Gold is exceptionally resistant to corrosion, a property that has made it one of the most treasured materials throughout human history. Corrosion is the natural process of a refined metal deteriorating chemically by reacting with its environment to revert back to a more stable state, like an ore. This chemical stability is why pure gold maintains its brilliant luster without rusting or tarnishing, unlike most other metals.
Understanding Corrosion
Corrosion is fundamentally an electrochemical process where metals lose electrons to an oxidizing agent, such as oxygen or moisture, in a reaction called oxidation. This process often involves the formation of miniature electrical cells on the metal’s surface, where one area acts as the anode (losing electrons) and another as the cathode (gaining electrons). The common example of iron rusting involves iron atoms reacting with oxygen and water to form hydrated iron oxide, a flaky, reddish-brown material. This natural degradation occurs because most metals are more chemically stable as compounds than in their pure, elemental form. For instance, silver tarnishes when it reacts with sulfur compounds in the air, forming a black layer of silver sulfide.
The Concept of Noble Metals
Gold belongs to a select group known as the noble metals, which are characterized by their outstanding resistance to chemical attack, oxidation, and corrosion. The reason for gold’s extreme stability is rooted deep within its atomic structure. Gold atoms possess an electron configuration that makes their outermost electrons tightly bound to the nucleus. Unlike reactive metals that readily give up their electrons to form chemical bonds, gold’s electrons are significantly less available for reaction. This reluctance to lose electrons means gold does not easily combine with oxygen, sulfur, or water, which are the main triggers for corrosion and tarnish in other metals. This chemical stubbornness is why gold remains largely unaffected by common acids or exposure to air and moisture over long periods.
Relativistic Effects
An advanced concept known as relativistic effects contributes to this stability by altering the behavior of gold’s electrons. Because the inner electrons of a heavy atom like gold move at speeds close to the speed of light, their mass effectively increases, pulling the outer electron closer to the nucleus. This strong attraction makes the single outer electron less prone to participating in chemical reactions, thus granting gold its legendary inertness.
Purity and Reactivity Exceptions
While pure gold (24-karat) is highly resistant, its resistance can be reduced when it is alloyed with other metals to improve hardness or change color. Gold alloys, such as 14-karat gold, contain a significant percentage of less noble metals like copper or silver. These base metals in the alloy are susceptible to oxidation and tarnishing, which can cause the gold piece to discolor over time. The resulting discoloration is a reaction of the non-gold constituents, not the gold itself, with the gold matrix remaining stable.
Gold can be dissolved by a few highly aggressive chemical agents. The most famous of these is aqua regia, a powerful mixture of nitric and hydrochloric acids, which is capable of completely dissolving gold. This dissolution occurs because the acids work together: nitric acid oxidizes the gold, and the chloride ions from the hydrochloric acid immediately form a highly stable, soluble gold complex. Gold can also be dissolved using cyanide solutions in the presence of oxygen, a method historically used in large-scale mining operations.