Gold
Gold is a chemical element with symbol Au (from Latin:aurum) and atomic number 79. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable and ductile metal. Chemically, gold is a transition metal and a group 11 element.
Identity
- Atomic mass
- 196.967 u
- Category
- Transition metals
- Group
- 11
- Period
- 6
- Block
- d
- Oxidation States
- +1, +3
- Phase (25 °C)
- Solid
- Appearance
- metallic yellow
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1 4f14 5d10
- Short configuration
- [Xe] 4f14 5d10 6s1
- Electrons per shell
- 2, 8, 18, 32, 18, 1
- Electronegativity (Pauling)
- 2.54
- Electron affinity
- 223 kJ/mol
- 1st ionization energy
- 890 kJ/mol
- Ionization energies
- 890 · 1980 kJ/mol
Physical properties
- Melting point
- 1064.2 °C (1337 K)
- Boiling point
- 2969.8 °C (3243 K)
- Density
- 19.3 g/cm³
- Molar heat capacity
- 25.4 J/(mol·K)
Chemical behavior of gold
Gold has an anomalous electron configuration ([Xe] 4f¹⁴ 5d¹⁰ 6s¹). It is the most noble, inert and oxidation-resistant metal in the entire periodic table. Its most common oxidation states are +1 (aurous) and +3 (auric). Due to intense quantum relativistic effects that contract the 6s orbital and unshield the 5d orbitals, gold absorbs photons in the blue band of the visible spectrum, conferring its unmistakable, exclusive bright golden-yellow color. It does not react with oxygen, water or pure acids, but dissolves in aqua regia forming chloroauric acid, and in aerated alkaline cyanide solutions.
Reactivity
Au + HNO₃ + 4 HCl → H[AuCl₄] + NO + 2 H₂O
It dissolves in aqua regia through the combined action of the oxidizing nitric acid and the complexing hydrochloric acid, forming soluble tetrachloroauric acid of golden-yellow hue.
4 Au + 8 NaCN + O₂ + 2 H₂O → 4 Na[Au(CN)₂] + 4 NaOH
Elsner's reaction; finely ground gold dissolves in dilute sodium cyanide solutions with constant aeration to form the soluble dicyanoaurate complex.
2 Au + 3 Cl₂ → 2 AuCl₃
It reacts with dry chlorine gas at about 150-200 °C forming ruby-red gold(III) chloride.
2 H[AuCl₄] + 3 H₂O₂ → 2 Au (colloidal) + 8 HCl + 3 O₂
Mild reduction of auric salts with citrate or peroxide to synthesize stable suspensions of intense red gold nanoparticles.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Tetrachloroauric acid | HAuCl₄·4H₂O | Fundamental precursor for electrolytic gilding baths, gold nanoparticle synthesis and chemical catalysts |
| Potassium dicyanoaurate | K[Au(CN)₂] | Coordination salt used on a massive scale in the electroplating industry for gold coatings on electronic contacts |
| Gold(III) chloride | AuCl₃ | Red solid used as a homogeneous catalyst in fine organic synthesis for alkyne cyclization reactions |
| Sodium aurothiomalate | C₄H₄AuNa₂O₄S | Injectable antirheumatic drug (chrysotherapy) historically used to reduce joint inflammation in rheumatoid arthritis |
Electron shell distribution
View in the atom builderIsotopes
Natural gold is rigorously monoisotopic, made up 100% of the stable isotope gold-197 (¹⁹⁷Au, with 118 neutrons). Among its more than thirty synthetic radioisotopes, gold-198 (¹⁹⁸Au, half-life of 2.69 days) stands out: it emits beta and gamma radiation and has been used as a therapeutic radioisotope in brachytherapy for the localized treatment of prostate cancer and neck tumors.
Occurrence and production
It is a scarce element in the Earth's crust (about 0.004 ppm or 4 ppb). Due to its chemical inertia, it is the native metal par excellence, found free in hydrothermal quartz veins and in alluvial river-gravel deposits as dust, flakes and nuggets. It also occurs in tellurides such as calaverite (AuTe₂) and sylvanite. The largest producing countries are China, Australia, Russia, the United States and Canada. Large-scale extraction is carried out mainly by gold cyanidation on ore heaps or activated carbon adsorption (CIL/CIP).
History and origin of the name
It is the metal known to humanity from the most remote times (more than 6,000 years old in necropolises such as Varna in Bulgaria). Its unalterability, sun-like shine and ease of forging without melting made it the universal symbol of divinity, power and wealth in Ancient Egypt, Mesopotamia and the pre-Columbian civilizations. Its chemical symbol Au comes from the Latin word 'aurum' (shine of the dawn, resplendent), while its Spanish name derives from Vulgar Latin from the same root.
Applications and relevance of gold
Uses of the element
It is the most malleable and ductile metal that exists: a single ounce (31.1 grams) can be drawn into a wire over 80 kilometers long or beaten into gold-leaf sheets barely 100 nanometers thick. Over 40% goes to central-bank monetary reserves and investment bullion, and almost 50% to jewelry. In microelectronics it is the metal of bonding wires and motherboard contacts thanks to its zero corrosion; and in spaceflight it coats space helmet visors and the James Webb telescope mirrors to reflect infrared radiation.
Uses of its compounds
Functionalized gold nanoparticles are essential in rapid medical diagnostic strips (such as pregnancy tests and lateral-flow COVID-19 antigen tests); gold thiolate complexes are used in medicine against arthritis; and gold catalysts on titanium oxide oxidize carbon monoxide at room temperature.
Health, safety and environment
Biological role
Gold has no known biological function. The pure metal is completely inert and non-toxic, which is why it is used without risk in restorative dentistry (gold crowns) and in luxury gastronomy as a harmless food additive (E-175).
Safety
In metallic form it is totally safe. Soluble gold salts can cause allergic contact dermatitis, nephritis or liver disorders if administered prolongedly in medical chrysotherapy.
Environment
Gold's main environmental impact lies in its extraction techniques: mercury use in artisanal mining causes serious pollution of Amazonian and marine rivers, while industrial open-pit leaching requires strict management of cyanide tailings ponds to prevent seepage into aquifers.
Frequently asked questions about Gold
Why is gold yellow and not silvery like almost all metals?
Gold's golden color is a direct consequence of the speed of its electrons. With a very heavy nucleus containing seventy-nine protons, the innermost electrons orbit at speeds close to the speed of light. This relativistic phenomenon modifies the energy of its orbitals, making the metal absorb blue light and intensely reflect the yellow and red hues we see.
How do gold nanoparticles work in some rapid tests?
Some rapid tests use gold nanoparticles because they produce a visible color and can bind to molecules that recognize a specific substance, such as an antigen or a hormone. If the sample contains what the test looks for, the particles accumulate in one area and a colored line appears. Gold does not detect the disease by itself: accuracy depends on the test design and how it is performed.
What did the alchemy of 'drinkable gold' and modern medical chrysotherapy against arthritis consist of?
Medieval alchemists believed that a drinkable solution of pure gold would grant immortality and cure all diseases. Although ingested elemental gold is inert, in the 20th century medicine developed soluble thiolate gold complexes (such as aurothioglucose or auranofin) to treat severe rheumatoid arthritis. These drugs suppress the inflammatory activity of macrophages and lysosomal enzymes in the joints, forming the basis of the antirheumatic therapy known as chrysotherapy.
How much dissolved gold do Earth's oceans actually contain and why isn't it extracted?
Seawater contains gold dissolved in aqueous solution at an infinitesimal concentration of about thirteen parts per trillion (0.00001 milligrams per ton of water). Although the immensity of the oceans adds up to some twenty million tons of diluted gold in total, processing millions of liters of seawater to isolate a microscopic speck of metal would cost thousands of times more energy and money than the market value of the gold obtained, making it economically unviable.
Why are the mirrors of the James Webb space telescope coated with a thin film of gold?
The James Webb space telescope observes the deep universe mainly in the thermal infrared spectrum to capture the light stretched by cosmic expansion from the first galaxies. Gold is the best known reflector of infrared radiation, reaching reflectivity above 98% between 0.6 and 28 micrometers. Each of its eighteen beryllium mirrors carries a microscopic gold layer barely one hundred nanometers thick, sealed with transparent silica to prevent scratches.
How far can a gold-leaf sheet be stretched?
Gold is the most malleable metal that exists. Artisans can hammer a small nugget into gold-leaf sheets so thin they measure barely a ten-thousandth of a millimeter (about four hundred atoms thick). At that level of fineness, gold becomes translucent and lets a soft greenish light pass through, making it ideal for gilding altars and domes with very little weight.
Why was a gold foil used in Rutherford's experiment?
In Rutherford's experiment, alpha particles were fired at an extremely thin gold foil. Most went through the foil, but a few were deflected far off course, revealing that almost all the mass and positive charge of the atom are concentrated in a diminutive nucleus. Gold could be made into extremely thin foils, which allowed studying the passage of the particles. The experiment changed the model of the atom.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.