The Chemical Nature of Gold
Gold, represented by the chemical symbol Au (from the Latin aurum), is a precious metal renowned globally for its distinct properties. It belongs to Group 11 and Period 6 of the periodic table. Unlike many other elements, gold exhibits exceptionally low chemical reactivity, making it a highly valued material across various cultures and industries, from ancient Egyptian artifacts to modern electronics.
Reactivity with Common Substances
Gold’s chemical inertness is one of its most defining characteristics. This property stems from its electron configuration and high ionization energy, meaning it requires significant energy to remove its outer electrons, which are involved in chemical bonding.
Reactivity with Water
Gold does not react with water. Whether it is pure deionized water, seawater, or acidic rain, gold remains unaffected. It will not corrode, rust, or tarnish when exposed to water, even over geological timescales. This resistance to oxidation by water is a key reason for its longevity and its use in applications requiring extreme durability, such as certain aerospace components.
Reactivity with Air
Gold also displays no reaction with oxygen or other gases present in the air under normal atmospheric conditions. This means gold does not tarnish or corrode when exposed to air, unlike metals such as iron (which rusts) or silver (which tarnishes black due to reaction with sulfur compounds in the air). The pristine appearance of gold artifacts excavated from ancient burial sites, such as those from the Moche civilization in Peru or the Viking hoards in Scandinavia, centuries after their creation, serves as a testament to this remarkable stability.
Safety and Other Properties
Understanding the potential hazards and physical attributes of elements is crucial for safe handling and application.
Toxicity
Metallic gold, in its solid elemental form, is considered non-toxic. It is biologically inert, meaning it does not readily react with biological systems in the human body. This property allows for its extensive use in jewelry worn directly against the skin, dental fillings, and even some medical treatments, such as certain injections for rheumatoid arthritis where gold compounds are used (though these compounds can have different toxicological profiles than the elemental metal).
Radioactivity
Naturally occurring gold is stable and not radioactive. The most abundant isotope, Gold-197 ($^{197}$Au), has a stable nucleus. While some artificial radioactive isotopes of gold can be produced in laboratories (e.g., Gold-198 for medical imaging or therapy), these are not found naturally and elemental gold is not inherently radioactive.
Flammability
Gold is a metal and is not flammable. It does not burn or catch fire. Like all metals, it can melt when heated to very high temperatures (its melting point is 1064 degrees Celsius or 1948 degrees Fahrenheit), and it can vaporize at even higher temperatures, but it does not combust in the way organic materials do.
Famous Chemical Reaction: Dissolving Gold with Aqua Regia
Despite its general inertness, gold can be chemically dissolved under specific, highly corrosive conditions. The most famous example is its reaction with aqua regia, a Latin term meaning “royal water.” Aqua regia is a freshly prepared mixture of concentrated nitric acid ($HNO_3$) and concentrated hydrochloric acid ($HCl$), typically in a 1:3 molar ratio.
The reaction proceeds in two main steps:
- Nitric acid acts as a powerful oxidizing agent, oxidizing a minute amount of gold to gold(III) ions ($Au^{3+}$): $Au(s) + 3NO_3^-(aq) + 6H^+(aq) \rightarrow Au^{3+}(aq) + 3NO_2(g) + 3H_2O(l)$ or $Au(s) + NO_3^-(aq) + 4H^+(aq) \rightarrow Au^{3+}(aq) + NO(g) + 2H_2O(l)$
- The hydrochloric acid then reacts with these gold(III) ions to form tetrachloroaurate(III) complex ions ($[AuCl_4]^-$): $Au^{3+}(aq) + 4Cl^-(aq) \rightarrow [AuCl_4]^-(aq)$
The formation of the stable tetrachloroaurate(III) complex ion effectively removes the $Au^{3+}$ ions from solution, shifting the equilibrium of the first reaction to the right. This allows more gold to be oxidized, enabling the complete dissolution of gold. This process has been historically significant in gold refining and the analysis of gold purity.