Gold (Au) is widely known as a noble metal, a term reflecting its remarkable resistance to chemical change. Gold does not readily react with oxygen or water, meaning it will not rust or tarnish under normal environmental conditions. This chemical inertness is the primary reason gold maintains its appearance and has been prized for millennia, but specific, powerful chemical agents can dissolve and change the element.
Understanding Gold’s Chemical Stability
Gold’s exceptional stability is rooted in its atomic structure. The atom possesses a high first ionization energy, which is the substantial energy required to remove its outermost electron to form a positive ion. Because gold does not willingly give up its electrons, it resists most chemical reactions, especially oxidation.
Contributing to this resistance is the relativistic effect, which is particularly pronounced in heavy elements like gold. The high speed of gold’s innermost electrons causes a contraction of the outer s-orbitals, pulling the valence electrons closer to the nucleus. This tighter binding makes the single valence electron more difficult to remove, inhibiting its ability to participate in reactions with common substances like most acids, bases, or oxygen.
Gold’s resistance to oxidation is why it is often found in its pure metallic state in nature, rather than as a compound. This inherent stability also prevents gold from reacting with common single acids, such as hydrochloric acid or nitric acid, when they are used alone. The metal’s high resistance to corrosion and oxidation is a defining feature that sets it apart from more base metals like iron or copper.
The Unique Action of Aqua Regia
The reputation of gold as chemically indestructible was challenged by the discovery of aqua regia, meaning “royal water,” which dissolves the metal. Aqua regia is a highly corrosive mixture of concentrated nitric acid (\(\text{HNO}_3\)) and concentrated hydrochloric acid (\(\text{HCl}\)), typically prepared in a 1:3 volume ratio. Neither acid can dissolve gold alone, but their combination creates a powerful chemical synergy.
The dissolution process relies on a two-step mechanism driven by the distinct roles of the two acids. Nitric acid acts as a potent oxidizing agent, converting the neutral gold metal (\(\text{Au}\)) into gold ions (\(\text{Au}^{3+}\)). Although nitric acid alone dissolves only a negligible amount of gold before the reaction stalls, this initial oxidation step is crucial.
Hydrochloric acid supplies a high concentration of chloride ions (\(\text{Cl}^-\)) to the solution. These chloride ions immediately react with the newly formed gold ions to create the tetrachloroaurate anion (\(\text{AuCl}_4^-\)). The formation of this complex effectively removes the \(\text{Au}^{3+}\) ions from the solution, shifting the chemical equilibrium and allowing the nitric acid to keep oxidizing more gold metal. This coordinated action is what permits the full dissolution of gold, forming chloroauric acid (\(\text{HAuCl}_4\)) in the final solution.
Reactions with Halogens and Cyanide Compounds
Beyond aqua regia, gold can be chemically altered by highly electronegative elements and specialized industrial compounds. Gold reacts directly with halogens, such as chlorine (\(\text{Cl}_2\)) and bromine (\(\text{Br}_2\)), which are powerful oxidizing agents. When heated, gold metal reacts with chlorine gas to form gold(III) chloride (\(\text{AuCl}_3\)) and with bromine to form gold(III) bromide (\(\text{AuBr}_3\)). Resulting gold halides are often unstable and decompose upon further heating. Fluorine, the most electronegative element, is even more reactive, readily forming gold fluorides.
For commercial extraction, gold is primarily dissolved using solutions containing cyanide compounds, such as sodium cyanide (\(\text{NaCN}\)), in a process known as cyanidation. This technique is used globally to recover gold from low-grade ores. The reaction requires the presence of oxygen (\(\text{O}_2\)) and occurs in an alkaline solution. The cyanide ions (\(\text{CN}^-\)) and oxygen work together to complex the gold, forming a soluble dicyanoaurate(I) complex (\(\text{Au}(\text{CN})_2^-\)). The gold metal is oxidized from its zero oxidation state to the \(+1\) state, a process governed by the Elsner equation. This complexation is highly efficient, allowing for the dissolution of gold even at very low concentrations, which is then recovered from the solution, often using activated carbon.