Gold (Au, Z = 79): The Metal of Eternity and Wealth

Role of Gold in Astrophysics and Cosmochemistry

Synthesis in Cataclysmic Events

Gold is one of the most prestigious elements produced by the universe. Unlike iron or silicon, it cannot be synthesized in the cores of ordinary stars by nuclear fusion. Its creation requires extreme conditions, primarily the r-process (rapid neutron capture). This process occurs during some of the most violent events in the cosmos:

• Neutron star mergers: Now considered the main source of gold in the universe. When two neutron stars collide, phenomenal amounts of neutron-rich matter are ejected, allowing the rapid synthesis of heavy elements such as gold, platinum, and uranium.
• Type II supernovae (core-collapse): The shock waves generated during the explosion of massive stars briefly create intense neutron fluxes, enabling the r-process.
• Magnetorotational supernovae: A particular type of supernova with extreme magnetic fields and rotation, also capable of producing elements via the r-process.

Every gold jewel thus contains atoms forged in the cosmic violence of events that occurred billions of years ago.

Cosmic Abundance and Terrestrial Rarity

The cosmic abundance of gold is about 1.0×10⁻¹² that of hydrogen in number of atoms, making it about 6 times rarer than platinum and thousands of times rarer than iron. Its rarity on Earth is further accentuated by its siderophile nature (affinity for iron). During the differentiation of the Earth into layers (core, mantle, crust), most of the gold present in the primitive planet migrated to the iron core. The gold we mine today likely comes from a late addition of chondritic material (meteorites) after the formation of the core, which "repainted" the Earth's surface with precious metals.

Gold as a Geochemical Tracer

The distribution of gold in terrestrial rocks follows complex laws related to hydrothermal, magmatic, and sedimentary processes. Gold anomalies serve as the main guide for mining exploration. Isotopic ratios of gold (notably \(^{197}\mathrm{Au}/^{195}\mathrm{Pt}\)) are studied to understand the origin of deposits and the processes of continent formation.

History of the Discovery and Use of Gold

Etymology and Symbolism

The chemical symbol Au comes from the Latin "aurum", meaning "shining dawn" or "light of dawn". This term evokes the characteristic color and luster of the metal. In almost all cultures, gold has symbolized purity, divinity, power, and immortality, due to its unalterability. Its name in various languages (gold, oro, zoloto) resonates with wealth and prestige.

Gold in Antiquity

Gold is the first metal known and used by humanity, dating back to the Chalcolithic (Copper Age), around 5000-4000 BCE. It was found in its native state in rivers, making it easy to recover without complex metallurgy. The Egyptians used it for sumptuous purposes (Tutankhamun's mask, tombs), the Mesopotamians used it in jewelry and trade, and pre-Columbian cultures (Incas, Aztecs) revered it as the "sweat of the sun".

Alchemy and the Quest for Gold

For centuries, alchemy sought to transform "base" metals (such as lead) into gold using the philosopher's stone. Although chemically futile, this quest nevertheless laid the foundations for modern experimental chemistry. The understanding that gold was a fundamental chemical element (incapable of being created or destroyed by chemical means) was a crucial step in the development of science.

The Great Gold Rushes

The discovery of new deposits has several times upended the global economy: California (1848), Australia (1851), Klondike (1896), South Africa (Witwatersrand, 1886). These rushes accelerated the colonization of territories, developed mining technologies, and influenced international monetary flows.

Deposits and Modern Production

Gold is present in different forms:

• Native gold: In nuggets, flakes, or inclusions in quartz veins (lodes). This is the historical form.
• Alluvial deposits (placers): Detrital gold concentrated by erosion in river sands.
• "Carlin-type" or "invisible gold" deposits: Gold is finely disseminated in sedimentary rocks, often associated with arsenic and sulfur.
• Porphyry deposits: Associated with magmatic intrusions.

The main producing countries are China (the world's leading producer), Australia, Russia, the United States, and Canada. Annual mining production is about 3,000 to 3,500 tons. South Africa, once the leader, has seen its production decline. Recycling (old jewelry, electronic waste) represents an additional important source. The price of gold, set on the London and New York markets, fluctuates according to geopolitical, economic, and monetary factors.

Structure and Fundamental Properties of Gold

Classification and Atomic Structure

Gold (symbol Au, atomic number 79) is a transition metal of the 6th period, located in group 11 of the periodic table, along with copper and silver, with which it shares some chemical similarities (the "coinage metals" group). Its atom has 79 protons, usually 118 neutrons (for the stable isotope \(^{197}\mathrm{Au}\)), and 79 electrons with the electronic configuration [Xe] 4f¹⁴ 5d¹⁰ 6s¹. This configuration with a complete d¹⁰ shell and a single s electron is the origin of its color and properties.

Characteristic Physical Properties

Gold is a bright yellow metal, very dense, extremely malleable and ductile, and an excellent conductor.

• Distinctive yellow color: Unique among pure metals, due to relativistic electronic transitions (Einstein effects).
• High density: 19.32 g/cm³ at 20 °C.
• Exceptional malleability and ductility: 1 gram of gold can be hammered into a 1 m² sheet (gold leaf 0.1 µm thick) or drawn into a wire over 2 km long.
• Excellent conductor of heat and electricity (third after silver and copper).
• Inalterability: Resists oxidation and corrosion under normal conditions.
• Moderate melting point for a precious metal: 1064.18 °C.

Gold crystallizes in a face-centered cubic (FCC) structure.

Transformation Points

Gold melts at 1064.18 °C (1337.33 K) and boils at 2970 °C (3243 K). Its relatively low melting point facilitated its processing from antiquity.

Chemical Reactivity (very low)

Gold is the noblest metal along with platinum and a few others. It is practically inert under ambient conditions:

• Air and oxygen: No reaction at any temperature.
• Water: No reaction.
• Simple acids: Insoluble in hydrochloric, sulfuric, nitric, or hydrofluoric acid, even when concentrated.
• Aqua regia: The only common reagent capable of dissolving it, forming chloroauric acid (HAuCl₄).
• Alkaline cyanides in the presence of oxygen: Forms soluble complexes [Au(CN)₂]⁻. This is the basis of cyanidation, the main method for extracting gold from low-grade ores.
• Halogens: Reacts with chlorine to form AuCl₃ and with bleach.
• Mercury: Forms an amalgam (liquid alloy), historically used in mining extraction.

Summary of Physical Characteristics

Density: 19.32 g/cm³.
Melting point: 1337.33 K (1064.18 °C).
Boiling point: 3243 K (2970 °C).
Crystal structure: Face-centered cubic (FCC).
Electronic configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s¹.
Main oxidation states: +1 and +3.

Table of Gold Isotopes

Electronic Configuration and Electron Shells of Gold

N.B.:
Electron shells: How electrons are organized around the nucleus (new window).

Gold has 79 electrons distributed over six electron shells. Its electronic configuration [Xe] 4f¹⁴ 5d¹⁰ 6s¹ has a particularity: the 5d subshell is completely filled (10 electrons), while a single electron occupies the 6s shell. This can also be written as: K(2) L(8) M(18) N(32) O(18) P(1), or in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s¹.

Detailed Structure of the Shells

K shell (n=1): 2 electrons (1s²).
L shell (n=2): 8 electrons (2s² 2p⁶).
M shell (n=3): 18 electrons (3s² 3p⁶ 3d¹⁰).
N shell (n=4): 32 electrons (4s² 4p⁶ 4d¹⁰ 4f¹⁴).
O shell (n=5): 18 electrons (5s² 5p⁶ 5d¹⁰).
P shell (n=6): 1 electron (6s¹).

Valence Electrons and Oxidation States

Gold has 11 valence electrons if we count the electrons in the 5d and 6s shells (10+1). Chemically, gold is less reactive than copper or silver in its group. Its most common oxidation states are +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds).

• Gold(I) (Au⁺): Often present in linear complexes, such as gold(I) cyanide K[Au(CN)₂] used in electroplating. Gold(I) compounds are often unstable and tend to disproportionate into Au(0) and Au(III).
• Gold(III) (Au³⁺): The most stable and common state. Forms square planar complexes, such as chloroauric acid (HAuCl₄) or gold(III) chloride (AuCl₃).
• Colloidal state (Au⁰ in nanoparticles): Gold nanoparticles (1-100 nm) exhibit unique optical properties (red, purple colors) due to surface plasmon resonance, absent in bulk gold.

Gold can also form unusual oxidation states such as +5 and even -1 (alkali aurides such as CsAu).

Chemical Reactivity of Gold

Reaction with Air and Oxygen

Gold is totally inert to oxygen, nitrogen, carbon dioxide, and other atmospheric gases at all temperatures. It does not form oxides, which explains why ancient gold objects found are perfectly intact, without any trace of tarnish or corrosion.

Reaction with Water and Acids

Gold is insoluble in all simple acids. This property is proverbial: "as unassailable as gold".

Its only major chemical weakness is aqua regia (a 1:3 mixture of concentrated nitric and hydrochloric acids). The reaction produces tetrachloroauric(III) acid, HAuCl₄:
Au + HNO₃ + 4 HCl → HAuCl₄ + NO + 2 H₂O

Gold also dissolves in aqueous solutions of sodium or potassium cyanide in the presence of oxygen, forming the soluble dicyanoaurate(I) complex, [Au(CN)₂]⁻. This is the principle of cyanidation, used to extract gold from very low-grade ores.

N.B. :
, or royal water, is a corrosive mixture of concentrated nitric acid (HNO₃) and concentrated hydrochloric acid (HCl) in a typical ratio of 1:3. Its ability to dissolve gold and platinum, otherwise resistant to separate acids, is explained by the in situ formation of chlorine (Cl₂) and nitrosyl chloride (NOCl), which oxidize these metals into soluble complex ions (such as [AuCl₄]⁻). Used since alchemy for the purification of precious metals, it still plays a crucial role in metallurgy, microelectronics, and analytical chemistry.

Reactions with Halogens and Other Elements

• Chlorine: Reacts at moderate temperature to form AuCl₃ (gold(III) chloride).
• Bromine and iodine: React to form AuBr₃ and AuI (the latter is unstable).
• Fluorine: Reacts to form AuF₃ and AuF₅ (gold(V) fluoride is a very powerful oxidant).
• Sulfur and selenium: Do not react directly, but gold sulfides and selenides can be prepared by other means.
• Mercury: Easily forms an amalgam (liquid alloy) at room temperature. This property was used to recover gold from auriferous sands (amalgamation).

Important Compounds

• Chloroauric acid (HAuCl₄): Yellow-red liquid, main precursor for electroplating and nanoparticle synthesis.
• Potassium gold(I) cyanide (K[Au(CN)₂]): Very toxic white salt, electrolyte for gold plating.
• Gold nanoparticles (Au⁰): Colloidal suspensions with vivid colors (red, purple) depending on size and shape, used in biomedicine and catalysis.
• Gold(III) chloride (AuCl₃): Red solid, chlorinating agent in organic synthesis.

Industrial and Technological Applications of Gold

• In jewelry and goldsmithing (about 50% of global demand), in the form of alloys with silver, copper, palladium (karats);
• As a store of value and currency (ingots, investment coins, central bank reserves);
• In electronics for contacts, connectors, and high-reliability integrated circuits (corrosion resistance, excellent conductivity);
• As a protective and conductive coating (gold plating) on electronic components, medals, and space equipment;
• In dentistry for crowns, bridges, and inlays (gold-palladium-silver alloys);
• In nanotechnology and biomedicine: gold nanoparticles for imaging (contrast), detection (biosensors), photothermal therapy for cancer, and drug delivery;
• As a catalyst in the chemical industry, especially for the selective oxidation of carbon monoxide (CO) and the production of vinyl chloride;
• In glass and ceramics as a pigment (colloidal gold red or "ruby gold") and for mirror-effect or heated glazing;
• For bonding wires in microprocessors and memory chips;
• In space equipment as a reflective coating on helmet visors, satellites, and the James Webb Space Telescope (sunshield);
• As a reference standard in electrochemistry (saturated calomel electrode, often with a gold contact);
• In pharmacy: gold salts (chrysotherapy) for the treatment of rheumatoid arthritis (declining use);
• In conductive inks for printed electronics and flexible circuits;
• For decoration on porcelain and glass (gilding);
• In fundamental research: as a target for the production of heavy nuclei in nuclear physics.

Key Applications: Jewelry, Electronics, and Medicine

Jewelry and Karats

Pure gold (24 karats) is too soft for jewelry. It is alloyed with other metals to increase its hardness and change its color:

• Yellow gold: Alloy with silver and copper.
• White gold: Alloy with palladium, nickel, or silver, often rhodium-plated for a bright white finish.
• Rose/red gold: Higher copper content.
• Green gold: Alloy with silver.

The fineness (purity) is expressed in karats (1 karat = 1/24) or in thousandths (e.g., 750/1000 gold = 18 karats).

High-Reliability Electronics

The exceptional properties of gold make it an indispensable material in high-end electronics:

• Conductivity: Excellent, with very low resistance even at thin thicknesses.
• Corrosion resistance: Gold contacts do not corrode, ensuring reliable connection for decades, even in humid environments.
• Ease of bonding: Easily soldered and deposited by electroplating.

It is found in motherboard connectors, computer chips (bonding wires), mobile phone contacts, and military and space equipment where reliability is critical.

Gold Nanoparticles in Biomedicine

This is one of the most promising fields. Gold nanoparticles (1-100 nm) exhibit unique optical properties (surface plasmon resonance): they strongly absorb and scatter light in the visible and near-infrared range. Applications:

• Diagnostic imaging: Contrast agent for optical coherence tomography (OCT) or photoacoustic imaging.
• Photothermal therapy: Injected nanoparticles accumulate in tumors. When illuminated by a near-infrared laser (which penetrates tissues), they locally heat up and destroy cancer cells.
• Drug delivery vectors: Nanoparticles can be functionalized to deliver chemotherapy drugs directly to the tumor.
• Rapid diagnostic tests: Such as in pregnancy tests or biosensors, where the color change of nanoparticle aggregates indicates a positive result.

Economic and Financial Role

Gold as Currency and Safe Haven

For millennia, gold has served as the basis for monetary systems (gold standard). Although demonetized today, it remains the safe haven asset par excellence:

• Protection against inflation: Its value tends to increase when the purchasing power of paper currencies falls.
• "Safe haven" asset: In times of geopolitical or financial crisis, investors turn to gold.
• Portfolio diversification: Gold has a low correlation with stocks and bonds.

Central banks hold enormous gold reserves (about 35,000 tons in total) as a guarantee of stability.

Gold Markets

The price of gold is set twice a day in London (London Gold Fixing) and is traded continuously on exchanges such as COMEX in New York. There is also a vast physical market (ingots, coins) and derivative financial products (ETFs, futures).

Toxicology, Environment, and Recycling

Toxicity

Bulk metallic gold is inert and non-toxic. It can be worn, touched, and even ingested (gold leaf in pastry) without danger. However:

• Soluble gold salts: Compounds such as gold(III) chloride (AuCl₃) are corrosive and toxic.
• Cyanides used in extraction: Extremely toxic to humans and the environment, requiring very strict management.
• Chrysotherapy: Injectable gold salts for rheumatoid arthritis can have serious side effects (dermatitis, nephropathy, myelosuppression).
• Nanoparticles: Their potential long-term toxicity is the subject of in-depth studies, although gold is biocompatible.

Environmental Impact of Mining

Gold mining, especially artisanal and small-scale mining (ASM), can have devastating impacts:

• Use of mercury: Amalgamation releases mercury into the environment, contaminating waterways and causing poisoning (Minamata disease) in local populations.
• Use of cyanide: Leaks from leaching ponds can contaminate groundwater.
• Deforestation, erosion, ecosystem disruption.
• Mining waste (tailings, residues): Often contain arsenic and other heavy metals.

The "Fair Gold" initiative and other certifications attempt to promote more responsible mining practices.

Recycling

Gold is the recycling champion: nearly 30% of the gold used each year comes from recycling. It can be recycled indefinitely without loss of quality. Sources include:

• Old jewelry (the most important source).
• Waste electrical and electronic equipment (WEEE): Increasingly important "urban mines" (phones, computers).
• Dental and medical waste.
• Industrial waste (electroplating sludge, used catalysts).

Recycling is carried out by refiners who melt down waste, purify gold by electrolysis or chemical attack, and recast it into high-purity ingots (999.9/1000).

Outlook

Gold will remain a strategic material:

• Emerging technologies: Its role in nanomedicine, green catalysis, and quantum electronics could grow.
• Space exploration: M-type (metallic) asteroids could contain astronomical amounts of gold and other precious metals, opening a distant but fascinating perspective.
• Enduring cultural and financial value: The universal appeal of gold, a symbol of eternity and success, shows no sign of fading.