Gold is a chemical element with the symbol Au (from Latin: aurum "gold") and an atomic number of 79. Gold is a dense, soft, shiny metal and the most malleable and ductile metal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a transition metaland a group 11 element. With exception of the noble gases, gold is the least reactive chemical element known. It has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning ofrecorded history.
Gold resists attacks by individual acids, but it can be dissolved by the aqua regia, so named because it dissolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, formingamalgam alloys. Gold 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.
The native metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, usually with tellurium. Gold standards have been the most common basis for monetary policies throughout human history, being widely supplanted by fiat currency only in the late 20th century. Gold has also been frequently linked to a wide variety of symbolisms and ideologies. A total of 165,000 tonnes of gold have been mined in human history, as of 2009. This is roughly equivalent to 5.3 billion troy ounces or, in terms of volume, about 8500 m3, or acube 20.4 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 lead to many uses of gold, including electric wiring, colored glass production and even gold leaf eating.
While pure gold is yellow in color, colored gold can be developed into various colors. These colors are generally obtained by alloying gold with other elements in various proportions.
For example, alloys which are mixed 14 parts gold to 10 parts alloy create 14-karat gold, 18 parts gold to 6 parts alloy creates 18 karat, and so on. This is often expressed as the result of the ratio, i.e.: 14/24 equals 0.585 and 18/24 is 0.750. There are hundreds of possible alloys and mixtures, but in general the addition of silver will color gold white, and the addition of copper will color it red. A mix of around 50/50 copper and silver gives the range of yellow gold alloys the public is accustomed to seeing in the marketplace. A small amount (0.2%) of zinc can be added to harden the alloy.
The most common grades of gold, in addition to pure 24K, are 22K (92%), 18K (75%), 14K (58%) and 9K (38%).
Colored golds can be classified to three groups:
- the Au-Ag-Cu system, producing white, yellow, green and red golds; typically malleable alloys
- the intermetallic compounds, producing blue and purple golds, as well as other colors. These are typically brittle but can be used as gems and inlays
- the surface oxide layers, such as black gold; mechanical properties depend on the bulk alloy, and the colored surface is prone to wear.
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