Gold has maintained its value and luster throughout human history, standing out among metals. Unlike iron that rusts or silver that dulls, pure gold remains virtually untouched over time. This enduring quality is rooted in the metal’s unique atomic structure. To understand why gold retains its brilliance, we must first examine the chemical processes that cause other common metals to degrade.
Understanding Tarnish and General Corrosion
Tarnish is a form of mild surface corrosion that causes metals to develop a dull, darkened film. This discoloration occurs when the metal surface reacts with elements in the surrounding environment, primarily oxygen or sulfur compounds in the air. For example, silver readily reacts with hydrogen sulfide gas to create a black layer of silver sulfide, which is the familiar tarnish seen on silverware.
This reaction involves metal atoms readily giving up electrons to surrounding elements, forming new chemical compounds. Iron reacts with oxygen and moisture to form iron oxide, commonly known as rust. Copper reacts with carbon dioxide and water to form a greenish-blue layer of copper carbonate, often called a patina. Tarnish on these metals is a clear visual indication that a chemical reaction has taken place.
The Unique Chemical Stability of Gold
Gold’s exceptional stability stems from its status as a “noble metal,” a classification given to metals highly resistant to chemical attack and oxidation. Pure gold (Au) possesses an atomic structure that makes it reluctant to participate in chemical reactions. Gold’s electrons are tightly bound to the nucleus, a property related to its heavy atomic weight.
This tight electron binding means gold atoms do not easily lose electrons to form chemical bonds with common atmospheric elements like oxygen or sulfur. Gold exhibits a very high reduction potential, preferring to remain in its metallic state rather than forming compounds like oxides or sulfides. The metal ignores the presence of moisture and reactive gases that degrade other elements.
The conditions required to force gold into a chemical reaction are far beyond everyday life. Gold is only reliably dissolved by aqua regia, a highly corrosive mixture of nitric acid and hydrochloric acid. This extreme inertness is why 24-karat gold can be exposed to air and water for centuries without losing its characteristic bright color.
The Role of Alloys in Gold Discoloration
While pure 24-karat gold is impervious to tarnish, the gold used in jewelry is an alloy mixed with other metals like copper or silver. Alloying increases hardness and durability, as pure gold is too soft for practical use. These added components are measured by the karat system, where lower karats indicate a higher percentage of non-gold metals.
These base metals are prone to reacting with the environment, leading to discoloration. For instance, copper can react with oxygen to form copper oxide, or silver can react with sulfur to form silver sulfide. This chemical change on the surface of the non-gold components is what is perceived as the jewelry tarnishing.
The lower the karat rating, the more susceptible the piece is to visible discoloration. When gold jewelry appears dull or darkened, the gold itself is not tarnishing; the less stable alloy metals are undergoing a chemical change.