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| General |
| Name, Symbol,
Number |
gold, Au, 79 |
| Chemical series |
transition metals |
| Group, Period,
Block |
11, 6, d |
| Appearance |
metallic yellow
 |
| Standard atomic weight |
196.966569(4) g·mol−1 |
| Electron configuration |
[Xe] 4f14 5d9 6s1 |
| Electrons per shell |
2, 8, 18, 32, 18, 1 |
| Physical properties |
| Phase |
solid |
| Density (near r.t.) |
19.3 g·cm−3 |
| Liquid density at m.p. |
17.31 g·cm−3 |
| Melting point |
1337.33 K
(1064.18 °C, 1947.52 °F) |
| Boiling point |
3129 K
(2856 °C, 5173 °F) |
| Heat of fusion |
12.55 kJ·mol−1 |
| Heat of vaporization |
324 kJ·mol−1 |
| Heat capacity |
(25 °C) 25.418 J·mol−1·K−1 |
Vapor pressure
| P(Pa) |
1 |
10 |
100 |
1 k |
10 k |
100 k |
| at T(K) |
1646 |
1814 |
2021 |
2281 |
2620 |
3078 |
|
| Atomic properties |
| Crystal structure |
cubic face centered |
| Oxidation states |
−1, 1, 2, 3, 4, 5, 6, 7
(amphoteric oxide) |
| Electronegativity |
2.54 (scale Pauling) |
| Ionization energies |
1st: 890.1 kJ/mol |
| 2nd: 1980 kJ/mol |
| Atomic radius |
135 pm |
| Atomic radius (calc.) |
174 pm |
| Covalent radius |
144 pm |
| Van der Waals radius |
166 pm |
| Miscellaneous |
| Magnetic ordering |
no data |
| Electrical resistivity |
(20 °C) 22.14 n Ω·m |
| Thermal conductivity |
(300 K) 318 W·m−1·K−1 |
| Thermal expansion |
(25 °C) 14.2 µm·m−1·K−1 |
| Speed of sound (thin rod) |
(r.t.) (hard-drawn)
2030 m·s−1 |
| Young's modulus |
78 GPa |
| Shear modulus |
27 GPa |
| Bulk modulus |
220 GPa |
| Poisson ratio |
0.44 |
| Mohs hardness |
2.5 |
| Vickers hardness |
216 MPa |
| Brinell hardness |
? 2450 MPa |
| CAS registry number |
7440-57-5 |
| Selected isotopes |
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| References |
Gold (IPA: /ˈgold/) is a chemical element with the symbol Au (from the
Latin aurum) and atomic number 79. It is a highly
sought-after precious metal which, for many centuries, has been used as money, a store of value and in jewelry.
The metal occurs as nuggets or grains in rocks, underground "veins" and in alluvial deposits. It is one of the coinage metals. Gold is dense,
soft, shiny and the most malleable and ductile of the
known metals. Pure gold has an attractive bright yellow color.
Gold forms the basis for a monetary standard used by the International Monetary Fund (IMF) and the Bank for International Settlements (BIS). The ISO currency
code of gold bullion is XAU. Modern industrial uses include
dentistry and electronics, where gold has traditionally
found use because of its good resistance to oxidative corrosion.
Chemically, gold is a trivalent and univalent transition metal. Gold does not react
with most chemicals, but is attacked by chlorine, fluorine,
aqua regia and cyanide. Gold dissolves in mercury, forming amalgam alloys, but does not react with it. Gold is
insoluble in nitric acid, which will dissolve silver and base metals, and this is the basis
of the gold refining technique known as "inquartation and parting". Nitric acid has long been used to confirm the presence of
gold in items, and this is the origin of the colloquial term "acid test," referring to a gold standard test for genuine
value.
Characteristics
Gold is the most malleable and ductile metal; a
single gram can be beaten into a sheet of one square meter,
or an ounce into 300 square feet. Gold leaf can be beaten
thin enough to become translucent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.
Gold readily forms alloys with many other metals. These alloys can be produced to increase the hardness or to create exotic
colors (see below). Native gold contains usually eight to ten percent silver, but often much more
— alloys with a silver content over 20% are called electrum. As the amount of silver increases,
the color becomes whiter and the specific gravity becomes lower.
Gold is a good conductor of heat and electricity, and is
not affected by air and most reagents. Heat,
moisture, oxygen, and most corrosive agents have very little
chemical effect on gold, making it well-suited for use in coins and jewelry; conversely, halogens will chemically alter gold, and
aqua regia dissolves it via formation of the chloraurate ion.
Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3
(gold(III) or auric compounds). Gold ions in solution are readily reduced and precipitated out as gold metal by adding any other metal as the reducing agent. The added
metal is oxidized and dissolves allowing the gold to be displaced from solution and be recovered
as a solid precipitate.
Recent research undertaken by Sir Frank Reith of the Australian National University shows that microbes play an important role
in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial
deposits.[1]
High quality pure metallic gold is tasteless, in keeping with its resistance to corrosion (it is metal ions which confer taste
to metals).
In addition, gold is very dense, a cubic meter weighing 19300 kg. By comparison, the density
of lead is 11340 kg/m³, and the densest element, Iridium, is 22650
kg/m³.
Toxicity
Generally gold is not harmful if consumed and is sometimes used as a food decoration in the form of gold leaf. However, consumption and accumulation of large amounts of gold (or gold compounds) in the body can still be toxic and the symptoms are similar to those of heavy metal poisoning.
Applications
As the metal
Medium of monetary exchange. In various countries, gold is used as a standard for monetary exchange, in coinage and in jewelry.
Pure gold is too soft for ordinary use and is typically hardened by alloying with copper or other base metals. The gold content
of gold alloys is measured in carats (k), pure gold being designated as 24k.
Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness. Modern collector/investment bullion coins
(which do not require good mechanical wear properties) are typically 24k, although the American Gold Eagle and British gold sovereign
continue to be made at 22k, on historical tradition. The Canadian Gold Maple
Leaf coin contains the highest purity gold of any popular bullion coin, at 99.999% (.99999 fine). Several other 99.99%
pure gold coins are currently available, including Australia's Gold Kangaroos (first appearing in 1986 as the Australian Gold Nugget, with the kangaroo theme appearing in 1989), the several coins of the
Australian Lunar Calendar series, and the Austrian Philharmonic. In 2006, the U.S. Mint began production of the American Buffalo gold bullion coin also at 99.99% purity.
Today, gold has fallen out of use in coins made for general circulation.
Jewelry. Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry,
altering its hardness and ductility, melting point, color and other properties. Alloys with lower "k", typically 22k, 18k, 14k or
10k, contain higher percentages of copper, silver or other base metals in the alloy. Copper is
the most commonly used base metal, yielding a redder metal. Eighteen carat gold containing 25% copper is found in antique and
Russian jewelry and has a distinct, though not dominant, copper cast, creating rose gold.
Fourteen carat gold-copper alloy is nearly identical in color to certain bronze alloys, and both
may be used to produce police and other badges. Blue gold can be made by alloying with iron and purple gold can be made by alloying with aluminum,
although rarely done except in specialized jewelry. Fourteen and eighteen carat gold alloys with silver alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with
palladium or nickel. White 18 carat gold containing 17.3%
nickel, 5.5% zinc and 2.2% copper is silver in appearance. Nickel is toxic, however, and its release from nickel white gold is
controlled by legislation in Europe. Alternative white gold alloys are available based on palladium, silver and other white
metals (World Gold Council), but the palladium alloys are more expensive than those using nickel. High-carat white gold alloys
are far more resistant to corrosion than are either pure silver or sterling silver. The Japanese craft of Mokume-gane exploits the
colour contrasts between laminated colored gold alloys to produce decorative wood-grain effects.
The 220 kg Gold brick displayed in Chinkuashi Gold Museum, Taiwan.
Other uses:
- In medieval times, gold was often seen as beneficial for the health, in the belief that something that rare and beautiful
could not be anything but healthy. Even some modern esotericists and forms of
alternative medicine assign metallic gold a healing power. Some gold salts do have
anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions. However,
only salts and radioisotopes of gold are of pharmacological value, as elemental (metallic) gold is inert to all chemicals it
encounters inside the body.
- Gold leaf, flake or dust is used on and in some gourmet foodstuffs, notably sweets and drinks as decorative
ingredient.[2] Gold flake was used by the nobility in
Medieval Europe as a decoration in foodstuffs and drinks, in the form of leafs, flakes or dust,
either to demonstrate the host's wealth or in the honest belief that something that valuable and rare must be beneficial for
one's health.
- Gold solder is used for joining the components of gold jewelry by high-temperature hard
soldering or brazing. If the work is to be of hallmarking
quality, gold solder must match the carat weight of the work, and alloy formulae are manufactured in most industry-standard carat
weights to color match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as
Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting
points, goldsmiths can assemble complex items with several separate soldered joints.
- Gold can be used in food and has the E Number 175. Goldwasser (German: "Goldwater") is a traditional herbal
liqueur produced in Gdańsk, Poland and Schwabach, Germany and contains
flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf. However, since metallic
gold is inert to all body chemistry, it adds no taste nor has it any other nutritional effect and leaves the body unaltered.
- Dentistry. Gold alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges. The gold alloys' slight
malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally
more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as
incisors is favored in some cultures and discouraged in others.
- Gold can be made into thread and used in embroidery.
- Gold is ductile and malleable, meaning it can be
drawn into very thin wire and can be beaten into very thin sheets known as gold leaf.
- Gold produces a deep, intense red color when used as a coloring agent in glass.
- In photography, Gold toners are used to shift the color of silver bromide black and white prints towards brown or blue tones,
or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak publish formulae for several
types of gold toners, which use gold as the chloride (Kodak, 2006).
- Electronics. The concentration of free electrons in gold metal is 5.90×1022 cm-3. Gold is highly
conductive to electricity, and has been used for electrical wiring in some high energy applications (silver is even more
conductive per volume, but gold has the advantage of corrosion resistance). For example, gold electrical wires were used during
some of the Manhattan Project's atomic experiments, but large high current silver
wires were used in the calutron isotope separator magnets in the project.
- Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other
environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a
thin layer coating electrical connectors of all kinds, thereby ensuring good connection. For example, gold is used in the
connectors of the more expensive electronics cables, such as audio, video and USB
cables. The benefit of using gold over other connector metals such as tin in these applications, is
highly debated. Gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply
a marketing ploy. However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive
atmospheres, and in use for contacts with a very high failure cost (certain computers,
communications equipment, spacecraft, jet aircraft
engines) remains very common, and is unlikely to be replaced in the near future by any other metal.
- Besides sliding electrical contacts, gold is also used in electrical contacts because of its
resistance to corrosion, electrical
conductivity, ductility and lack of toxicity.[3] Switch contacts are generally subjected to more intense
corrosion stress than are sliding contacts.
- Colloidal gold (Colloidal sols of gold nanoparticles) in water are intensely red-colored, and can be made
with tightly-controlled particle sizes up to a few tens of nm across by reduction of gold chloride with citrate or ascorbate ions. Colloidal gold is used in research applications in
medicine, biology and materials science. The technique of immunogold labeling exploits the ability of the gold particles to
adsorb protein molecules onto their surfaces. Colloidal gold particles coated with specific antibodies can be used as probes for
the presence and position of antigens on the surfaces of cells (Faulk and Taylor 1979). In ultrathin sections of tissues viewed
by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the antigen (Roth et al. 1980). Colloidal gold is also the form of gold used as gold paint on ceramics prior to firing.
- Gold, or alloys of gold and palladium, are applied as conductive coating to biological
specimens and other non-conducting materials such as plastics and glass to be viewed in a scanning electron microscope. The coating, which is usually applied by sputtering with an argon plasma, has
a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earth, and its very high
density provides stopping power for electrons in the SEM's electron beam, helping to limit
the depth to which the electron beam penetrates the specimen. This improves definition of the position and topography of the
specimen surface and increases the spatial resolution of the image. Gold also produces a high output of secondary electrons when irradiated by an electron beam, and these low-energy electrons are the most
commonly-used signal source used in the scanning electron microscope.
- Many competitions, and honors, such as the Olympics and the Nobel Prize, award a gold medal to the winner.
- As gold is a good reflector of both infrared and visible light, it is used for the protective coatings on many artificial
satellites and in infrared protective faceplates in thermal protection suits and astronauts'
helmets.
- Gold is used as the reflective layer on some high-end CDs.
- The isotope gold-198, (half-life: 2.7 days) is used in some cancer treatments and for treating other diseases.[4]
- Automobiles may use gold for heat insulation. McLaren F1 uses gold foil in the engine
compartment.[5]
As gold chemical compounds
Gold is attacked by and dissolves in alkaline solutions of potassium or sodium cyanide, and
gold cyanide is the electrolyte used in commercial electroplating of gold onto base metals and electroforming. Gold
chloride (chloroauric acid) solutions are used to make colloidal gold by reduction with citrate
or ascorbate ions. Gold chloride and gold oxide are used to make
highly-valued cranberry or red-colored glass, which, like colloidal gold sols, contains
evenly-sized spherical gold nanoparticles.
History
Gold has been known and highly-valued since prehistoric times. It may have been the first
metal used by humans and was valued for ornamentation and rituals. Egyptian hieroglyphs from as early as 2600 BC describe gold, which king Tushratta of the Mitanni claimed was "more plentiful than dirt" in
Egypt.[6] Egypt and Nubia
had the resources to make them major gold-producing areas for much of history. Gold is also mentioned several times in the
Old Testament, and is included with the gifts of the magi in
the first chapters of Matthew New Testament The south-east corner of the Black Sea was famed for its gold. Exploitation is said to date from the time of Midas, and this gold was important in the establishment of what is probably the world's earliest coinage in
Lydia between 643 and 630 BC.
The Mali Empire in Africa was famed throughout the old world for its large amounts of
gold. Mansa Musa, ruler of the empire (1312–1337) became famous throughout the old world for
his great hajj to Mecca in 1324. When he passed through
Cairo in July of 1324, he was reportedly accompanied by a camel
train that included thousands of people and nearly a hundred camels. He gave away so much gold that it took over a decade
for the economy across North Africa to recover, due to the rapid inflation that it initiated.[7] A contemporary Arab historian remarked;
| “ |
Gold was at a high price in Egypt until they came in that year. The mithqal did not
go below 25 dirhams and was generally above, but from that time its value fell and it cheapened in price and has remained cheap
till now. The mithqal does not exceed 22 dirhams or less. This has been the state of affairs for about twelve years until this
day by reason of the large amount of gold which they brought into Egypt and spent there [...] |
” |
|
—Chihab Al-Umari[8]
|
The European exploration of the Americas was fueled in no small part by reports of the gold ornaments displayed in great
profusion by Native American peoples, especially in Central America, Peru, and Colombia.
Although the price of some platinum group metals can be much higher, gold has long been considered the most desirable of
precious metals, and its value has been used as the standard for many currencies (known as the gold standard) in history. Gold has been used
as a symbol for purity, value, royalty, and particularly roles that combine these properties. Gold as a sign of wealth and
prestige was made fun of by Thomas More in his treatise Utopia. On that imaginary island, gold is so abundant that it is used to make chains for slaves,
tableware and lavatory-seats. When ambassadors from other countries arrive, dressed in ostentatious gold jewels and badges, the
Utopians mistake them for menial servants, paying homage instead to the most modestly-dressed of their party.
There is an age-old tradition of biting gold in order to test its authenticity. Although this is certainly not a professional
way of examining gold, the bite test should score the gold because gold is considered a soft metal according to the
Mohs' scale of mineral hardness. The purer the gold the easier it should
be to mark it. Painted lead can cheat this test because lead is softer than gold (and may invite a small risk of lead poisoning if sufficient lead is absorbed by the biting).
This 156 ounce nugget was found by an individual prospector in the Southern California Desert using a metal detector.
Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever
produced has been extracted since 1910.[9] It has been estimated that all the gold in the world that has ever been refined would
form a single cube 20 m (66 ft) on a side (equivalent to 8000 m³).[9]
One main goal of the alchemists was to produce gold from other substances, such as
lead — presumably by the interaction with a mythical substance called the philosopher's stone. Although they never succeeded in this attempt, the alchemists promoted an
interest in what can be done with substances, and this laid a foundation for tod
