Gold is a metallic element, its chemical symbol is Au, and its atomic number is 79. The simple substance (free form) of gold, commonly known as gold, is a popular precious metal, which has been used as currency, preservation and jewelry for many centuries. In nature, gold exists in the form of simple substance in gold nuggets or rocks, underground veins and alluvium. Gold is also one of the currency metals. Gold, a simple substance, is solid at room temperature, with high density, softness, brightness, corrosion resistance and the highest ductility and ductility among known metals. Gold is a transition metal, which can form trivalent and monovalent positive ions after dissolution. Gold will not react with most chemicals, but will be corroded by chlorine, fluorine, aqua regia and cyanide. Gold can be dissolved by mercury to form amalgam (but this is not a chemical reaction); Nitric acid can dissolve silver and base metals, but not gold. The above two properties become the basis of gold refining technology, which are called "separate casting" and "separate casting" respectively. (This entry focuses on the chemical element properties of gold. For social and other aspects (such as price, collection significance, etc. ) for its elemental forms, see the golden entry. )