Gold

From Wikipedia, the free encyclopedia

"Element 79" redirects here. For the short story and anthology by Fred Hoyle, see Element 79 (anthology).

This article is about the element. For the color, see Gold (color). For other uses, see Gold (disambiguation).

Gold,  ₇₉Au

General properties
Name, symbol	gold, Au
Pronunciation	/ˈɡoʊld/ GOHLD
Appearance	metallic yellow
Gold in the periodic table
Ag ↑ Au ↓ Rg platinum ← gold → mercury
Atomic number (Z)	79
Group, block	group 11, d-block
Period	period 6
Element category	transition metal
Standard atomic weight (Ar)	196.966569(5)[1][2]
Electron configuration	[Xe] 4f14 5d10 6s1
Electrons per shell	2, 8, 18, 32, 18, 1
Physical properties
Phase	solid
Melting point	1337.33 K ​(1064.18 °C, ​1947.52 °F)
Boiling point	3243 K ​(2970 °C, ​5378 °F)
Density near r.t.	19.30 g/cm3
when liquid, at m.p.	17.31 g/cm3
Heat of fusion	12.55 kJ/mol
Heat of vaporization	342 kJ/mol
Molar heat capacity	25.418 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1646 1814 2021 2281 2620 3078
Atomic properties
Oxidation states	5, 3, 2, 1, −1, −2, −3 ​(an amphoteric oxide)
Electronegativity	Pauling scale: 2.54
Ionization energies	1st: 890.1 kJ/mol 2nd: 1980 kJ/mol
Atomic radius	empirical: 144 pm
Covalent radius	136±6 pm
Van der Waals radius	166 pm
Miscellanea
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	2030 m/s (at r.t.)
Thermal expansion	14.2 µm/(m·K) (at 25 °C)
Thermal conductivity	318 W/(m·K)
Electrical resistivity	22.14 nΩ·m (at 20 °C)
Magnetic ordering	diamagnetic[3]
Magnetic susceptibility (χmol)	−28.0·10−6 cm3/mol (at 296 K)[4]
Tensile strength	120 MPa
Young's modulus	79 GPa
Shear modulus	27 GPa
Bulk modulus	180 GPa[5]
Poisson ratio	0.4
Mohs hardness	2.5
Vickers hardness	188–216 MPa
Brinell hardness	188–245 MPa
CAS Number	7440-57-5
History
Naming	from Latin aurum, meaning gold
Discovery	In the Middle East (before 6000 BCE)
Most stable isotopes of gold
iso NA half-life DM DE (MeV) DP 195Au syn 186.10 d ε 0.227 195Pt 196Au syn 6.183 d ε 1.506 196Pt β− 0.686 196Hg 197Au 100% is stable with 118 neutrons 198Au syn 2.69517 d β− 1.372 198Hg 199Au syn 3.169 d β− 0.453 199Hg
view talk edit | references | in Wikidata

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. It is one of the least reactive chemical elements and is solid under standard conditions. Gold often occurs in free elemental (native) form, as nuggets or grains, in rocks, in veins, and in alluvial deposits. It occurs in a solid solution series with the native element silver (as electrum) and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides).
Gold's atomic number of 79 makes it one of the higher numbered, naturally occurring elements. It is thought to have been produced in supernova nucleosynthesis, from the collision of neutron stars,^[6] and to have been present in the dust from which the Solar System formed. Because the Earth was molten when it was formed, almost all of the gold present in the early Earth probably sank into the planetary core. Therefore, most of the gold that is present today in the Earth's crust and mantle is thought to have been delivered to Earth later, by asteroid impacts during the Late Heavy Bombardment, about 4 billion years ago.^[7][8]
Gold is resistant to most acids, though it does dissolve in aqua regia, a mixture of nitric acid and hydrochloric acid, which forms a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to refine gold and to confirm the presence of gold in metallic objects, giving rise to the term acid test. Gold also dissolves in alkaline solutions of cyanide, which are used in mining and electroplating. Gold dissolves in mercury, forming amalgam alloys, but this is not a chemical reaction.
Historically, the value of gold was rooted in its relative rarity, easy handling and minting, easy smelting and fabrication, resistance to corrosion and other chemical reactions (nobility), and distinctive color^{[citation needed]}. As a precious metal, gold has been used for coinage, jewelry, and other arts throughout recorded history. In the past, a gold standard was often implemented as a monetary policy, but gold coins ceased to be minted as a circulating currency in the 1930s, and the world gold standard was abandoned for a fiat currency system after 1976.
A total of 186,700 tonnes of gold is in existence above ground, as of 2015.^[9] The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.^[10] Gold's high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity have led to its continued use in corrosion resistant electrical connectors in all types of computerized devices (its chief industrial use). Gold is also used in infrared shielding, colored-glass production, gold leafing, and tooth restoration. Certain gold salts are still used as anti-inflammatories in medicine. As of 2014, the world's largest gold producer by far was China with 450 tonnes.^[11]