Gold is admired for its enduring luster and unchanging appearance, unlike many other metals that degrade over time. This remarkable stability allows gold to retain its brilliance without tarnishing or corroding. Understanding the reasons for this unique property reveals fundamental principles of chemistry.
The Chemistry of Rust
Rust is a specific form of corrosion that primarily affects iron and its alloys, such as steel. Chemically, rust is composed of iron oxides, predominantly hydrated iron(III) oxide (Fe$_2$O$_3$$\cdot$xH$_2$O) and iron(III) oxide-hydroxide. This reddish-brown substance forms when iron reacts with oxygen in the presence of water or moisture, a process known as oxidation. The formation of rust is an electrochemical reaction where iron atoms lose electrons to oxygen.
Unlike some other metal oxides that form a protective layer, rust is porous and flaky. This means rust does not adhere tightly to the iron surface, allowing air and water to continue reaching the underlying metal. Consequently, the iron can progressively corrode and degrade completely over time. While other metals also undergo oxidation, the resulting compounds are typically referred to as corrosion rather than rust, which is a term specifically reserved for iron oxidation.
Why Gold Resists Reaction
Gold’s exceptional resistance to rust and other forms of degradation stems from its distinctive chemical makeup. Its atoms possess a highly stable electron configuration, where the outermost electrons are tightly bound to the nucleus. Specifically, gold’s electron arrangement includes a filled 5d orbital and a single electron in the 6s orbital (5d$^{10}$ 6s$^{1}$). This configuration makes it energetically unfavorable for gold atoms to lose or share their electrons in chemical reactions.
This inherent stability is why gold is classified as a “noble metal.” Noble metals are a group of metallic elements recognized for their outstanding resistance to oxidation, corrosion, and chemical reactions under most environmental conditions. Gold stands out even among noble metals as one of the least reactive elements on the periodic table. Its atoms do not readily combine with highly reactive elements like oxygen or sulfur, which are common culprits in the degradation of other metals.
Gold’s Broader Non-Corrosive Properties
Beyond its resistance to rusting, gold exhibits a broad spectrum of non-corrosive properties that contribute to its lasting appeal. The metal does not readily react with most acids, bases, or common atmospheric chemicals. This chemical inertness prevents the surface darkening known as tarnishing, which affects many other metals like silver. Pure gold, in particular, is virtually immune to tarnishing because it does not form compounds with oxygen or moisture.
Gold’s enduring non-reactivity has made it a preferred choice for jewelry and coinage for millennia. In modern applications, its resistance to corrosion and excellent electrical conductivity make it indispensable in electronics, including computer components and high-end technical equipment. Gold alloys, however, can sometimes show signs of tarnish or corrosion depending on the other metals mixed with the gold.