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CHEMICAL-PHYSICAL PROPERTIES
- Atomic number: 79
- Atomic weight: 196.967
- Electronic configuration: [Xe] 4f14 5d10 6s1
- Atomic radius (nm) 0.134
- Electrode standard potentials or Eo(Au/Au+, V); +1.68
- " " (Au+/Au+++); +1.40
- Melting point (C); 1064.4
- Boiling point (C); 2970
- Density (g/cc); 19.32
- Oxidation Numbers; +1, +3

PRESENCE IN NATURE AND PRODUCTION
Gold (Au) is known by humans since about 4000 B.C, and always considered the KING of metals, as we well know; for it too many people would make the worst things and I don't add anything else...

It can be found nearly always in the metallic elementary state among various minerals or in the alluvial deposits of rivers in the form of little scales.
Ores containing Au in compounds are very rare and we can report mainly the AuTe2, (GOLD TELLURIDE).
In fact, its electrode potentials, so positive, show that it has very little tendency to give its electrons (e-).
Its presence in the lithosphere is about 0.0000005% and it's extracted form ores even when they contain it only for 0.00001%.
The minerals containing Au are enriched and concentrated by flotation in water by means of particular foams that can separate only the materials containing gold.
Then, the metal is extracted in solution of CN- (cyanide ions) that form very stable complexes with Au, lowering much its oxidation potential to -0.67 V, so that it can easily oxidized by AIR bubbling in the solution.

This process is made in big pools of cyanide solutions and these industrial plants need extreme safety protections, to avoid dispersions of cyanide solution in the working places or in the environment.

So, the soluble complex [Au(CN)2]- is formed.
Adding metallic ZINC (Zn), this reduces Au to the metallic state

2 [Au(CN)2]- + Zn ->
and this is washed with H2SO4 to eliminate the Zn traces and finally purified with electrolytic processes.

CHEMICAL PROPERTIES
Gold is a tender metal, if pure, but extremely resistant to the chemical attack and this is one of the features that make it so precious.
Only the mix HCl/HNO3 can dissolve it, not the two acids separately, because Au is stable in solution only when it forms complexes in solution, like Au(Cl)4-, containing Au+++ and HNO3 can perform the oxidation only if this complex is formed with a lower
Eo = 0.96 V:

Au + 4 Cl- + 3 NO3- -> [AuCl4-] + 9 H2O + 3 NO2

In solution, all salts of Au+++ give hydrolysis, reacting with water to form the insoluble HYDROXIDE:

Au+++ + 6H2O -> Au(OH)3 + 3 H3O+

USES

Of course, Au is used in jewelry and coins production at 18 carats (75%) in many alloys with COPPER, SILVER, NICKEL,... that increase its hardness.
Then is very used in dentures, to cover objects with a thin Au layer by means of GALVANIC processes and in the electronic, space, and chemical industry, because it's an excellent conductor and gives a great resistance to corrosion.
It can be produced also in very thin films (few microns); in these conditions, Au appear green and it's used for ex., among glass layers for cars and buildings thanks to its property to stop the thermal IR radiations of the Sun.

Learn more about this author, Aldo Bonincontro.
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