Gold has been coveted across civilizations, not just for its striking color and rarity, but for its remarkable permanence. Its untarnished gleam, often recovered from ancient shipwrecks or tombs, showcases an innate resistance to decay that few other metals possess. This enduring quality explains why gold has historically been a store of value and a symbol of lasting purity.
Understanding the Process of Rusting
Rusting is a specific type of corrosion, defined as the natural disintegration of a material due to chemical reactions with its environment. Rust is hydrated iron(III) oxide, a compound formed when iron reacts with oxygen and water. This reaction is classified as a reduction-oxidation (redox) process, which involves the transfer of electrons between atoms.
In this process, iron acts as the reducing agent, losing electrons (oxidation). Oxygen acts as the oxidizing agent, gaining these electrons (reduction). Water is necessary because it helps dissolve oxygen and allows the free movement of ions, accelerating the electron transfer. The reddish-brown substance we call rust is the final, stable compound formed from the iron, oxygen, and water.
The Chemistry of Gold’s Non-Reactivity
Gold’s resistance to corrosion is a direct consequence of its unique atomic structure and its classification as a “noble metal.” Noble metals are characterized by their exceptional chemical stability and low reactivity under most environmental conditions. Gold’s atoms hold onto their outermost electrons with unusual strength, a phenomenon explained by a high ionization energy.
Ionization energy is the amount of energy required to remove an electron from an atom. Gold requires significant energy to give up its electrons, making it highly unwilling to participate in the oxidation half of a redox reaction. This reluctance means gold does not readily bond with highly reactive elements like oxygen, sulfur, or water, which drive the rusting process in other metals.
The stability of the gold atom is further enhanced by its electronic configuration, where its electron orbitals are effectively filled, resulting in a stable outer shell. For a metal to rust, it must surrender electrons to oxygen, but gold’s tightly bound electrons make this transfer energetically unfavorable under normal atmospheric conditions. Because gold cannot easily be oxidized, it remains in its elemental, metallic form, preserving its luster and physical integrity.
When Gold Does Corrode
While gold resists atmospheric corrosion, its resistance is relative, not absolute, and it can be dissolved under highly specific and aggressive chemical conditions. The most famous example is aqua regia, a potent mixture of concentrated nitric acid and hydrochloric acid. The nitric acid acts as a powerful oxidizing agent, while the hydrochloric acid provides chloride ions that stabilize the gold ions, pulling the gold atoms apart.
Another method used to dissolve gold is the cyanidation process, widely employed in mining. In this process, gold reacts with an alkaline cyanide solution in the presence of oxygen and water to form a highly soluble gold-cyanide complex. These reactions demonstrate that gold is not entirely inert, but requires specialized and powerful chemical agents to force the oxidation process.
When gold jewelry appears tarnished, the gold itself is not corroding. Jewelry is typically an alloy, meaning it is a mixture of gold and less noble metals like copper or silver to increase hardness. The tarnish that appears on the surface is the corrosion of these other metals, which have reacted with sulfur or oxygen in the air.