|Classification:||Gold is a transition metal|
|Melting point:||1064.18 oC, 1337.33 K|
|Boiling point:||2850 oC, 3123 K|
|Neutrons in most abundant isotope:||118|
|Electron configuration:||[Xe] 4f14 5d10 6s1|
|Density @ 20oC:||19.32 g/cm3|
|Atomic volume:||10.2 cm3/mol|
|Structure:||fcc: face-centered cubic|
|Specific heat capacity||0.128 J g-1 K-1|
|Heat of fusion||12.550 kJ mol-1|
|Heat of atomization||368 kJ mol-1|
|Heat of vaporization||334.40 kJ mol-1|
|1st ionization energy||890.1 kJ mol-1|
|2nd ionization energy||1980 kJ mol-1|
|3rd ionization energy||–|
|Electron affinity||222.752 kJ mol-1|
|Minimum oxidation number||-1|
|Min. common oxidation no.||0|
|Maximum oxidation number||5|
|Max. common oxidation no.||3|
|Electronegativity (Pauling Scale)||2.54|
|Polarizability volume||6.1 Å3|
|Reaction with air||none|
|Reaction with 15 M HNO3||none|
|Reaction with 6 M HCl||none|
|Reaction with 6 M NaOH||none|
|Atomic radius||135 pm|
|Ionic radius (1+ ion)||151 pm|
|Ionic radius (2+ ion)||–|
|Ionic radius (3+ ion)||99 pm|
|Ionic radius (1- ion)||–|
|Ionic radius (2- ion)||–|
|Ionic radius (3- ion)||–|
|Thermal conductivity||318 W m-1 K-1|
|Electrical conductivity||48.8 x 106 S m-1|
|Freezing/Melting point:||1064.18 oC, 1337.33 K|
Discovery of Gold
Humans have known gold and treasured it since prehistoric times.
Who discovered gold? We do not know, it was discovered before there were written records. We know there is a good chance the discoverer found it in a river bed.
Gold is usually associated with rocks such as quartz and pyrites. As these rocks undergo weathering or erosion by water, the gold can be washed into rivers allowing people to find it easily.
Currently we can trace our use of gold back to at least 6200 years ago. A variety of objects made of gold have been found in Bulgaria from 4500-4000 BC. (1) (shown left).
Gold artifacts dated to 5000 years ago have been found in Egyptian tombs; gold was already being beaten into sheets, foil and wire in Egypt at this time. (2) The Egyptian word for gold was ‘nub’ and is related to Nubia, the land south of Egypt where much of Egypt’s gold was obtained.
Gold in the native state (i.e. found naturally) is usually mixed with other metals, such as silver. Its purity can be increased by depletion guilding or refining – a significant step forward in technology. (3)
Gold of 98% purity has been found in Nahal Qunah in the ancient kingdom of Israel, dating from about 6000 years ago. (4), (5)
Analyses of gold from ancient Egypt indicates refining began there about 2500 years ago. (6)
The legendary wealth of King Croesus of Lydia (now in modern Turkey) came from refining gold found in several local rivers. (7), (8)
Gold became the basis of money in many ancient civilizations, and even today most countries maintain large reserves of gold for financial credibility. Most modern currencies, however, are not tied to gold as they were in the days of the Gold Standard, described by economist John Maynard Keynes as “a barbarous relic.” (9)
In ancient times people viewed gold as the perfect substance. Alchemists began a relentless but fruitless pursuit spanning many centuries, trying to discover how to transform other metals into gold.
Although alchemy failed in its goal, the experimental techniques alchemists developed were important to the early chemists.
Constant failure to produce gold simply meant substances were not yet pure enough, or the perfect proportions had not yet been found.
Alchemist’s concepts of mercury and sulfur were different from ours; while they did include the substances we call mercury and sulfur, they also encompassed other metals and substances.
The word ‘gold’ is an Anglo-Saxon word, similar to the Anglo-Saxon word for yellow, ‘geolo.’ It is believed to have come from the Sanskrit ‘jval’ meaning ‘to shine.’ (10)
The chemical symbol Au comes from ‘aurum’ the Latin word for gold. (Aurora was goddess of dawn or the morning glow.) (11)
Interesting Facts about Gold
- Gold is the most ductile of all metals, meaning it is the most suitable for drawing into wires. Amazingly, one ounce of gold can be drawn into a wire 1250 miles long (thickness 1 micron). This means that you could make a gold wire long enough to go around the earth with just 20 ounces of gold. Using metric units, one gram of gold could be drawn to a length of 66 km.
- Gold is also the most malleable of all metals, meaning it can be beaten into thinner sheets than any other metal. Gold can be beaten without any special difficulty to a thickness of 0.1 micron. A stack of one thousand sheets of 0.1 micron gold leaf is the same thickness as a typical piece of printer paper.
- Gold is one of the densest elements. A cube of gold with each side just 14.2 inches long would weigh a ton. (37.27 cm sides give a metric ton.) The six metals denser than gold are Os, Ir, Pt, Re, Np and Pu.
- The concentration of gold below Earth’s crust is about 100 times higher than the concentration in it.
- ‘The acid test’ has become part of everyday speech. It means a test whose result is absolutely certain. The first acid test was a drop of nitric acid on metal. Gold does not dissolve in nitric acid, so if a metal reacts with nitric acid, it is certainly not gold. The ‘acid test’ became popular in the 1849 Californian gold rush, when all sorts of shady characters tried their hand at selling fake gold.
- The total mass of gold ever extracted from Earth is 170 000 metric tons (at the beginning of 2012). This amount of gold would fill three and a half Olympic swimming pools. About 2500 metric tons of gold is now mined every year. Two-thirds of all the gold ever taken from the earth has been taken since 1950. (12) (13)
- Want to get rich quick? Nobel prize winning chemist Fritz Haber did, but not for personal gain; Haber tried to help the German economy by extracting gold from seawater, but could not do it profitably. As recently as the 1980s the oceans were thought to contain about 4 kg of gold for every cubic kilometer of water – that’s almost 1 kg or 2 lb of gold each for everyone on Earth today. Estimates of ocean gold keep falling, however, and it now seems likely that each cubic kilometer of ocean contains just 30 grams of gold (Nozaki, 1992). That’s 1 ounce of gold in every 264 billion gallons of water. If you could process seawater equal in volume to draining Lake Ontario, you would capture 48 kg of gold, worth less than $3 million at today’s gold price. You cannot put that amount of water – 1600 cubic kilometers – through any chemical extraction process for $3 million. But perhaps you have an idea?
- In the world-changing Gold Foil Experiment, Ernest Rutherford and his coworkers Hans Geiger and Ernest Marsden fired alpha particles at gold leaf. The experiment found that alpha particles were deflected as they passed through the gold more than they ought to have been if the gold atoms were made of smoothly spread matter. The gold leaf used in the experiment had been pressed to about 0.6 micrometers thick – that’s a thickness of about 2000 gold atoms. By 1911 Rutherford had concluded that atoms consist of a tiny, dense point of positive charge surrounded mostly by empty space in which negatively charged electrons are present.
- In the fourth century BC the Greek mathematician and philosopher Plato wrote: “all the gold which is under or upon the earth is not enough to give in exchange for virtue.”
Appearance and Characteristics
Gold is considered to be non-toxic.
Gold is an excellent conductor of heat and electricity.
It is a soft, yellow, metal with a beautiful lustrous sheen. It is the most malleable and ductile of all the elements and a single gram can be beaten into a sheet one square meter sheet of gold leaf.
Gold has a very high density, 19.32 g/cm3. (A tennis ball sized sphere of gold would weigh about 5.7 pounds (2.6 kilograms). Gold is not the densest natural element, however. That honor belongs to osmium, followed very closely by iridium. Tennis ball sized spheres of these elements would each weigh about 6.8 pounds (3.1 kilograms).
Gold is unaffected by air, water, alkalis and all acids except aqua regia (a mixture of hydrochloric acid and nitric acid) which can dissolve gold. Gold does react with halogens. It will, for example, react very slowly with chlorine gas at room temperature to form gold chloride, AuCl3. If gold chloride is heated gently, it will decompose to release the pure elements again.
Unusually for a metal, gold can also form compounds (aurides) in which its oxidation number is negative (-1). For example, gold can combine with cesium to form cesium auride, CsAu, and rubidium to form rubidium auride, RbAu. These are ionic compounds with non-metallic properties in which the Cs or Rb ions are charged +1 while the Au atoms are charged 1-.
Uses of Gold
Gold is widely used in jewelry and coinage. It is also used in dental work as crowns, as gold plating for decoration and as gold thread in embroidery work. The gold content in alloys is usually measured in carats (k), with pure gold defined as 24k.
Many satellites carry gold-coated mylar sheets as a solar heat shield because gold is an excellent reflector of radiation and unreactive. Similarly astronaut’s helmet visors are coated with a thin layer of gold to guard against dangerous effects of solar radiation.
Gold is used widely in microelectronic circuits to ensure reliable, corrosion-resistant and static-free performance.
The isotope 198Au, with a half-life of 2.7 days, is used for treating cancers – especially of the bladder, cervix, and prostate.
Gold flake is added to some gourmet sweets and drinks.
Chloroauric acid (HAuCl4) is used in photography for toning the silver image.
Abundance and Isotopes
Abundance earth’s crust: 4 parts per billion by weight, 0.4 parts per billion by moles
Abundance solar system: 1 part per billion by weight, 10 parts per trillion by moles
Cost, pure: $5540 per 100g
Cost, bulk: $3800 per 100g
Source: Gold is found underground and in rivers. The river deposits arise when gold that was rock-bound is released by erosion of the surrounding rock by running water.
Gold is usually found as a metal alloyed to some degree with silver or sometimes with mercury as an amalgam. Gold nuggets found on Earth range from sizeable nuggets through tiny grains in alluvial (river) deposits to microscopic pieces in rocks.
Commercially, gold is purified by cyaniding, amalgamating with mercury, or smelting processes. Further refining, which produces nearly pure gold, is usually by electrolysis.
Isotopes: Gold has 35 isotopes whose half-lives are known, with mass numbers 171 to 205. Naturally occurring gold consists of its one stable isotope, 197Au.
1. Varna Museum of Archeology – The Middle Eneolithic Age
2. T.G.H. James, The British Museum, Gold Technology in Ancient Egypt: Mastery of Metal Working Methods., 1972, Gold Bulletin V, p42.
3. S. La Niece, Depletion Guilding from Third Millennium BC UR., Iraq, 1995, Vol. 57, p41-47.
5. Encyclopedia of Prehistory: South and Southwest Asia, Volume 8 By Peter Neal Peregrine, Melvin Ember, Human Relations Area Files, inc
6. A. Lucas, Ancient Egyptian Materials and Industries., 1948, p263, St Ann’s Press.
7. John N. Wilford, Ancient King’s Legendary Gold., 2000.
8. Cindy L. Nimchuk, University of Toronto, Bryn Mawr Classical Review., 2001.
9. Personal Wealth, Reparations, Probability and the Gold Standard, John Maynard Keynes – 1919 to 1926.
10. Eric. J. Holmyard, Makers of Chemistry., 1931, Oxford at the Clarendon Press. p163.
11. Vivi Ringnes, Origin of the Names of Chemical Elements, J. Chem. Educ., 1989, 66 (9), p731.
12. USGS Minerals 2012 (pdf download)
13. Gold.org Facts
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