Gold [Au]
Characteristics
An: 79 N: 118
Am: 196.96655 g/mol
Group No: 11
Group Name: Coinage metal
Block: d-block Period: 6
State: solid at 298 K
Colour: gold (!) Classification: Metallic
Boiling Point: 3129K (2856oC)
Melting Point: 1337.33K (1064.18oC)
Density: 19.3g/cm3
Discovery Information
Who: Known to the ancients. 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. Eqyptian hieroglyphs from as early as 2600BC mention gold. 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.
Name Origin
Gold from old English word geolo (yellow); Au from Latin: aurum (gold). “Gold” in different languages.
Sources
Found in veins in the crust, with copper ore and natively. Major producers include South Africa, Canada, the United States and Western Australia.
Around 1400 tons are produced each year.
Abundance
Universe: 0.0006 ppm (by weight)
Sun: 0.001 ppm (by weight)
Carbonaceous meteorite: 0.17 ppm
Earth’s Crust: 0.011 ppm
Seawater: 5 x 10-5 ppm
Human: 100 ppb by weight; 3 ppb by atoms
Uses
Pure gold is too soft for ordinary use and is hardened by alloying with silver, copper, and other metals. These alloys are mostly used in jewellery and coinage.
White gold (an alloy of gold with platinum, palladium, nickel, and/or zinc) serves as a substitute for solid platinum.
Gold is used in restorative dentistry especially in tooth restorations such as crowns and permanent bridges as its slight maliablity makes a superior molar mating surface to other teeth, unlike a harder ceramic crown.
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. 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 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.
During the 19th century, gold rushes occurred whenever large gold deposits were discovered. The first major gold strike in the United States occurred in a small north Georgia town called Dahlonega. Further gold rushes occurred in California, Colorado, Otago, Australia, Witwatersrand, Black Hills, and Klondike.
Because of its historically high value, much of the gold mined throughout history is still in circulation in one form or another.
Notes
On the 13th of March 2008 the price of gold reached $1000 per troy ounce (31.1035g) for the first time in history. This works out at $32150 per kilogram!
It is the most malleable and ductile metal known; a single gram can be beaten into a sheet of one square meter, or an ounce into 300 square feet.
Supposedly around half of the world’s supply of gold is stored in the United States Treasury Department’s gold depository in Fort Knox Kentucky, which is considered to be one of the most secure buildings in the world.
Because gold is traded like currencies, it has it’s own ISO currency code, XAU (USD = US dollars, GBP = GB Pounds sterling etc.).
Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910. 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 (8000 m3).
At the end of 2001, it was estimated that all the gold ever mined totalled only 145,000 tonnes.
Gold Compounds
Auranofin C20H35AuO9PS+
An organogold compound classified by the World Health Organization as an antirheumatic agent.
Aurothioglucose AuSC6H11O5
A derivative of the sugar glucose, it is used to treat rheumatoid arthritis.
Sodium aurothiomalate C4H3AuNaO4S
an organogold compound used for its antirheumatic effects to treat rheumatoid arthritis. In the United Kingdom only this intramuscular injection drug and the orally taken Auranofin are used medically.
Reactions of Gold
Reactions with water
Gold does not react with water, under any circumstances.
Reactions with air
Under normal conditions gold will not react with air.
Reactions with halogens
Gold reacts with chlorine and bromine to form trihalides and with iodine to form a monohalide.
2Au(s) + 3Cl2(g) –> 2AuCl3(s)
2Au(s) + 3Br2(g) –> 2AuBr3(s)
2Au(s) + I2(g) –> 2AuI(s)
Reactions with acids
Gold can be dissolved with a mixture of hydrochloric acid (HCl) and concentrated nitric acid (HNO3), in a ration of 3:1. This combination is known as aqua regia, “royal water“.
Reactions with bases
Gold will not react with aqueous bases.
Occurrence of Gold
Economic gold extraction can be achieved from ore grades as little as 0.5g/1000kg (0.5 parts per million, ppm) on average in large easily mined deposits. Typical ore grades in open-pit mines are 1-5 g/1000 kg (1-5 ppm), ore grades in underground or hard rock mines are usually at least 3 g/1000 kg (3 ppm) on average. Since ore grades of 30g/1000kg (30 ppm) are usually needed before gold is visible to the naked eye, in most gold mines the gold is invisible.
Since the 1880s, South Africa has been the source for a large proportion of the world’s gold supply. Production in 1970 accounted for 79% of the world supply, producing about 1,000 tonnes. However, production in 2005 was just 294 tonnes according to the British Geological Survey. This sharp decline was due to the increasing difficulty of extraction and changing economic factors affecting the industry in South Africa.
The city of Johannesburg was built atop the world’s greatest gold finds. Gold fields in the Free State and Gauteng provinces are deep and require the world’s deepest mines. The Second Boer War of 1899-1901 between the British Empire and the Afrikaner Boers was at least partly over the rights of miners and possession of the gold wealth in South Africa.
Other major producers are United States, Australia, China and Peru. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States. In South America, the controversial project Pascua Lama aims at exploitation of rich fields in the high mountains of Atacama Desert, at the border between Chile and Argentina. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining.
After initial production, gold is often subsequently refined industrially by the Wohlwill process or the Miller process. Other methods of assaying and purifying smaller amounts of gold include parting and inquartation as well as cuppelation, or refining methods based on the dissolution of gold in aqua regia.
The world’s oceans hold a vast amount of gold, but in very low concentrations (perhaps 1-2 parts per billion). A number of people have claimed to be able to economically recover gold from sea water, but so far they have all been either mistaken or crooks. Reverend Prescott Jernegan ran a gold-from seawater swindle in America in the 1890s. A British fraud ran the same scam in England in the early 1900s.
Fritz Haber (the German inventor of the Haber process) attempted commercial extraction of gold from sea water in an effort to help pay Germany’s reparations following the First World War. Unfortunately, his assessment of the concentration of gold in sea water was unduly high, probably due to sample contamination. The effort produced little gold and cost the German government far more than the commercial value of the gold recovered. No commercially viable mechanism for performing gold extraction from sea water has yet been identified. Gold synthesis is not economically viable and is unlikely to become so in the foreseeable future.
The average gold mining and extraction costs are $238 per troy ounce but these can vary widely depending on mining type and ore quality. In 2001, global mine production amounted to 2,604 tonnes, or 67% of total gold demand in that year. At the end of 2001, it was estimated that all the gold ever mined totalled 145,000 tonnes.
Isotopes of Gold
195Au [116 neutrons]
Abundance: synthetic
Half life: 186.10 days [ Electron Capture ]
Decay Energy: 0.227 MeV
Decays to 195Pt.
196Au [117 neutrons]
Abundance: synthetic
Half life: 6.183 days [ Electron Capture ]
Decay Energy: 1.506 MeV
Decays to 196Pt.
Half life: 6.183 days [ beta- ]
Decay Energy: 0.686 MeV
Decays to 196Hg.
197Au [118 neutrons]
Abundance: 100%
Stable with 118 neutrons
198Au [119 neutrons] Abundance: synthetic
Half life: 2.69517 [ beta- ]
Decay Energy: 1.372 MeV
Decays to 198Hg.
199Au [120 neutrons] Abundance: synthetic
Half life: 3.169 days [ beta- ]
Decay Energy: 0.453 MeV
Decays to 199Hg.