Gold is a chemical element with the symbol Au (from Latin: aurum, "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 metal
occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold 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 161,000 tonnes of gold have been mined in human history, as of 2009. This
is roughly equivalent to 5.175 billion troy ounces or, in terms of volume, about
8,333 cubic meters.
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
Chemically, gold is a transition metal and can form trivalent and univalent cations
in solutions. Compared with other metals, pure gold is chemically least reactive,
but it is attacked by aqua regia (a mixture of acids), forming chloroauric acid,
but not by the individual acids, and by alkaline solutions of cyanide. Gold dissolves
in mercury, forming amalgam alloys, but does not react with it. Gold is insoluble
in nitric acid, which dissolves silver and base metals. This property is exploited
in the gold refining technique known as "inquartation and parting". Nitric acid 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.
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. 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 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). 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
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). 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.
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 (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 aluminium, iron, indium
and other elements can produce more unusual colors of gold
for various applications.