Gold is a chemical element with symbol Au and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright reddish yellow, soft and ductile metal. Chemically, gold is a group 11 element, it is solid under standard conditions. Gold occurs in free elemental form, as nuggets or grains, in rocks, in veins, in alluvial deposits, it occurs in a solid solution series with the native element silver and naturally alloyed with copper and palladium. Less it occurs in minerals as gold compounds with tellurium. 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 dissolves in alkaline solutions of cyanide, which are used in mining and electroplating.
Gold dissolves in mercury, forming amalgam alloys. A rare element, gold is a precious metal, used for coinage and other arts throughout recorded history. In the past, a gold standard was implemented as a monetary policy, but gold coins ceased to be minted as a circulating currency in the 1930s, the world gold standard was abandoned for a fiat currency system after 1971. A total of 186,700 tonnes of gold exists above ground, as of 2015; the world consumption of new gold produced is about 50% in jewelry, 40% in investments, 10% in industry. Gold's high malleability, resistance to corrosion and most other chemical reactions, conductivity of electricity have led to its continued use in corrosion resistant electrical connectors in all types of computerized devices. Gold is used in infrared shielding, colored-glass production, gold leafing, tooth restoration. Certain gold salts are still used as anti-inflammatories in medicine; as of 2017, the world's largest gold producer by far was China with 440 tonnes per year.
Gold is the most malleable of all metals. It can be drawn into a monoatomic wire, stretched about twice before it breaks; such nanowires distort via formation and migration of dislocations and crystal twins without noticeable hardening. A single gram of gold can be beaten into a sheet of 1 square meter, an avoirdupois ounce into 300 square feet. Gold leaf can be beaten thin enough to become semi-transparent; the transmitted light appears greenish blue, because gold reflects yellow and red. Such semi-transparent sheets strongly reflect infrared light, making them useful as infrared shields in visors of heat-resistant suits, in sun-visors for spacesuits. Gold is a good conductor of electricity. Gold has a density of 19.3 g/cm3 identical to that of tungsten at 19.25 g/cm3. By comparison, the density of lead is 11.34 g/cm3, that of the densest element, osmium, is 22.588±0.015 g/cm3. Whereas most metals are gray or silvery white, gold is reddish-yellow; this color is determined by the frequency of plasma oscillations among the metal's valence electrons, in the ultraviolet range for most metals but in the visible range for gold due to relativistic effects affecting the orbitals around gold atoms.
Similar effects impart a golden hue to metallic caesium. Common colored gold alloys include the distinctive eighteen-karat rose gold created by the addition of copper. Alloys containing palladium or nickel are important in commercial jewelry as these produce white gold alloys. Fourteen-karat gold-copper alloy is nearly identical in color to certain bronze alloys, both may be used to produce police and other badges. White gold alloys can be made with nickel. Fourteen- and eighteen-karat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. Blue gold can be made by alloying with iron, purple gold can be made by alloying with aluminium. Less addition of manganese, aluminium and other elements can produce more unusual colors of gold for various applications. Colloidal gold, used by electron-microscopists, is red. Gold has only one stable isotope, 197Au, its only occurring isotope, so gold is both a mononuclidic and monoisotopic element. Thirty-six radioisotopes have been synthesized, ranging in atomic mass from 169 to 205.
The most stable of these is 195Au with a half-life of 186.1 days. The least stable is 171Au. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, β+ decay; the exceptions are 195Au, which decays by electron capture, 196Au, which decays most by electron capture with a minor β− decay path. All of gold's radioisotopes with atomic masses above 197 decay by β− decay. At least 32 nuclear isomers have been characterized, ranging in atomic mass from 170 to 200. Within that range, only 178Au, 180Au, 181Au, 182Au, 188Au do not have isomers. Gold's most stable isomer is 198m2Au with a half-life of 2.27 days. Gold's least stable isomer is 177m2Au with a half-life of only 7 ns. 184m1Au has three decay paths: β+ decay, isomeric
Norman Neill Greenwood FRS CChem FRSC was an Australian-British chemist and Emeritus Professor at the University of Leeds. He is best known for the innovative textbook Chemistry of the Elements, co-authored with Alan Earnshaw, first published in 1984. After attending University High School, Greenwood read Chemistry at the University of Melbourne and graduated with a BSc in 1945 and an MSc in 1948. In 1948, he was awarded the Exhibition of 1851 Scholarship to enable him to read for a PhD at Sidney Sussex College, Cambridge under the supervision of Harry Julius Emeléus, he received the PhD in 1951. Greenwood was a senior research fellow at the Atomic Energy Research Establishment from 1951 until 1953 when he was appointed a lecturer at the University of Nottingham, his first PhD student at Nottingham was Kenneth Wade. Professor William Wynne-Jones, the Chairman of the School of Chemistry at Kings College, recruited Greenwood to the first established chair of inorganic chemistry in the country in 1961.
Greenwood was appointed professor and head of the Department of Inorganic and Structural Chemistry at the University of Leeds in 1971, a post which he held until his retirement in 1990 when he was given the title emeritus professor. Greenwood was elected a fellow of the Royal Society in 1987, his wide-ranging researches in inorganic and structural chemistry have made major advances in the chemistry of boron hydrides and other main-group element compounds. He pioneered the application of Mössbauer spectroscopy to problems in chemistry, he was a prolific writer and inspirational lecturer on chemical and educational themes, has held numerous visiting professorships throughout the world. He was appointed by NASA as principal investigator in the study of lunar rocks, he served as chairman of the IUPAC Commission on Atomic Weights from 1970 to 1975 and as president of the IUPAC Inorganic Chemistry Division. Greenwood, N. N.. Principles of Atomic Orbitals – Monograph for Teachers. Royal Society of Chemistry.
P. 48. ISBN 9780854040285. Greenwood, N. N.. Ionic crystals, lattice defects and nonstoichiometry. Butterworths. P. 194. Greenwood, N. N. C.. Mössbauer Spectroscopy. Chapman and Hall. P. 659. Greenwood, Norman N.. Chemistry of the Elements. Butterworth-Heinemann. P. 1340. ISBN 978-0-08-037941-8. Greenwood, N. N.. Recollections of a Scientist Volume 1. Boyhood and Youth in Australia. Xlibris Corporation. P. 288. ISBN 1-4691-7935-0. Greenwood, N. N.. Recollections of a Scientist, Volume 2: Expanding Horizons: England and Europe. Xlibris Corporation. P. 438. ISBN 978-1477151860. Editor: Spectroscopic Properties of Inorganic and Organometallic Compounds, Royal Society of Chemistry, Volume 1 to Volume 9 Norman Greenwood tells his life story at Web of Stories
Gold fluoride, AuF3, is an orange solid that sublimes at 300 °C. It is a powerful fluorinating agent. AuF3 can be prepared by reacting AuCl3 with F2 or BrF3; the crystal structure of AuF3 consists of spirals of square-planar AuF4 units