenever large gold
   deposits were discovered, including the California, Colorado, Otago,
   Australian, Witwatersrand, Black Hills, and Klondike gold rushes.

   Because of its historically high value, much of the gold mined
   throughout history is still in circulation in one form or another.

Occurrence

   A gold nugget
   Enlarge
   A gold nugget
   Raw gold from California (top) and Australia (bottom), showing
   octahedral formations
   Enlarge
   Raw gold from California (top) and Australia (bottom), showing
   octahedral formations

   Due to its relative chemical inertness, gold is usually found as the
   native metal or alloy. Occasionally large accumulations of native gold
   (also known as nuggets) occur, but usually it occurs as minute grains.
   These grains occur between mineral grain boundaries or as inclusions
   within minerals. Common gold associations are quartz often as veins and
   sulfide minerals. The most common sulfide associations are pyrite,
   chalcopyrite, galena, sphalerite, arsenopyrite, stibnite and
   pyrrhotite. Rarer mineral associations are petzite, calaverite,
   sylvanite, muthmannite, nagyagite and krennerite.

   Gold is widely distributed in the Earth's crust at a background level
   of 0.03 g/1000 kg (0.03 ppm by weight). Hydrothermal ore deposits of
   gold occur in metamorphic rocks and igneous rocks; placer deposits
   originate from these sources.
   Gold in a quartz vein
   Enlarge
   Gold in a quartz vein

   The primary source of gold is usually igneous rocks or surface
   concentrations. A deposit usually needs some form of secondary
   enrichment to form an economically viable ore deposit: either chemical
   or physical processes like erosion or solution or more generally
   metamorphism, which concentrates the gold in sulfide minerals or
   quartz. There are several primary deposit types, common ones are termed
   reef or vein. Primary deposits can be weathered and eroded, with most
   of the gold being transported into stream beds where it congregates
   with other heavy minerals to form placer deposits. In all these
   deposits the gold is in its native form. Another important ore type is
   in sedimentary black shale and limestone deposits containing finely
   disseminated gold and other platinum group metals.

   Gold occurs in sea water at 0.1 to 2 mg/ t (0.1 to 2 ppb by weight)
   depending on sample location.

Production

   The entrance to an underground gold mine in Victoria, Australia
   Enlarge
   The entrance to an underground gold mine in Victoria, Australia
   Gold ore
   Enlarge
   Gold ore

   Economic gold extraction can be achieved from ore grades as little as
   0.5 g/1000 kg (0.5 ppm) on average in large easily mined deposits.
   Typical ore grades in open-pit mines are 1–5 g/1000 kg (1-5 ppm), ore
   grades in underground or hard rock mines are usually at least 3 g/1000
   kg (3 ppm) on average. Since ore grades of 30 g/1000 kg (30 ppm) are
   usually needed before gold is visible to the naked eye, in most gold
   mines you will not see any gold.

   Since the 1880s, South Africa has been the source for a large
   proportion of the world’s gold supply. Production in 1970 accounted for
   79% of the world supply, producing about 1,000 tonnes. However,
   production in 2004 was 342 tonnes. This decline was due to the
   increasing difficulty of extraction and changing economic factors
   affecting the industry in South Africa.

   The city of Johannesburg was built atop the world's greatest gold
   finds. Gold fields in the Free State and Gauteng provinces are deep and
   require the world's deepest mines. The Second Boer War of 1899– 1901
   between the British Empire and the Afrikaner Boers was at least partly
   over the rights of miners and possession of the gold wealth in South
   Africa.
   Gold nuggets From Arizona
   Enlarge
   Gold nuggets From Arizona

   Other major producers are Canada, United States and Australia. Mines in
   South Dakota and Nevada supply two-thirds of gold used in the United
   States. Siberian regions of Russia also used to be significant in the
   global gold mining industry. Kolar Gold Fields in India is another
   example of a city being built on the greatest gold deposits in India.
   In South America, the controversial project Pascua Lama aims at
   exploitation of rich fields in the high mountains of Atacama, at the
   border between Chile and Argentina. Today about one-quarter of the
   world gold output is estimated to originate from artisanal or small
   scale mining.

   After initial production, gold is often subsequently refined
   industrially by the Wohlwill process or the Miller process. Other
   methods of assaying and purifying smaller amounts of gold include
   parting and inquartation as well as cuppelation, or refining methods
   based on the dissolution of gold in aqua regia.

   The world's oceans hold a vast amount of gold, but in very low
   concentrations (perhaps 1-2 parts per billion). Fritz Haber (the German
   inventor of the Haber process) attempted commercial extraction of gold
   from sea water in an effort to help pay Germany's reparations following
   the First World War. Unfortunately, his assessment of the concentration
   of gold in sea water was unduly high, probably due to sample
   contamination. The effort produced little gold and cost the German
   government far more than the commercial value of the gold recovered. No
   commercially viable mechanism for performing gold extraction from sea
   water has yet been identified. Gold synthesis is not economically
   viable and is unlikely to become so in the foreseeable future.

   The average gold mining and extraction costs are $238 per troy ounce
   but these can vary widely depending on mining type and ore quality. In
   2001, global mine production amounted to 2,604 tonnes, or 67% of total
   gold demand in that year. At the end of 2001, it was estimated that all
   the gold ever mined totalled 145,000 tonnes.

Price

   LBMA USD morning price fixings ($US per troy ounce) since 2001
   Enlarge
   LBMA USD morning price fixings ($US per troy ounce) since 2001

   Like other precious metals, gold is measured by troy weight and by
   grams. When it is alloyed with other metals the term carat or karat is
   used to indicate the amount of gold present, with 24 karats being pure
   gold and lower ratings proportionally less. The purity of a gold bar
   can also be expressed as a decimal figure ranging from 0 to 1, 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, a certain weight of gold was given the name
   of a unit of currency. For a long period, the United States government
   set the value of the US dollar so that one troy ounce was equal to
   $20.67 ($664.56/kg), but in 1934 the dollar was revalued to $35.00 per
   troy ounce ($1125.27/kg). By 1961 it was becoming hard to maintain this
   price, and a pool of US and European banks agreed to manipulate the
   market to prevent further currency devaluation against increased gold
   demand.

   On 17 March 1968, economic circumstances caused the collapse of the
   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.
   Central banks still hold historical gold reserves as a store of value
   although the level has generally been declining. The largest gold
   depository in the world is that of the U.S. Federal Reserve Bank in New
   York.

   Since 1968 the price of gold on the open market has ranged widely, with
   a record high of $850/oz ($27,300/kg) on 21 January 1980, to a low of
   $252.90/oz ($8,131/kg) on 21 June 1999 (London Fixing). On 11 May 2006
   the London gold fixing was $715.50/oz ($23,006/kg).

   Over 2005 the World Gold Council estimated total global gold supply to
   be 3,859 tonnes and demand to be 3,754 tonnes, giving a surplus of 105
   tonnes.

Compounds

   Although gold is a noble metal, it can form many compounds, gold(III)
   chloride (AuCl[3]) and chlorauric acid (HAuCl[4]) being the most
   common. Gold compounds can be gold(I) (aurous, univalent, +1), gold(II)
   (organometallic compounds such as [Au(CH[2])[2]P(C[6]H[5])[2]][2]Cl[2]
   contain divalent gold, +2) or gold(III) (auric, trivalent, +3). Gold
   also can under extreme conditions form a +5 and even a +7 state with
   fluorine (gold pentafluoride, AuF[5] and the extremely rare gold
   heptafluoride, AuF[7]), as well as a -1 state. Such compounds
   containing the Au^- anion are called aurides and include caesium
   auride, CsAu, rubidium auride, RbAu, and tetramethylammonium auride,
   (CH[3])[4]N^+ Au^-.

   Gold also forms:
     * The AuCl[4]^- ion after dissolving in aqua regia
     * Gold halides (F,Cl,Br,I)
     * Gold chalcogenides (O, S, Se,Te)
     * Gold cyanides, with the Au(CN)[2]^– ion, such as potassium gold
       cyanide KAu(CN)[2]
     * Gold cluster compounds
     * Gold hydrazide: an olive-green powder, AuN[2]H[3], one of several
       explosive compounds known archaically as aurum fulminans

   Some gold compounds are notable for forming what's known as the
   aurophilic bond, a weak interaction comparable in strength to the
   hydrogen bond.

Isotopes

   There is one stable isotope of gold, and 18 radioisotopes with ^195Au
   being the most stable with a half-life of 186 days.

   Gold has been proposed as a " salting" material for nuclear weapons
   (cobalt is another, better-known salting material). A jacket of natural
   gold, irradiated by the intense high-energy neutron flux from an
   exploding thermonuclear weapon, would transmute into the radioactive
   isotope Au-198 with a half-life of 2.697 days and produce approximately
   .411 MeV of gamma radiation, significantly increasing the radioactivity
   of the weapon's fallout for several days. Such a weapon is not known to
   have ever been built, tested, or used.

Medicine

   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. Gold used in dentistry is widely regarded as the
   safest form of restorative material, as well as the most successful. As
   has recently been discovered, solutions containing special sugar-coated
   gold nano-particles can be used as toxin- indicators.

   Aurum metallicum (metallic gold) is also used as a homeopathic remedy,
   claimed to be helpful for people with a depressive nature.

Symbolism

   Three Gold Sovereigns with a Krugerrand
   Enlarge
   Three Gold Sovereigns with a Krugerrand
   Swiss minted 1 kg gold bar.
   Enlarge
   Swiss minted 1 kg gold bar.

   Gold has been associated with the extremities of utmost evil and great
   sanctity throughout history. In the Book of Exodus, the Golden Calf is
   a symbol of idolatry and rebellion against God. In Communist
   propaganda, the golden pocket watch and its fastening golden chain were
   the characteristic accessories of the class enemy, the bourgeois and
   the industrial tycoons. Credit card companies associate their product
   with wealth by naming and colouring their top-of-the-range cards
   “gold;” although, in an attempt to out-do each other, platinum (and the
   even-more-elite black card) has now overtaken gold.

   On the other hand in the Book of Genesis, Abraham was said to be rich
   in gold and silver. Eminent orators such as John Chrysostom were said
   to have a “mouth of gold with a silver tongue.” Gold is associated with
   notable anniversaries, particularly in a 50-year cycle, such as a
   golden wedding anniversary, golden jubilee, etc.

   Great human achievements are frequently rewarded with gold, in the form
   of medals and decorations. Winners of races and prizes are usually
   awarded the gold medal (such as the Olympic Games and the Nobel Prize),
   while many award statues are depicted in gold (such as the Academy
   Awards, the Golden Globe Awards the Emmy Awards, the Palme d'Or, and
   the British Academy Film Awards).

   Medieval kings were inaugurated under the signs of sacred oil and a
   golden crown, the latter symbolizing the eternal shining light of
   heaven and thus a Christian king's divinely inspired authority. Wedding
   rings are traditionally made of gold; since it is long-lasting and
   unaffected by the passage of time, it is considered a suitable material
   for everyday wear as well as a metaphor for the relationship. In
   Orthodox Christianity, the wedded couple is adorned with a golden crown
   during the ceremony, an amalgamation of symbolic rites.

   The symbolic value of gold varies greatly around the world, even within
   geographic regions. For example, gold is quite common in Turkey but
   considered a most valuable gift in Sicily.

   From most ancient times, gold has been connected to religion and
   spirituality, especially associated with the Sun. It was also seen as
   the best material to decorate religious imagery, all over history.
   Retrieved from " http://en.wikipedia.org/wiki/Gold"
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