Mixed valence compounds

Well-defined cluster compounds are numerous.^[36] In such cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C₆H₅)₃)}₆²⁺. Gold chalcogenides, such as gold sulfide, feature equal amounts of Au(I) and Au(III).

Color

Different colors of Ag-Au-Cu alloys

Whereas most metals are gray or silvery white, gold is slightly reddish yellow.^[41] This color is determined by the density of loosely bound (valence) electrons; those electrons oscillate as a collective "plasma" medium described in terms of a quasiparticle called a plasmon. The frequency of these oscillations lies in the ultraviolet range for most metals, but it falls into the visible range for gold due to subtle relativistic effects that affect the orbitals around gold atoms.^[42][43] Similar effects impart a golden hue to metallic caesium.
Common colored gold alloys include the distinctive eighteen-karat rose gold created by the addition of copper. Alloys containing palladium or nickel are also important in commercial jewelry as these produce white gold alloys. Fourteen-karat gold-copper alloy is nearly identical in color to certain bronze alloys, and both may be used to produce police and other badges. White gold alloys can be made with palladium or nickel. Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. Blue gold can be made by alloying with iron, and purple gold can be made by alloying with aluminium. Less commonly, addition of manganese, aluminium, indium and other elements can produce more unusual colors of gold for various applications.^[34]
Colloidal gold, used by electron-microscopists, is red if the particles are small; larger particles of colloidal gold are blue.

Isotopes

Main article: Isotopes of gold

Gold has only one stable isotope, 197Au, which is also its only naturally occurring isotope, so gold is both a mononuclidic and monoisotopic element. Thirty-six radioisotopes have been synthesized ranging in atomic mass from 169 to 205. The most stable of these is 195Au with a half-life of 186.1 days. The least stable is 171Au, which decays by proton emission with a half-life of 30 µs. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, and β⁺ decay. The exceptions are 195Au, which decays by electron capture, and 196Au, which decays most often by electron capture (93%) with a minor β⁻ decay path (7%).^[44] All of gold's radioisotopes with atomic masses above 197 decay by β⁻ decay.^[45]
At least 32 nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range, only 178Au, 180Au, 181Au, 182Au, and 188Au do not have isomers. Gold's most stable isomer is 198m2Au with a half-life of 2.27 days. Gold's least stable isomer is 177m2Au with a half-life of only 7 ns. 184m1Au has three decay paths: β⁺ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths.^[45]

Modern applications

The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.^[10]

Jewelry

Main article: Jewelry

Moche gold necklace depicting feline heads. Larco Museum Collection. Lima-Peru

Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower karat rating, typically 22k, 18k, 14k or 10k, contain higher percentages of copper or other base metals or silver or palladium in the alloy.^[34] Nickel is toxic, and its release from nickel white gold is controlled by legislation in Europe.^[34] Palladium-gold alloys are more expensive than those using nickel. High-karat white gold alloys are more resistant to corrosion than are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects.
By 2014, the gold jewelry industry was escalating despite a dip in gold prices. Demand in the first quarter of 2014 pushed turnover to $23.7 billion according to a World Gold Council report.

Investment

Gold prices (US$ per troy ounce), in nominal US$ and inflation adjusted US$.

Main article: Gold as an investment

The ISO 4217 currency code of gold is XAU.^[46]
Many holders of gold store it in form of bullion coins or bars as a hedge against inflation or other economic disruptions. Modern bullion coins for investment or collector purposes do not require good mechanical wear properties; they are typically fine gold at 24k, although the American Gold Eagle and the British gold sovereign continue to be minted in 22k (0.92) metal in historical tradition, and the South African Krugerrand, first released in 1967, is also 22k (0.92).^[47]
The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin, at 99.999% or 0.99999, while the popular issue Canadian Gold Maple Leaf coin has a purity of 99.99%. In 2006, the United States Mint began producing the American Buffalo gold bullion coin with a purity of 99.99%. The Australian Gold Kangaroos were first coined in 1986 as the Australian Gold Nugget but changed the reverse design in 1989. Other modern coins include the Austrian Vienna Philharmonic bullion coin and the Chinese Gold Panda.