Gold

Gold
Atomic number	79
Symbol	Au
Group	11 (coinage metals)
Electron configuration	[Xe] 4f14 5d10 6s1
Crystal structure	FCC
Period	6
Main isotopes	197Au
Phase STP	Solid
Block	d
Oxidation states	+1, +3
Wikidata	Q897

Gold is a heavy, dense transition metal with the symbol Au (from Latin aurum, “shining dawn”) and atomic number 79. In its pure form it is a solid at STP with a distinctive lustrous yellow color – an appearance unique among metals. Gold is a Group 11 metal (the coinage metals, with Cu and Ag) in Period 6 of the periodic table. It has a filled 5d subshell and a single 6s valence electron (electron configuration [Xe] 4f^14 5d^10 6s^1). This heavy nucleus causes strong relativistic effects on the outer electrons, which help explain gold’s unusual properties (notably its color and chemical inertness). The most common oxidation states are +1 and +3 (unlike copper and silver, which mostly show +1 and +2). Gold metal is solid under normal conditions (melting point ~1064 °C) and is extraordinarily unreactive: it is not attacked by air or water at room temperature and resists corrosion.

Overview and Key Facts

Gold (Au) is a dense, malleable transition metal with a bright yellow luster. It is one of the less reactive metals (“noble”), meaning it does not oxidize or tarnish in air and is resistant to most acids. Gold’s atomic number is 79, and its atomic weight is about 196.97 u. In the periodic table it lies in Group 11 (coinage metals) and the d-block of Period 6. At room temperature it is a typical solid metal (face-centered cubic crystal structure) with a density of about 19.3 g/cm³ (19,300 kg/m³) – one of the highest of all elements. Key attributes include a Pauling electronegativity of about 2.54, a first ionization energy of 890 kJ/mol, and an atomic (empirical) radius on the order of 144 picometers. Gold’s characteristic yellow color and chemistry are strongly influenced by relativistic effects on its 6s and 5d electrons. It has one stable isotope, ^197Au, and common oxidation states of +1 and +3 (the gold(I) and gold(III) ions).

Atomic Structure and Electron Configuration

Gold atoms have 79 electrons arranged in shells with the configuration [Xe] 4f^14 5d^10 6s^1. This means the outermost shell has one 6s electron outside a filled 5d subshell. The valence shell is therefore the 6s orbital (with the 5d10 core beneath it). On the periodic trend charts, gold’s atomic radius is roughly 144 pm, which is smaller than expected for its period due to relativistic contraction of the 6s orbital. Similarly, its 6s electron is bound relatively tightly, giving a high first ionization energy (890 kJ/mol, or ~9.22 eV) – higher than silver or copper. Gold’s electronegativity (Pauling scale ~2.54) is fairly large for a metal, also reflecting relativistic stabilization of its s-electron.

Relativistic effects (from fast-moving inner electrons near the massive gold nucleus) cause the 6s orbital to contract and the 5d orbitals to shift in energy. One consequence is that gold absorbs violet/blue light more than silver does, giving bulk gold its distinctive yellow color. These relativistic shifts also contribute to gold’s increased inertness and unusual chemical preferences. For example, gold more readily forms Au(III) compounds relative to lighter coinage metals, and it has a stable +1 oxidation state where even copper(+1) is prone to disproportionate. In Group 11, gold is the least reactive (noblest) of the three; copper and silver are more easily oxidized.

'''''(Image: Atomic illustration of gold with electron shells) The gold atom (Z=79) has 79 electrons in shells 2-8-18-32-18-1 Xe]4f^14 5d^10 6s^1). Relativistic contraction of the 6s orbital (red) helps explain gold’s unique yellow color and chemistry..

Periodic trends around gold show its properties bridging those of silver and the next row transition metals. Its ionic radius as Au^+ is about 137 pm (six-coordinate) and as Au^3+ about 85 pm (six-coordinate). Compared to Ag (EN ~1.93, IE 732 kJ/mol) and Cu (EN ~1.90, IE 745 kJ/mol), gold’s higher values fit the pattern of increased effective nuclear charge and relativistic effects down the group.

Isotopes and Nuclear Properties

Gold has only one stable (and naturally occurring) isotope, ^197Au, which has a nuclear spin of 3/2. All other gold isotopes are radioactive. The longest-lived radioisotope is ^195Au (half-life ~186 days), but by far the most commonly encountered in applications is ^198Au (half-life 2.7 days), which decays by β^– emission to ^198Hg (stable). ^198Au emits gamma rays (~411 keV) as it decays, and it has been used in radiotherapy (for example, as Au nanoparticles or seeds for targeted irradiation) and as a radiotracer in biomedicine. Other notable gold radioisotopes include ^199Au (half-life ~3.14 days) and ^200Au (half-life ~2.4 days). Virtually no radioactive gold is found in nature; any ^197Au transmuted by cosmic rays is negligible. Gold’s nuclear properties (single stable isotope, moderate mass number) give it a large magnetic moment (nuclear μ=+0.59 μN) useful in nuclear magnetic resonance studies of compounds. However, gold isotopes are not used for geological dating or environmental tracing, since there are no long-lived parent-daughter systems involving gold.

Allotropes and Typical Compounds

Gold is a pure elemental metal and does not have allotropes in the way carbon or phosphorus do (there are no alternative crystalline forms besides the common metallic one). However, gold forms different structures at the nanoscale. For example, gold nanoparticles (metal clusters on the order of 1–100 nm) exhibit unique optical and chemical behavior not seen in bulk gold. These nanoparticles can appear red, purple, or blue in solution (instead of yellow), due to plasmonic (surface resonance) effects – small gold colloids absorb and scatter light differently as particle size changes. The image below illustrates how varying the surface coating (glutathione) on gold sols changes their color.

'''''(Image: Vials of gold nanoparticle solutions) *Solutions of gold nanoparticles (with various coatings) showing different colors. Gold at the nanoscale can appear red or purple, a consequence of size-dependent optical (“plasmon”) effects.

Chemically, gold’s “typical” compounds fall into two main oxidation states:

• *Gold(I) compounds (Au¹⁺): Gold(I) is a closed-shell d^10 configuration, and Au(I) centers are usually linear two-coordinate in complexes (e.g. [AuCl] complexes) or form dinuclear units (like Au^+(AuCl2^–) if bridging halides). Common gold(I) compounds include gold(I) chloride (AuCl, a pale yellow solid, also called aurous chloride) and potassium dicyanoaurate(I) K[Au(CN)2], which is the form of gold in many extraction processes. Gold(I) also forms complexes with phosphine ligands (e.g. AuCl(PPh3)). These Au(I) compounds are often easily oxidized to Au(III) or disproportionate (2 Au^+ → Au^0 + Au^3+).

• Gold(III) compounds (Au³⁺): Gold(III) has a d^8 configuration and is typically found in square-planar coordination. Gold(III) chloride (AuCl3) is a red-brown solid (often existing as Au2Cl6) which dissolves in water to give dark red chloroauric acid (HAuCl4). Chloroauric acid (HAuCl4) and its salts (e.g. NaAuCl4) are common sources of Au(III) in solution. Gold(III) also forms oxides (Au2O3 is a dark brown solid that decomposes above 160 °C) and hydroxides (colloidal Au(OH)3). Highly oxidizing conditions or fluorine can produce gold fluorides; e.g., AuF5 (gold(V) fluoride) is known as a powerful oxidizer and AuF4 (Au(III) fluoride) is also known.

Gold can also exhibit the −1 oxidation state in “auride” compounds such as cesium auride (CsAu), where a full d^10 shell accepts an extra electron. These are very rare. Gold hydrides (AuH and related species) can be detected in the gas phase or under special conditions, but no stable gold polyhydride is known at standard conditions. Gold does not form stable simple sulfides or chlorides beyond those listed above; for example, if Au^+ and a halogen are mixed, it tends to form AuCl or AuCl3 rather than Au2S? Sulfide minerals of gold do exist (e.g. calaverite AuTe2, aurostibnite AuSb) but these are rare—gold is most often found as the native element.

Gold’s bonding is highly polarizable: it forms coordination complexes with soft ligands (like thioethers or phosphines) readily. A well-known example is gold(I) thiolate clusters (Aun(SR)m) which have applications in nanotech. Gold also forms complex ions such as [AuCl4]^– (tetrachloroaurate(III)) and [Au(CN)2]^– (dicyanoaurate(I)). These complexes are key in extraction and plating processes. In solid salts (e.g. KAuCl4), the AuCl4^– anion is square-planar around Au(III).

Relaxed metals: gold does not dissolve in any non-oxidizing acids or bases. It is famously soluble in “aqua regia” (a 3:1 mixture of concentrated HCl and HNO3). In aqua regia, nitric acid oxidizes Au to Au^3+ and chloride provides ligands, producing soluble [AuCl4]^–. The net result is that aqua regia dissolves gold to form chloroauric acid. Gold will not dissolve in plain HCl or HNO3 alone. It also does not react with sulfuric or hydrofluoric acids. In base, gold metal likewise is inert; concentrated cyanide in presence of O2 dissolves gold (via formation of [Au(CN)2]^–). Reactivity is so low that gold is often stored in inert containers (e.g. plastic) and used as electrodes in aggressive environments.

Physical Properties

Bulk gold is a soft, extremely malleable and ductile metal. It has a face-centered cubic (FCC) crystal structure (lattice constant ~4.08 Å). A single gram of gold can be drawn into tens of meters of wire or hammered into a large thin sheet. Gold’s Brinell hardness is very low (about 25 HB), reflecting its softness. It has a high density (~19.3 g/cm³ at 20 °C) owing to its heavy atomic mass and tight packing; this is roughly twice the density of lead. Its melting point is 1064.18 °C (1337.33 K) and its boiling point 2856 °C (3129 K). For temperature-dependent phases, gold thermally expands very little up to its melting point.

Gold is an excellent conductor of both electricity and heat. Its electrical resistivity at 20 °C is about 2.44×10^–8 Ω·m (20.4 nΩ·m), lower than most metals except silver and copper. This means gold is used for plating contacts that must remain conductive over time, despite gold being less conductive than copper or silver by about 30%. Thermally, gold’s conductivity is about 318 W·m^–1·K^–1 – quite high among metals, so gold is often used in heat sinks and reflective coatings. Gold is chemically inert enough that these conductivities remain constant over time in air.

Magnetically, bulk gold is diamagnetic (it has no unpaired electrons and is weakly repelled by a magnetic field). Its magnetic susceptibility is about –3.5×10^–5 (SI, dimensionless). Gold does not retain any magnetism.

Optically, a thick layer of gold is shiny and yellow. Reflectance of gold in visible light is around 90% in the red to infrared, but it reflects strongly in the green/blue as well, giving it that sunlit golden appearance. Thin films of gold are transparent red or purple because the apparent color comes from reflected ambient light rather than transmitted. Gold has characteristic spectral lines (e.g. at 242.8 nm, 267.6 nm in the UV) that are used in atomic spectroscopy for analysis or calibration.

Typical values and properties (all given in SI or commonly used units): density ~19,320 kg/m³, melting T 1337 K, boiling T 3129 K, thermal conductivity ~318 W/(m·K), and specific heat ~129 J/(kg·K) at room temperature. Its atomic polarizability is large, consistent with its heavy mass and outer d-electrons.

Chemical Reactivity and Trends

Gold is one of the least reactive metals. It sits near the bottom of the electrochemical series: its standard reduction potentials are very positive (E°[Au^3+ + 3e → Au] ≈ +1.50 V, E°[Au^+ + e → Au] ≈ +1.85 V, all vs. SHE). This means gold metal can be oxidized only by very strong oxidizers. Gold does not corrode or oxidize in air – even at high temperatures gold forms only a very thin oxide layer (Au2O3) that decomposes easily.

In terms of redox behavior, gold most commonly forms +1 and +3 oxidation states. Gold(III) compounds (e.g. AuCl3, Au(OH)3) tend to be stronger oxidizers than gold(I) compounds; Au(III) in water usually exists as [AuCl4]^– or [Au(OH)4]^– (tetrahedral). Gold(I) is relatively inert: AuCl and [Au(CN)2]^– are stable in air. Gold(II) is virtually unknown because Au^2+ disproportionates (2 Au^2+ → Au^1+ + Au^3+). In a solution, Au(I) often forms linear, two-coordinate complexes (classically [AuCl2]^–) or bridged complexes (Au(I)-Au(I) bonds or aurophilic interactions). Au(III), a d^8 metal, forms square-planar complexes analogous to Pt(II).

Gold is not an acid or base, and it does not dissolve in non-oxidizing acids. It will not react with HCl or H2SO4. In contrast, nitric acid alone (a strong oxidizer) will oxidize gold to Au^3+, but usually the dissolved Au^3+ quickly forms a stable Au salt and precipitates or forms insoluble oxide. Aqua regia (mixture of HCl and HNO3) dissolves gold because Cl^– ligands stabilize Au^3+ as [AuCl4]^–. In basic solutions, gold metal is also inert; however, gold hydroxide Au(OH)3 can precipitate from very strong solutions and is amphoteric (dissolves in excess base to [Au(OH)4]^–).

Gold’s reactions with non-metals are limited. It does not react with water or most gases. At red-hot temperatures gold can react with oxygen to form Au2O3 or Au2O (both unstable). It can be attacked by halogens: chlorine or bromine gas will form AuCl3 or AuBr3 at high temperatures. Iodine forms AuI (gold(I) iodide) and AuI3 (actually AuI3 is unstable and exists as [AuI2][I], i.e. Au(III) with I^–). Fluorine reacts with gold under extreme conditions to give gold fluorides, up to AuF5 (gold(V) fluoride) or AuF3 (gold(III) fluoride). Sulfur or hydrogen sulfide do not react with gold metal, which is why gold often occurs with sulfide minerals but not chemically bound to them (it usually stands out as free metallic grains). Gold readily forms an amalgam with mercury (Hg), a historically important reaction for extracting gold and noted in artisanal mining.

In galvanic series terms, gold is at the noble end: it will not corrode, but if electrically connected to a less noble metal it will act as the cathode and protect the less noble metal (the other metal will corrode first). This makes gold plating useful for corrosion-resistant electrical contacts. Complex formation: gold forms stable complexes with soft ligands (thiourea, cyanide, phosphines). For example, in the cyanide process 4 Au + 8 CN^– + O2 + 2 H2O → 4 [Au(CN)2]^– + 4 OH^–, gold is leached as the dicyanoaurate(I) complex.

Overall reactivity trend: gold is more inert than silver or copper. In reactivity series terms it is below hydrogen, meaning it cannot reduce H^+ from acids. Its chemistry is dominated by complex formation and the relative stability of Au(III) vs Au(I). For instance, AuCl3 can oxidize iodide to iodine while itself being reduced to AuCl2^– (with Au(I)).

Occurrence and Production

Gold is a rare element in the universe. Nucleosynthesis of gold occurs in neutron-rich astrophysical events (rapid r-process), such as neutron-star mergers and certain supernovae. Observations of gravitational-wave events (e.g. kilonovae) confirm that heavy elements like gold are produced in cosmic collisions. Even so, gold’s cosmic abundance is very low (roughly 10^–10 relative to hydrogen atoms).

On Earth’s crust, the average gold concentration is only a few parts per billion by weight. Gold is siderophile and chalcophile, so it tends to bond with sulfides and sink into planetary cores during formation; this makes crustal gold scarce. What is found in the crust is mostly concentrated by geological processes. The richest gold deposits are in hydrothermal quartz veins (often in orogenic belts), in weathered (lateritic) deposits, and as placer deposits (gold grains eroded from veins and concentrated in stream gravels). Gold is almost always found as the native metal rather than in compounds – it is chemically inert enough to survive as flakes, wires or nuggets in quartz or river gravels. Placer gold is particularly common in some countries (e.g. parts of Australia, California’s historical gold country, and in African stream beds).

Major gold-producing regions include: the Witwatersrand basin in South Africa (historically the largest source), the Carlin Trend in Nevada (USA), Australian deposits (Kalgoorlie, etc.), and the Jiaodong area in Shandong, China. In recent years China, Russia, and Australia have been the world’s top producers of gold (each mining on the order of 300–380 tonnes per year). Other significant producers include Canada, the United States, Ghana, Mexico, Peru, and Uzbekistan, each mining on the order of 100–200 tonnes per year. South Africa’s production has declined from its mid-20th-century peak, though it still has vast reserves.

In 2024, global mined gold was around 3,600 tonnes (a record level), supplemented by another few hundred tonnes recycled from scrap. Gold metal also exists in stockpiles: roughly 205,000 tonnes of gold have been mined in human history, most held as jewelry, bullion or reserves (about one-fifth of that is held by central banks and governments). For perspective, all the gold ever refined could make a cube about 18–20 m on a side.

Major gold ores include native gold itself and tellurides (e.g. calaverite AuTe2), sulfides like auriferous pyrite (FeS2 with small Au inclusions), and some arsenides. Extraction is done by crushing ore and either direct processing (carbon-in-pulp cyanidation) or flotation of sulfide concentrate followed by leaching. The ore is leached, typically with dilute cyanide under aerated, alkaline conditions, to give soluble [Au(CN)2]^–, then gold is recovered by adsorption onto activated carbon or zinc cementation, and finally refined. Other methods include thiosulfate leaching (for acid-sensitive ores) and, historically, mercury amalgamation (now limited due to toxicity). Refining of impure gold (doré bars) often involves the Miller process (chlorine gas blown through molten gold to remove impurities) followed by the Wohlwill electrolytic process to reach 99.999% purity.

The largest gold deposits (in terms of reserves) are in Australia (estimated ~10–12 thousand tonnes), followed by Russia (~8–11 thousand tonnes). The USA, South Africa, Canada, and Ghana each have several thousand tonnes of reserves. In late 2024 new discoveries (e.g. the “Wangu” deposit in Hunan, China) were reported with on the order of 1,000 tonnes of gold, highlighting the still-burgeoning reserves in Asia. The world’s top gold miners (2024) were China (~380 t), Russia (~330 t), Australia (~284 t), Canada (~202 t), USA (~158 t), Ghana (~141 t), Mexico (~140 t), Indonesia (~140 t), Peru (~137 t), and Uzbekistan (~129 t). Current major mining companies include Newmont (US), Barrick (Canada), Polyus (Russia), AngloGold Ashanti (South Africa), and Zijin Mining (China).

Applications and Technology

Gold’s unique combination of properties has made it valuable in many fields. By far the largest use is in jewelry and ornaments (roughly half of gold demand), due to its beauty and tarnish resistance. Another major use is in investment and reserve assets – coins, bars, and financial holdings (about 40% of annual demand). In electronics, gold is prized for its excellent conductivity and corrosion resistance. Its stable oxide layer and non-tarnish mean gold-plated contacts or wires remain reliable for decades. Gold is widely used in connectors, switch and relay contacts, printed circuit board contacts, and bonding wires in microchips. For instance, many smartphones and computers have thin layers of gold plating on their connectors. Gold also forms ohmic contacts and gate electrodes in certain semiconductor devices. In telecommunications and aerospace, gold-coated materials reflect infrared radiation and provide thermal control (for example, the Visor coating on astronaut helmets is a gold film that reflects harmful solar rays).

In catalysis, gold was historically not used (it was thought inert), but since the 1980s minute gold nanoparticles on supports have been found to catalyze useful reactions. Nano-gold catalysts can oxidize carbon monoxide at surprisingly low temperatures and enable selective oxidations (e.g., propylene to propylene oxide, or CO oxidation for air purification). Gold combined with palladium (Au–Pd catalysts) is used industrially for vinyl acetate production (an intermediate for plastics), taking advantage of gold’s low “light-off” (activation) temperature relative to other metals. Other gold catalysts can functionalize organic molecules (e.g. activating alkynes) in green chemistry contexts. Tissue-compatible gold (in inert form) is used in medicine too: e.g. radioactive ^198Au colloids for cancer therapy, gold-coated devices in implants. In dentistry, gold alloys and plating are used for crowns and bridges due to biocompatibility.

In advanced technology, gold nanoparticles are used in biomedicine (drug delivery, diagnostics, photothermal therapy) and in quantum dot displays (bedazzling TV screens with gold quantum dots), exploiting their optical properties. Gold is also used in glassmaking to impart red coloration (as in the famous Lycurgus Cup, where colloidal gold makes the glass appear red). In nanotechnology, gold-thiomers or gold- DNA conjugates are used in sensors and imaging. In metrology, gold wires serve in precision resistors and standards due to stable resistivity. Gold also has a role in catalyzing fuel cell reactions and in some high-end battery contacts. Despite its cost, new applications keep emerging due to gold’s reliability and unique nanoscale and surface chemistry.

In electronics and connectors, gold thickness is often just a few microns or less – a tiny fraction of annual production goes into these high-tech uses. Nevertheless, any given smartphone may contain tens of milligrams of gold. Gold’s role in aerospace, such as fully reflective coatings on instruments or fine wires in satellites, is also critical for reliability. As a chemical reagent, gold salts (like AuCl3, HAuCl4) are used in analytical chemistry (to determine impurities or in gold etching) and in synthetic chemistry to mediate organic transformations.

In summary, gold is not a workhorse metal (like Fe or Al), but its niche uses rely on properties no other material can fully replace. The electronics, medical, and jewelry industries drive demand for high-purity gold, while investment demand (coins, bullion, reserves) is tied to its status as a monetary and “safe haven” metal. Gold prices (on the order of US$60,000–70,000 per kilogram in 2024) reflect its rarity and desirability.

Biology, Environment, and Safety

Gold has virtually no known biological role. It is non-toxic to cells in the elemental form or inert compounds (unlike heavy metals like lead or mercury). However, some gold compounds have bioactivity: for instance, the gold(I) drug auranofin was used to treat rheumatoid arthritis. Gold isotopes have been explored in nuclear medicine for targeting tumors (radiolabeled gold nanoparticles). Metallic gold (fine dust) is chemically inert internally, but ingestion or inhalation of soluble gold compounds can cause toxicity; for example, gold salts can damage kidneys or skin and cause allergic reactions. Occupational exposure to gold dust/fumes (rare outside mining/refining) may cause itching, mouth sores or lung irritation; permissible exposure limits tend to be very low (on the order of μg/m^3) mainly due to precaution. In general, gold metal poses minimal chemical hazard and is often considered biologically inert.

Environmentally, gold is not reactive and does not participate in amplification or degradation cycles. It does not dissolve appreciably in water or soil under natural conditions. Gold mining and processing, however, can have environmental impacts: cyanide leaching and mercury use in artisanal mining can harm aquatic ecosystems, and mining operations can lead to habitat destruction. Gold itself, as an element, will persist in soils and sediments if deposited (unlike some pollutants that degrade). Gold plating and nanoparticles released into the environment from electronic waste or biomedical uses are currently subject to study, but no acute toxicity to wildlife from metallic gold is known.

Since gold is so stable, its toxicity is mainly a concern for compounds. For pure gold, there is no meaningful exposure limit, as normal human contact (e.g. with jewelry) is safe. Regulatory handling focuses on finely divided gold dust: it is essentially inert but respirable particles should be minimized by wearing masks/gloves. As a guideline, the U.S. OSHA permissible exposure limit for gold dust is on the order of 100 μg/m^3 (10-hour TWA) for similar metals, mainly to prevent nuisance dust inhalation.

History and Etymology

Gold has been known since prehistoric times; it was among the first metals exploited by humans. Early gold artifacts date back to at least 4000 BCE (in Sumer, Egypt, and the Indus Valley). Its distinctive color and malleability made it prized for jewelry, religious objects, and coinage. Ancient societies (Egyptians, Incas, Chinese, etc.) associated gold with gods and immortality. The name “gold” comes from Old English *geolu, meaning yellow, and the chemical symbol Au from Latin aurum. The Latin name suggests “dawn” or “shining,” reflecting gold’s sheen. In mythology, gold was a symbol of the sun; in alchemy it was the prime material (sometimes identified with perfection or the goal of transmutation).

The understanding of gold as an element evolved over time. Alchemists in medieval times learned to dissolve gold in aqua regia and to precipitate it out. The modern isolation of gold came by simple physical means (panning and smelting), and its discovery was not tied to a particular scientist. Gold’s place in chemistry was formalized by Lavoisier, who recognized it as one of the elements.

The term Aurum itself has Proto-Indo-European roots related to shining. The symbol Au and the name “aurum” were standardized by IUPAC by the early 19th century. “Gold” as a word is Germanic (compare German Gold, Dutch goud). The chemical distinction of gold likely began with the notion of “Nobili metals” in the 18th century (metals that could not be oxidized by simple acids). The notion of oxidation states came later, when Humphry Davy and others isolated new metal salts.

In 1868, Robert Bunsen and Peter T. Cleve first prepared chloroauric acid (HAuCl4), laying groundwork for modern gold chemistry. In 1856, Michael Faraday famously prepared a gold monolayer on an insulating sheet from solution, using this to estimate Avogadro’s number by measuring film thickness – an early example of nanoscience.

Gold played a pivotal role in physics: Ernst Rutherford’s famous 1911 experiment on atomic structure used thin gold foil to demonstrate the nuclear atom. In the 20th century, gold’s electron structure became a benchmark for relativistic quantum chemistry studies. Relativistic effects on gold were theoretically predicted by Pyykkö and others in the mid 20th century to account for gold’s color and chemistry, and later confirmed by spectroscopic data.

Gold has also driven exploration and history: the California Gold Rush (1848) and the Klondike (1896) reshaped societies. It was the basis of the “gold standard” in finance for over a century. Refining techniques like the Miller process (chlorine refining, developed around 1895) and Conrad’s refining process were major industrial advances. The story of synthetic gold (“chrysopoeia”) was the dream of alchemists.

Culturally, gold has been admired and used in art and ceremonies (crown jewels, religious objects, gilded architecture). Its stability meant that gold artifacts survived millennia, providing archeologists insights. Etymologically, gold’s name aurum is linked to words meaning “reddening” or “dawn” (Sanskrit hiraṇa-, Latin aurum, indicating shining). The quest to understand gold at the atomic level also influenced modern science: its spectral lines were used to define atomic models; its isotopes were studied in early nuclear physics; and it remains part of the educational narrative (it’s a classic example in textbooks of a relatively inert transition metal).

Data Table

Property / Identifier	Value and Notes
Atomic number (Z)	79
Symbol	Au
Category	Transition metal (Group 11, coinage metal)
Period, group, block	6th period; Group 11; d-block
Atomic weight (standard)	~196.97 (±0.00004) g·mol^–1
Electron configuration	[Xe] 4f^14 5d^10 6s^1
Valence electrons	6s^1 (with full 5d^10 core)
Common oxidation states	+1, +3 (also 0 as Au metal; −1 in aurides)
Electronegativity (Pauling)	2.54 (see note on relativistic effect)
Atomic radius	~144 pm (empirical covalent)
First ionization energy	890 kJ·mol^–1 (≈9.225 eV)
Density (20 °C, solid)	~19.32 g·cm^–3 (19,320 kg·m^–3)
Melting point	1337.33 K (1064.18 °C)
Boiling point	3129 K (2856 °C)
Crystal structure	Face-centered cubic (fcc)
Stable isotope	^197Au (100% natural abundance); nuclear spin 3/2
CAS Registry Number	7440-57-5 (industrial identifier)
Notable compounds (examples)	AuCl (gold(I) chloride), AuCl₃ (gold(III) chloride), [Au(CN)₂]⁻ (gold cyanide)
Appearance (bulk)	Bright metallic yellow (gold color)
Conductivity (electrical)	~4.10×10^7 S·m^–1 (room T)
Conductivity (thermal)	~318 W·m^–1·K^–1
Hazard class	Inert metal; toxicity minimal for solid gold

Notes: Data are approximate. Values vary slightly with source. Gold’s unique color, high density, and high conductivity are key identifiers. The electronegativity and ionization energy reflect its novelty among group 11 metals – higher than Ag or Cu. Solid gold exists only in the fcc crystal form under normal conditions (no polymorphs). Stable ^197Au is the only naturally occurring isotope; all others are radioactive with much shorter half-lives. The oxidation states +1 and +3 cover most known gold chemistry (Au(III) compounds are often written as Au(OH)3 or [AuCl_4]^–, etc.). The values above should be understood as representative for elemental gold in standard conditions.