Gold: reactions of elements
Gold is a soft metal that is largely unreactive. Gold jewellery can lie underground for thousands of years and emerge shiny after that lengthy exposure to air, water, and other chemicals in the environment. Gold does dissolve in some solvents as indicated below and does react with halogens.
The most common oxidation states of gold are Au(I) and Au(III) but there are a few examples of Au(V) (various anions [AuF6]- and [Au2F10]). The Au(III) ion has the d8 configuration which gives rise to many square planar complexes. The Au(I) configuration has the d10 configuration and is prone to the formation of linear structures. An important example is the cyanide complex [Au(CN)2]- which is one of the compounds involved in the gold extraction process from low grade ores. In addition the Au(-I) oxidation state (electronic confguration d10s2) is known and there are several examples of crystallographically characterised Au(-I) compounds such as [NMe4]Au.
Gold is the most electronegative of the d-block metals and its value of 2.4 is not too different from elements such as selenium (2.4), sulfur (2.5), and iodine (2.5). Gold also has quite a significant electron affinity (about 223 kJ mol-1, not much less than iodine (295 kJ mol-1). As a result the Au- ion is achievable and so CaAu, for instance, is more salt-like than like an alloy.
Reaction of gold with air
Gold metal is stable in air under normal conditions. However gold does dissolve in aqueous cyanide solutions in the presence of air.
Reaction of gold with water
Gold does not react with water.
Reaction of gold with the halogens
Gold metal reacts with chlorine, Cl2, or bromine, Br2, to form the trihalides gold(III) chloride, AuCl3, or gold(III) bromide, AuBr3, respectively. On the other hand, gold metal reacts with iodine, I2, to form the monohalide gold(I) chloride, AuI.
2Au(s) + 3Cl2(g) → 2AuCl3(s)
2Au(s) + 3Br2(g) → 2AuBr3(s)
2Au(s) + I2(g) → 2AuI(s)
Solutions of chlorine, Cl2, and trimethylammonium chloride, [NHMe3]Cl, in acetonitrile, MeCN, dissolve gold.
Reaction of gold with acids
Gold metal dissolves in aqua regia, a mixture of hydrochloric acid, HCl, and concentrated nitric acid, HNO3, in a 3:1 ratio. The name aqua regia was coined by alchemists because of its ability to dissolve gold - the "king of metals".
Reaction of gold with bases
Gold does not react with aqueous bases.