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 that is considered attractive, which is maintained without tarnishing in air or water. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements, 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, usually with tellurium.
Gold resists attacks by individual acids, but it can be dissolved by the 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; 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.
A gold coin is a coin made mostly or entirely of gold. In modern times, most gold coins are intended either to be sold to collectors, or to be used as bullion coins—coins whose nominal value is irrelevant and which serve primarily as a method of investing in gold.
Gold has been used as money for many reasons. It is fungible, with a low spread between the prices to buy and sell. Gold is also easily transportable, as it has a high value to weight ratio, compared to other commodities, such as silver. Gold can be divided into smaller units, without destroying its value; it can also be melted into ingots, and re-coined. The density of gold is higher than most other metals, making it difficult to pass counterfeits. Gold is extremely unreactive.