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Gold                                                                                                             

Standard Sate: solid  at 298 K 

Color: gold              

Gold is usually alloyed in jewellery to give it more strength, and the term carat describes the amount of gold present (24 carats is pure gold). It is the most malleable and ductile metal. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.

(Au), chemical element, a dense, lustrous, yellow
precious metal of Group Ib, Period 6, of the
periodic table. Gold has several qualities that
have made it exceptionally valuable throughout
history. It is attractive in colour and brightness,
durable to the point of virtual indestructibility,
highly malleable, and usually found in nature in a
comparatively pure form. The history of gold is
unequaled by that of any other metal because of
its value in the minds of men from earliest times.

For full treatment of the mining, recovery, and refining of gold, see
Industries, Extraction and Processing: Gold.

Gold is one of the heaviest of all metals. It is a good conductor of
heat and electricity. It is also soft and the most malleable and ductile
of metals; an ounce (28 g) can be beaten out to 187 square feet
(about 17 square m) in extremely thin sheets called gold leaf.

Because gold is visually pleasing and workable and does not tarnish
or corrode, it was one of the first metals to attract human attention.
Examples of elaborate gold workmanship, many in nearly perfect
condition, survive from ancient Egyptian, Minoan, Assyrian, and
Etruscan artisans, and gold has continued to be a highly favoured
material out of which to craft jewelry and other decorative objects.
(See metalwork; goldwork.) (See goldwork.)

Owing to its unique qualities, gold has been the one material that is
universally accepted in exchange for goods and services. In the form
of coins or bullion, gold has occasionally played a major role as a
high-denomination currency, although silver has generally been the
standard medium of payments in the world's trading systems. Gold
began to serve as backing for paper-currency systems when they
became widespread in the 19th century, and from the 1870s until
World War I the gold standard was the basis for the world's
currencies. Although gold's official role in the international monetary
system had come to an end by the 1970s, the metal remains a highly
regarded reserve asset, and approximately 45 percent of all the
world's gold is held by governments and central banks for this
purpose. Gold is still accepted by all nations as a medium of
international payment. (See also money.) (See money.)

Gold is widespread in low concentrations in all igneous rocks. Its
abundance in the Earth's crust is estimated at about 0.005 parts per
million. It occurs mostly in the native state, remaining chemically
uncombined except with tellurium, selenium, and possibly bismuth.
The element's only naturally occurring isotope is gold-197. Gold
often occurs in association with copper and lead deposits, and, though
the quantity present is often extremely small, it is readily recovered as
a by-product in the refining of those base metals. Large masses of
gold-bearing rock rich enough to be called ores are unusual. Two
types of deposits containing significant amounts of gold are known:
hydrothermal veins, where it is associated with quartz and pyrite
(fool's gold); and placer deposits, both consolidated and
unconsolidated, that are derived from the weathering of gold-bearing
rocks.

The origin of enriched veins is not fully known, but it is believed that
the gold was carried up from great depths with other minerals, at
least in partial solid solution, and later precipitated. The gold in rocks
usually occurs as invisible disseminated grains, more rarely as flakes
large enough to be seen, and even more rarely as masses or veinlets.
Crystals about 2.5 cm (1 inch) or more across have been found in
California. Masses, some on the order of 90 kg (200 pounds), have
been reported from Australia.

Alluvial deposits of gold found in or along streams were the principal
sources of the metal for ancient Egypt and Mesopotamia. Other
deposits were found in Lydia (now in Turkey) and the lands of the
Aegean and in Persia (now Iran), India, China, and other lands.
During the Middle Ages the chief sources of gold in Europe were the
mines of Saxony and Austria. The era of gold production that
followed the Spanish discovery of the Americas in the 1490s was
probably the greatest the world had witnessed to that time. The
exploitation of mines by slave labour and the looting of Indian palaces,
temples, and graves in Central and South America resulted in an
unprecedented influx of gold that literally unbalanced the economic
structure of Europe. From Christopher Columbus' discovery of the
New World in 1492 to 1600, more than 225,000 kg (8,000,000 ounces)
of gold, or 35 percent of world production, came from South
America. The New World's mines--especially those in
Colombia--continued into the 17th and 18th centuries to account for
61 and 80 percent, respectively, of world production; 1,350,000 kg
(48,000,000 ounces) were mined in the 18th century.

Russia became the world's leading producer of gold in 1823, and for
14 years it contributed the bulk of the world supply. During the
second era of expanding production (1850-75), more gold was
produced in the world than in all the years since 1492, primarily
because of discoveries in California and Australia. A third marked
increase (1890-1915) stemmed from discoveries in Alaska, Yukon
Territory, and South Africa. A major factor in the increase of the
world's supply of gold was the introduction in 1890 of the cyanide
process for the recovery of gold from low-grade ores and ores
containing minute, particle-sized gold. Gold production continued to
rise throughout the 20th century, partly because of the improvement in
recovery methods and partly because of the continual growth and
expansion of South Africa's gold-mining operations. (See gold rush.)

In the late 20th century four countries--South Africa, Russia, the
United States, and Australia--accounted for two-thirds of the gold
produced annually throughout the world. South Africa alone, with its
vast Witwatersrand mines, produces about one-third of the world's
gold.

Because pure gold is too soft to resist prolonged handling, it is usually
alloyed with other metals to increase its hardness for use in jewelry,
goldware, or coinage. Most gold used in jewelry is alloyed with silver,
copper, and a little zinc to produce various shades of yellow gold or
with nickel, copper, and zinc to produce white gold. The color of
these gold alloys goes from yellow to white as the proportion of silver
in them increases; more than 70 percent silver results in alloys that
are white. Alloys of gold with silver or copper are used to make gold
coins and goldware, and alloys with platinum or palladium are also
used in jewelry. The content of gold alloys is expressed in 24ths,
called karats; a 12-karat gold alloy is 50 percent gold, and 24-karat
gold is pure.

Because of its high electrical conductivity (71 percent that of copper)
and inertness, the largest industrial use of gold is in the electric and
electronics industry for plating contacts, terminals, printed circuits, and
semiconductor systems. Thin films of gold that reflect up to 98
percent of incident infrared radiation have been employed on satellites
to control temperature and on space-suit visors to afford protection.
Used in a similar way on the windows of large office buildings, gold
reduces the air-conditioning requirement and adds to the beauty. Gold
has also long been used for fillings and other repairs to teeth.

Compounds.


The characteristic oxidation states of gold are +1 (aurous
compounds) and +3 (auric compounds). Gold is more easily displaced
from solution by reduction than any other metal; even platinum will
reduce Au3+ ions to metallic gold.

Among the relatively few gold compounds of practical importance
are gold(I) chloride, AuCl; gold(III) chloride, or gold trichloride,
AuCl3; and chlorauric acid, HAuCl4. All three are involved in the
electrolytic refining of gold. Potassium cyanoaurate, K[Au(CN)2], is
the basis for most gold-plating baths (the solution employed when
gold is plated). The soluble salt sodium aurichloride, NaAuCl42H2O,
is used in the treatment of rheumatoid arthritis. Several organic
compounds of gold have industrial applications. For example, gold
mercaptides, which are obtained from sulfurized terpenes, are
dissolved in certain organic solutions and used for decorating china
and glass articles. atomic number 79 atomic weight 196.967 melting
point 1,063 C (1,945 F) boiling point 2,966 C (5,371 F) specific
gravity 19.3 (20 C) valence 1, 3 electronic config. 2-8-18-32-18-1

"gold" Encyclop�dia Britannica Online.

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