Characteristics

Gold is the most malleable of all metals; a single gram can be beaten into a sheet of 1 square meter, and an avoirdupois ounce into 300 square feet. Gold leaf can be beaten thin enough to become semi-transparent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.^[18] Such semi-transparent sheets also strongly reflect infrared light, making them useful as infrared (radiant heat) shields in visors of heat-resistant suits, and in sun-visors for spacesuits.^[19] Gold is a good conductor of heat and electricity.
Gold has a density of 19.3 g/cm³, almost identical to that of tungsten at 19.25 g/cm³; as such, tungsten has been used in counterfeiting of gold bars, such as by plating a tungsten bar with gold,^{[20][21][22][23]} or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.^[24] By comparison, the density of lead is 11.34 g/cm³, and that of the densest element, osmium, is 22.588 ± 0.015 g/cm³.^[25]

Chemistry

Gold(III) chloride solution in water

Although gold is the most noble of the noble metals,^[26][27] it still forms many diverse compounds. The oxidation state of gold in its compounds ranges from −1 to +5, but Au(I) and Au(III) dominate its chemistry. Au(I), referred to as the aurous ion, is the most common oxidation state with soft ligands such as thioethers, thiolates, and tertiary phosphines. Au(I) compounds are typically linear. A good example is Au(CN)₂⁻, which is the soluble form of gold encountered in mining. The binary gold halides, such as AuCl, form zigzag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.^[28]
Au(III) (auric) is a common oxidation state, and is illustrated by gold(III) chloride, Au₂Cl₆. The gold atom centers in Au(III) complexes, like other d⁸ compounds, are typically square planar, with chemical bonds that have both covalent and ionic character.
Gold does not react with oxygen at any temperature;^[29] similarly, it does not react with ozone^{[citation needed]}.
Some free halogens react with gold.^[30] Gold is strongly attacked by fluorine at dull-red heat^[31] to form gold(III) fluoride. Powdered gold reacts with chlorine at 180 °C to form AuCl₃.^[32] Gold reacts with bromine at 140 °C to form gold(III) bromide, but reacts only very slowly with iodine to form the monoiodide.
Gold does not react with sulfur directly,^[33] but gold(III) sulfide can be made by passing hydrogen sulfide through a dilute solution of gold(III) chloride or chlorauric acid.
Gold readily dissolves in mercury at room temperature to form an amalgam, and forms alloys with many other metals at higher temperatures. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors.^[34]
Gold reacts with potassium, rubidium, caesium, or tetramethylammonium, to form the respective auride salts, containing the Au⁻ ion. Caesium auride is perhaps the most famous.
Gold is unaffected by most acids. It does not react with hydrofluoric, hydrochloric, hydrobromic, hydriodic, sulfuric, or nitric acid. It does react with aqua regia, a mixture of nitric and hydrochloric acids, and with selenic acid. Aqua regia, a 1:3 mixture of nitric acid and hydrochloric acid, dissolves gold. Nitric acid oxidizes the metal to +3 ions, but only in minute amounts, typically undetectable in the pure acid because of the chemical equilibrium of the reaction. However, the ions are removed from the equilibrium by hydrochloric acid, forming AuCl₄⁻ ions, or chloroauric acid, thereby enabling further oxidation.
Gold is similarly unaffected by most bases. It does not react with aqueous, solid, or molten sodium or potassium hydroxide. It does however, react with sodium or potassium cyanide under alkaline conditions when oxygen is present to form soluble complexes.^[33]
Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds). Gold ions in solution are readily reduced and precipitated as metal by adding any other metal as the reducing agent. The added metal is oxidized and dissolves, allowing the gold to be displaced from solution and be recovered as a solid precipitate.