Gold is a chemical element with the symbol Au and atomic number 79. It is a dense, soft, malleable and ductile metal with a bright yellow color and luster, the properties of which remain without tarnishing when exposed to air or water. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements, and is solid under standard conditions. The metal therefore occurs often in free elemental (native) form, as nuggets or grains, in rocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, such as with tellurium as calaverite, sylvanite, or krennerite. As the metallic native element mineral , gold structurally belongs to the isometric copper group. It also forms a solid solution series with the native element silver (Ag) to which it is often naturally alloyed ( electrum). Other common natural gold alloys are with copper and palladium (Pd). Gold resists attacks by individual acids, but it can be dissolved by aqua regia (nitro- hydrochloric acid), so named because it dissolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. It dissolves in mercury, forming amalgam alloys; it 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, giving rise to the term acid test. This metal has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. In the past, the Gold standard has been implemented as a monetary policy, but it was widely supplanted by fiat currency starting in the 1930s. The last gold certificate and gold coin currencies were issued in the U.S. in 1932. In Europe, most countries left the gold standard with the start of World War I in 1914 and, with huge war debts, did not return to gold as a medium of exchange. The value of gold is rooted in its medium rarity, easily handling, easy smelting, non- corrosiveness, distinct color and non-reactiveness to other elements; qualities most other metals lack.