Gold

Gold,  ₇₉Au

Small sample of gold foil
General properties
Name, symbol	Gold, Au
Alternative name	Aurum
Appearance	Metallic dark yellow
Gold in the periodic table
Ag ↑ Au ↓ Rg Platinum ← Gold → Mercury
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)

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, catalyst and as exchange.

The symbol Au comes from the latin name of gold, aurum, and derivatives of this term are used in many countries as designation for gold, most often in Romance-speaking countries.

Properties

Chemical

Gold is very resistant to acid and alkali attacks and does not react with oxygen 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.

Gold resists the attack of molten sodium hydroxide, however, at temperatures above 700 °C, there is visible corrosion of the metal, and traces of gold flakes and gold oxide can be observed in the alkali melt. Small amounts of metallic sodium have also been observed, which rapidly form an alloy with the gold, which is stable enough that it doesn't readily react with water or acids.

Gold is unaffected by concentrated (40%) hydrofluoric acid at standard conditions.

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 (see Prospectors). 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.

Gold itself is usually found uncombined in nature, but when found as a chemical compound, it is most often combined with tellurium, in the form of calaverite and krennerite (two different polymorphs of AuTe₂), petzite (Ag₃AuTe₂) and sylvanite (AgAuTe₄).

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, sodium or potassium metabisulfite are commonly used as reducing agents, as they're cheap and readily available.

Projects

• Gold plating
• Make gold colloids
• Gold electrode in water electrolysis
• Catalyst
• Make jewelry
• Element collection

Handling

Safety

Pure gold is non-toxic and it's even used in medical implants. On the other hand, most gold 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. Though given the value of gold, it's best to keep it in a hidden place or a safe.

Disposal

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

Gallery

• 

  A cupel with a gold bead, courtesy by Cody.

References

Relevant Sciencemadness threads

• chemistry of GOLD, known reactions and techniques
• Gold Recovery Computer Scrap Complete Process