Gold
Gold (pronounced
/ˈɡoʊld/) is a
chemical element with the symbol Au (from its
Latin name aurum) and
atomic number 79. It is a highly sought-after
precious metal which has been used as
money, a
store of value and in
jewelery since the beginning of recorded history. The metal occurs as
nuggets or grains in rocks, underground "veins" and in
alluvial deposits. It is one of the
coinage metals. Gold is dense, soft, shiny and the most
malleable and
ductile substance known. Pure gold has a bright yellow color traditionally considered attractive.
Gold formed the basis for the
gold standard used before the collapse of the
Bretton Woods system. The
ISO currency code of gold
bullion is XAU.
Modern industrial uses include
dentistry and
electronics, where gold has traditionally found use because of its good resistance to
oxidative corrosion.
Chemically, gold is a
transition metal and can form trivalent and univalent cations upon solvation. Gold does not react with most chemicals, but is attacked by
chlorine,
fluorine,
aqua regia and
cyanide. Gold dissolves in
mercury, forming
amalgam alloys, but does not react with it. Gold is insoluble in
nitric acid, which will dissolve silver and base metals, and this is the basis of 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.
Characteristics
Electron shell diagram of gold
Gold is the most
malleable and
ductile metal; a single
gram can be beaten into a sheet of one
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.
Gold readily creates alloys with many other metals. These alloys can be produced to increase the hardness or to create exotic colors (see below). 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 via formation of the chloraurate ion.
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.
Recent research undertaken by Sir Frank Reith of the Australian National University shows that microbes play an important role in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.
[1]High quality pure metallic gold is tasteless; 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 19300
kg. By comparison, the density of
lead is 11340 kg/m³, and the second densest element,
iridium, is 22650 kg/m³. golds
