The question of whether gold oxidizes often arises when discoloration is observed on jewelry. Oxidation is a chemical reaction where a substance loses electrons, typically by reacting with oxygen, causing rust or tarnish on most metals. Pure gold, known as 24-karat gold, does not oxidize or tarnish under normal atmospheric conditions. However, this changes significantly when gold is mixed with other metals or subjected to aggressive chemical environments.
Why Gold Resists Atmospheric Oxidation
Gold is classified as a “noble metal,” signifying its profound resistance to chemical reactions like corrosion and oxidation. This stability is rooted in its unique atomic structure and electron configuration. Gold atoms hold their valence electrons—the outermost electrons involved in bonding—with exceptional tightness.
The energy required to strip an electron from a gold atom, known as its ionization energy, is extremely high. This makes the formation of gold oxide thermodynamically unfavorable at standard room temperature and atmospheric pressure. This chemical inertness prevents gold from readily transferring electrons with oxygen or water. Gold’s resistance to losing electrons gives it a high positive electrochemical potential, which is why it remains untarnished.
The Role of Alloys in Gold Tarnishing
The dark film that develops on gold jewelry is almost always a result of the other metals present in the piece. Pure gold is too soft for everyday wear, so it is alloyed with metals like silver, copper, and zinc to increase its hardness and durability. These alloys are measured in karats, and lower-karat gold, such as 10k or 14k, contain a higher percentage of these more reactive metals.
It is these base metals, not the gold itself, that react with the environment to form tarnish. Copper readily oxidizes when exposed to air and moisture. Silver reacts with sulfur compounds, like hydrogen sulfide gas in the atmosphere, a process known as sulfidation. This reaction forms a black silver sulfide layer on the jewelry surface, making pieces with higher proportions of copper and silver more susceptible to discoloration.
Conditions That Force Gold to React
While gold is highly stable, it is not completely invulnerable and can be forced to react under specific, non-atmospheric conditions. The most famous example is its dissolution in aqua regia, a powerful mixture of concentrated nitric acid and hydrochloric acid. This highly corrosive liquid dissolves gold because it both oxidizes the metal and forms a stable complex ion, which pulls the gold atoms into the solution.
Gold also reacts with strong oxidizing agents, such as halogens like chlorine and bromine. For instance, gold reacts directly with chlorine gas to form gold(III) chloride. Furthermore, in industrial processes like gold mining, the metal is dissolved using alkaline solutions containing cyanide in the presence of oxygen. These chemical environments are far removed from everyday conditions but demonstrate that gold’s inertness has its chemical limits.