Periodic table › Gold
1. Identity and classification
Dense, soft, malleable, and ductile metallic solid with a warm, lustrous yellow-gold color that resists tarnishing in air and water
- Name and symbol
- Gold (Gold) · Au
- Atomic number
- 79
- Relative atomic weight / mass number
- 196.97 ± 0.01
- Group · period · block
- 11 · 6 · d
- Category
- Transition Metal
- State near room temperature
- Solid
- Stable isotopes
- At least one stable isotope
- CAS number
- 7440-57-5
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 79 · 79
- Neutrons in reference isotope
- 118 (197Au)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹⁴ 5d¹⁰ 6s¹- Abbreviated configuration
[Xe]6s¹ 4f¹⁴ 5d¹⁰- Electrons per shell
- 2, 8, 18, 32, 18, 1
- Listed oxidation states
- +3, +1
- Electronegativity
- 2.54 (Pauling)
- First ionization energy
- 890.174 kJ/mol · 9.226 eV
- Electron affinity
- 222.785 kJ/mol
- Covalent radius
- 136 pm
- van der Waals radius
- 166 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 19.282 g/cm³
- Melting point
- 1337.33 K · 1064.18 °C · 1947.524 °F
- Boiling point
- 3129 K · 2855.85 °C · 5172.53 °F
- Crystal structure
- FCC
- Specific heat
- 0.129 J/(g·K)
- Thermal conductivity
- 318 W/(m·K)
- Enthalpy of fusion
- 12.55 kJ/mol
- Enthalpy of vaporization
- 324 kJ/mol
- Magnetic ordering
- Diamagnetic
- Mohs hardness
- 2.5
- Speed of sound
- 2030 m/s
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
Aqua regia can dissolve gold; most simple acids do not.
Selected compounds: HAuCl₄ (Chloroauric acid) · AuCl₃ (Gold(III) chloride) · KAu(CN)₂ (Potassium dicyanoaurate)
Reaction reference
Gold resists ordinary oxidation in air.
No ordinary reaction with water.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 197Au | Stable | 100% | Stable |
| 198Au | Radioactive | — | 2.6941 d |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- Known since antiquity; no exact discovery year
- Scientists associated with discovery
- Known since antiquity
- Location
- Ancient Egypt, Bulgaria (Varna) & India
- Origin
- Natural
- Listed crustal abundance
- 0.004 ppm
Minerals and naming reference
Native Gold (often alloyed with silver as electrum), Calaverite (AuTe₂), Sylvanite; quartz vein lodes and alluvial placer deposits
From Proto-Germanic '*gulþą' (yellow/shining); chemical symbol Au from Latin 'aurum' (shining dawn)
7. Applications
Fine jewelry (24K, 22K, 18K), wedding bands, sovereign bullion coins, architectural gold leaf (gilding)
Detailed applications by field
- everyday
- Fine jewelry (24K, 22K, 18K), wedding bands, sovereign bullion coins, architectural gold leaf (gilding)
- industry
- Central bank monetary reserves, infrared-reflective architectural glass coatings, precision electroplating
- electronics
- Corrosion-free wire bonding inside microchips, CPU socket pins, USB/HDMI connectors, and smartphone PCBs
- medicine
- Colloidal gold nanoparticles in rapid lateral-flow diagnostic tests, dental crowns, rheumatoid arthritis chrysotherapy (auranofin)
- energy
- Corrosion-resistant coatings on hydrogen fuel cell bipolar plates
- aerospace
- Infrared radiation shielding on the James Webb Space Telescope (JWST) primary mirror and astronaut helmet visors
- agriculture
- Gold nanoparticle biosensors for rapid detection of plant pathogens and soil toxins
- science
- Rutherford gold-foil alpha scattering experiment; scanning electron microscopy (SEM) sputter coating
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Biologically inert in metallic form; soluble gold(I) thiolates exhibit anti-inflammatory activity in joints, while gold nanoparticles serve as biocompatible drug-delivery and diagnostic carriers.