Difference between revisions of "Gold"
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{{Infobox element | {{Infobox element | ||
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==Properties== | ==Properties== | ||
===Chemical=== | ===Chemical=== | ||
| − | Gold is very resistant to acid and alkali attacks and does not react with | + | 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 + HNO<sub>3</sub> + 4 HCl → HAuCl<sub>4</sub> + NO + 2 H<sub>2</sub>O | :Au + HNO<sub>3</sub> + 4 HCl → HAuCl<sub>4</sub> + NO + 2 H<sub>2</sub>O | ||
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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.<ref>http://pubs.acs.org/doi/pdf/10.1021/ja01601a004</ref> | 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.<ref>http://pubs.acs.org/doi/pdf/10.1021/ja01601a004</ref> | ||
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| + | Gold is unaffected by concentrated (40%) [[hydrofluoric acid]] at standard conditions.<ref>https://www.youtube.com/watch?v=Ri8heWPz5zY</ref> | ||
===Physical=== | ===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/cm<sup>3</sup> is slightly higher than that of [[tungsten]] and [[uranium]]. | 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/cm<sup>3</sup> is slightly higher than that of [[tungsten]] and [[uranium]]. | ||
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Gold leaf, used in food decorations are also a source of gold, albeit the quantity is small and it's usually a gold alloy. | 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 [[Iron(III) chloride|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. | + | 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 [[Iron(III) chloride|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 AuTe<sub>2</sub>), petzite (Ag<sub>3</sub>AuTe<sub>2</sub>) and sylvanite (AgAuTe<sub>4</sub>). | 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 AuTe<sub>2</sub>), petzite (Ag<sub>3</sub>AuTe<sub>2</sub>) and sylvanite (AgAuTe<sub>4</sub>). | ||
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===Disposal=== | ===Disposal=== | ||
Due to gold's price and rarity, it's best to try and recycle as much gold as possible. | Due to gold's price and rarity, it's best to try and recycle as much gold as possible. | ||
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| + | ==Gallery== | ||
| + | <gallery widths="200" position="center" columns="4" orientation="none"> | ||
| + | Cupel with gold by Cody's Lab.jpg|A cupel with a gold bead, courtesy by Cody. | ||
| + | </gallery> | ||
==References== | ==References== | ||
Latest revision as of 20:30, 20 March 2023
 Small sample of gold foil | |||||
| General properties | |||||
|---|---|---|---|---|---|
| Name, symbol | Gold, Au | ||||
| Alternative name | Aurum | ||||
| Appearance | Metallic dark yellow | ||||
| Gold in the periodic table | |||||
| |||||
| 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.
Contents
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.[1]
Gold is unaffected by concentrated (40%) hydrofluoric acid at standard conditions.[2]
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.
