Gold

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Gold,  79Au
General properties
Name, symbol Gold, Au
Appearance Metallic dark yellow
Gold in the periodic table
Ag

Au

Rg
PlatinumGoldMercury
Atomic number 79
Standard atomic weight (Ar) 196.966569(5)
Group, block 11; d-block
Period period 6
Electron configuration [Xe] 4f14 5d10 6s1
per shell
2, 8, 18, 32, 18, 1
Physical properties
Metallic yellow
Phase Solid
Melting point 1337.33 K ​(1064.18 °C, ​1947.52 °F)
Boiling point 3243 K ​(2970 °C, ​5378 °F)
Density near r.t. 19.30 g/cm3
when liquid, at  17.31 g/cm3
Heat of fusion 12.55 kJ/mol
Heat of 342 kJ/mol
Molar heat capacity 25.418 J/(mol·K)
 pressure
Atomic properties
Oxidation states 5, 3, 2, 1, −1, −2, −3 ​(an amphoteric oxide)
Electronegativity Pauling scale: 2.54
energies 1st: 890.1 kJ/mol
2nd: 1980 kJ/mol
Atomic radius empirical: 144 pm
Covalent radius 136±6 pm
Van der Waals radius 166 pm
Miscellanea
Crystal structure ​​Face-centered cubic (fcc)
Speed of sound thin rod 2030 m/s (at )
Thermal expansion 14.2 µm/(m·K) (at 25 °C)
Thermal conductivity 318 W/(m·K)
Electrical resistivity 22.14 Ω·m (at 20 °C)
Magnetic ordering Diamagnetic
Tensile strength 120 MPa
Young's modulus 79 GPa
Shear modulus 27 GPa
Bulk modulus 180 GPa
Poisson ratio 0.4
Mohs hardness 2.5
Vickers hardness 188–216 MPa
Brinell hardness 188–245 MPa
CAS Registry Number 7440-57-5
History
Naming from Latin aurum (gold)
Discovery ~6000 BCE (Middle East)
· references

Gold is a chemical element with the symbol Au and atomic number 79. It is a transitional metal, part of Group 11, the same group as silver and copper. It's well known for its corrosion resistance and its high economic value. Gold is mainly used in jewels, electronics and as exchange.

Properties

Chemical

Gold is very resistant to acid and alkali attacks and does not react with oxgen or halogens at standard conditions. However a mixture of hydrochloric acid and nitric acid known as aqua regia will dissolve gold.

Au + HNO3 + 4 HCl → HAuCl4 + NO + 2 H2O

Gold can also be dissolved by cyanides, such as sodium cyanide, a process used in gold extraction, when the gold concentration is low. Mercury dissolves gold forming an amalgam.

Physical

Gold is a bright yellow dense, soft, malleable and ductile metal. Very pure gold (24 carat) is soft enough to be dent by biting it, a practice occasionally seen in gold diggers and Olympic athletes, who traditionally bit their gold medals. Gold is the most malleable of all metals, one gram can be beaten into a sheet of 1 square meter. It has high thermal and electric conductivity, properties that gives it many uses in electronics. Its density of 19.3 g/cm3 is slightly higher than that of tungsten and uranium.

Availability

Gold can be found in nature as nuggets, either pure or mixed with silver or platinum group metals. During the Gold Rush, very large nuggets were dug up from the rivers. Nowadays, nuggets tend to be rarer, instead grain sized gold is more often found, as previous extraction methods focused on large nuggets. Extracting gold from gold-rich soil/sand is very intensive and may not be 100% legal depending on where you live.

Gold can be extracted from jewelry, but doing so often destroys jewelry that would cost more than the gold it is made of. Gold bullions and coins are also a source of gold, albeit an expensive one.

Gold leaf, used in food decorations are also a source of gold, albeit the quantity is small and it's usually a gold alloy.

However the most sought source of gold are scrap electronics. Extracting gold from old electronics such as finger and socket contacts, pins, CPUs, RAM chips, board plating, adjustable switches, etc. is one of the most known aspects of amateur chemistry. Usually the older the electronic device is, the more gold it has. Extracting the gold is done by various methods: dissolving the copper circuit with a PCB etchant, such as ferric chloride and collecting the gold foil by filtering the solution, which is later purified by dissolving it in aqua regia and melted; dissolving the boards in cyanide solution, reducing the gold cyanide compound and melting the powder; dissolving the gold with mercury and extracting the gold; electrochemical separation. The amount of gold obtained is low, but it's a cheap source.

Preparation

Gold can be reduced from its salts by reducing it with a reducing compound. Since gold sits close to the bottom of the reactivity scale, any common metal will reduce it to its elemental form. In case of chloroauric acid, it is usually reduced with sodium metabisulfite.

Projects

  • Gold plating
  • Gold colloids
  • Gold electrode
  • Catalyst

Handling

Safety

Pure gold is non-toxic and it's even used in medical implants. However, its compounds, especially the salts are toxic and they should be handled with proper protection.

Storage

No special storage is required for bulk and powdered gold.

Disposal

Due to gold's price and rarity, it's best to try and recycle as much gold as possible.

References

Relevant Sciencemadness threads