Gold () is a chemical element with the symbol Au () and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation 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 traditionally considered attractive. It is one of the coinage metals and formed the basis for the gold standard used before the collapse of the Bretton Woods system in 1971. 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. At STP it is attacked by aqua regia, forming chloroauric acid and by alkaline solutions of cyanide but not by hydrochloric, nitric or sulphuric acids. 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 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.
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