Gold
It has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements solid under standard conditions. The metal therefore occurs often in free elemental (native) form, as nuggets or grains in rocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, usually with tellurium.
Gold resists attacks by individual acids, but it can be dissolved by the aqua regia (nitro-hydrochloric acid), so named because it dissolves gold. Gold also dissolves in alkaline solutions of cyanide, which have been used in mining. It dissolves in mercury, forming amalgam alloys; is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items, giving rise to the term the acid test.
This metal has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have sometimes been a monetary policies, but were widely supplanted by fiat currency starting in the 1930s. The last gold certificate and gold coin currencies were issued in the U.S. in 1932. In Europe, most countries left the gold standard with the start of World War I in 1914 and, with huge war debts, failed to return to gold as a medium of exchange.
A total of 171,300 tonnes of gold have been mined in human history, according to GFMS as of 2011. This is roughly equivalent to 5.5 billion troy ounces or, in terms of volume, about 8876 m3, or a cube 20.7 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry.
Besides its widespread monetary and symbolic functions, gold has many practical uses in dentistry, electronics, and other fields. Its high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity led to many uses of gold, including electric wiring, colored-glass production and gold leafing.
Most of the Earth's gold lies at its core, the metal's high density having made it sink there in the planet's youth. Virtually all of the gold that mankind has discovered is considered to have been deposited later by meteorites which contained the element, with the asteroid that formed Vredefort crater being implicated in the formation of the largest gold mining region on earth – Witwatersrand basin.
Occurrence
Gold's atomic number of 79 makes it one of the higher atomic number elements which occur naturally. Like all elements with atomic numbers larger than iron, gold is thought to have been formed from a supernova nucleosynthesis process. Their explosions scattered metal-containing dusts (including heavy elements like gold) into the region of space in which they later condensed into our solar system and the Earth. Because the Earth was molten when it was just formed, almost all of the gold present on Earth sank into the core. Most of the gold that is present today in the Earth's crust and mantle was delivered to Earth by asteroid impacts during the late heavy bombardment.
On Earth, gold is found in ores in rock formed from the Precambrian time onward. It most often occurs as a native metal, typically in a metal solid solution with silver (i.e. as a gold silver alloy). Such alloys usually have a silver content of 8–10%. Electrum is elemental gold with more than 20% silver. Electrum's color runs from golden-silvery to silvery, dependent upon the silver content. The more silver, the lower the specific gravity.
Native gold occurs as very small to microscopic particles embedded in rock, often together with quartz or sulfide minerals such as "Fool's Gold", which is a pyrite. These are called lode deposits. The metal in a native state is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits called placer deposits. Such free gold is always richer at the surface of gold-bearing veins[clarification needed] owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets.
Gold sometimes occurs combined with tellurium as the minerals calaverite, krennerite, nagyagite, petzite and sylvanite, and as the rare bismuthide maldonite (Au2Bi) and antimonide aurostibite (AuSb2). Gold also occurs in rare alloys with copper, lead, and mercury: the minerals auricupride (Cu3Au), novodneprite (AuPb3) and weishanite ((Au, Ag)3Hg2).
Recent research suggests that microbes can sometimes play an important role in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.
Gold in seawater
The world's oceans contain gold. Measured concentrations of gold in the Atlantic and Northeast Pacific are 50–150 fmol/L or 10–30 parts per quadrillion (about 10–30 g/km3). In general, gold concentrations for Atlantic and Pacific samples are the same (~50 fmol/L) but less certain. Mediterranean deep waters contain higher concentrations of gold (100–150 fmol/L) attributed to wind-blown dust and/or rivers. At 10 parts per quadrillion the Earth's oceans would hold 15,000 tonnes of gold. These figures are three orders of magnitude less than reported in the literature prior to 1988, indicating contamination problems with the earlier data.
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 acted in an intentional deception. Prescott Jernegan ran a gold-from-seawater swindle in the United States in the 1890s. A British fraudster ran the same scam in England in the early 1900s. Fritz Haber (the German inventor of the Haber process) did research on the extraction of gold from sea water in an effort to help pay Germany's reparations following World War I. Based on the published values of 2 to 64 ppb of gold in seawater a commercially successful extraction seemed possible. After analysis of 4,000 water samples yielding an average of 0.004 ppb it became clear that the extraction would not be possible and he stopped the project. 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.
| Symbol | Au | |
| Atomic Number | 79 | |
| Atomic Weight | 196.96654 | |
| Oxidation States | +1,+3 | |
| Electronegativity, Pauling | 2 | |
| State at RT | solid | |
| Melting Point, K | 1337.58 | |
| Boiling Point, K | 3080 |
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.
- 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
Harmful effects:
Gold is considered to be non-toxic.
Characteristics:
- 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.
