Gold is a dense, soft, highly ductile transition metal with bright yellow luster; it is chemically inert and used in jewelry, electronics, and as a monetary standard.
The electron configuration of gold is [Xe] 4f14 5d10 6s1. This unusual configuration results from the relativistic stabilization of the 6s orbital and the full filling of the 5d subshell, contributing to gold’s characteristic color and chemical stability.
Gold appears yellow because of relativistic effects on its electrons. The contraction of the 6s orbital and expansion of the 5d orbitals alter how gold absorbs and reflects light. It absorbs blue light and reflects red and yellow wavelengths, giving gold its distinctive metallic hue.
Gold is termed a noble metal because it is highly resistant to oxidation and corrosion. It does not react with oxygen or most acids. However, it dissolves in aqua regia (a mixture of hydrochloric and nitric acids), forming chloroauric acid (HAuCl4):
\(\mathrm{Au(s) + HNO_3 + 4\,HCl \rightarrow HAuCl_4 + NO + 2\,H_2O}\)
Gold commonly exhibits oxidation states of +1 and +3. Examples include:
These states are involved in many coordination and catalytic compounds of gold.
Gold is used in a wide range of applications:
Gold is an excellent electrical conductor because of its single free 6s electron that moves easily through the metallic lattice. Its resistance to oxidation ensures long-term conductivity, which is why it is used in connectors and contacts in high-end electronics.
Aqua regia is a mixture of concentrated hydrochloric acid (HCl) and nitric acid (HNO3) in a 3:1 ratio. Nitric acid oxidizes gold to Au3+, and hydrochloric acid provides chloride ions to form soluble chloroauric acid:
\(\mathrm{Au + 3\,NO_3^- + 6\,H^+ + 4\,Cl^- \rightarrow [AuCl_4]^- + 3\,NO_2 + 3\,H_2O}\)
Gold occurs naturally in its native metallic form (Au) and in alluvial deposits (river sediments). It can also be found in minerals like pyrite and quartz veins. It is typically extracted by mining and refined through cyanidation or smelting.
Gold is a soft, dense, malleable, and ductile metal with a bright yellow color. It has a melting point of 1064 °C, a boiling point of 2856 °C, and a density of 19.3 g/cm³. It can be hammered into sheets thinner than 0.0001 mm (gold leaf).
Metallic gold is non-toxic and inert, making it safe for use in jewelry and medical applications. However, some soluble gold compounds (like AuCl3) can be toxic and must be handled carefully. In medicine, controlled doses of gold salts are used for therapeutic purposes.
When gold reacts with chlorine gas at high temperatures, it forms gold(III) chloride:
\(\mathrm{2\,Au(s) + 3\,Cl_2(g) \rightarrow 2\,AuCl_3(s)}\)
This compound is used as a precursor in gold plating and catalysis.