Physics & Chemistry

Gold

May 10, 2011, 4:15 pm
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Gold is classified as a precious metal. The combination of gold’s relative scarcity and its evident beauty has made it an intrinsically valuable commodity throughout the history of humanity. It is most probably the oldest precious metal known to man. Wars have been fought over it and countless numbers have died trying to gain it or protect it.

Previous Element: Platinum

Next Element: Mercury
79

Au

196.97
Physical Properties
Color golden yellow
Phase at Room Temp. solid
Density (g/cm3) 19.32
Hardness (Mohs) 2.75
Melting Point (K) 1337.63
Boiling Point (K) 3081
Heat of Fusion (kJ/mol) 12.678
Heat of Vaporization (kJ/mol) 343
Heat of Atomization (kJ/mol) 366
Thermal Conductivity (J/m sec K) 318
Electrical Conductivity (1/mohm cm) 446.4
Source Gold bearing rock (misc)
Atomic Properties
Electron Configuration [Xe]6s14f145d10
Number of Isotopes 54 (1 natural)
Electron Affinity (kJ/mol) 222.752
First Ionization Energy (kJ/mol) 890.1
Second Ionization Energy (kJ/mol) 1980
Third Ionization Energy (kJ/mol) ---
Electronegativity 2.54
Polarizability (Å3) 6.1
Atomic Weight 196.97
Atomic Volume (cm3/mol) 10.2
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 144
Ionic Radius1+ (pm) 151
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) 99
Common Oxidation Numbers +3
Other Oxid. Numbers -1, +1, +2, +5
Abundance
In Earth's Crust (mg/kg) 4.0x10-3
In Earth's Ocean (mg/L) 4.0x10-6
In Human Body (%) 0%
Regulatory / Health
CAS Number 7440-57-5
OSHA Permissible Exposure Limit (PEL) No limts
OSHA PEL Vacated 1989 No limts
NIOSH Recommended Exposure Limit No limits
Sources:

Mineral Information Institute
Jefferson Accelerator Laboratory
EnvironmentalChemistry.com
 

Scientifically speaking, gold is an element, a metal, with an atomic number 79. Its physical and chemical properties make it ideal for a number of applications. It is very stable and as a result seldomly combines with other elements. In other words, it does not corrode or rust. It conducts electricity efficiently (only silver being a better conductor of electricity). It conducts heat very well. Gold is very malleable which means it can be hammered into shapes. Gold is so malleable that it can be hammered into a sheet so thin that light can pass through it. It is also ductile, which means it can be drawn into long, thin wires: a wire thread approximately 50 miles long can be drawn from a single troy ounce of gold (31.1 grams). It is also one of the densest metals: one cubic foot of gold weighs over 1200 pounds.

Name

Gold’s chemical symbol is Au. It comes from the Latin word aurum which means shining dawn, a reference to its bright yellow color and shiny luster. The English word gold has its origins in Middle English.

Sources

Gold is found in two major types of deposits. Lode deposits are deposits where gold is found in cracks and veins in rocks. These are also called vein deposits. The second type of gold deposit is called a placer deposit. Placer deposits, common in the Sierra Nevada of California, are formed by moving water that has eroded gold out of lode deposits. When the speed of the water in a river slows sufficiently, the heavy gold falls to the bottom and accumulates in the sand of the riverbed. A third major source of gold is as a by-product of copper and silver mining. Gold is so valuable that it is worth the effort to recover even minute amounts from copper and silver ore.

 

caption Gold. (Source:MII)

It is estimated that the total amount of gold yet to be retrieved from the Earth is 100,000 tons. South Africa is the world’s largest producer of gold and is estimated to have half of these gold resources. Both the United States and Brazil have significant amounts of the world’s gold resources. Approximately one-fifth of the total resources of gold in the world is by-product from copper and silver ores.

In the United States, Alaska and Nevada are the main producers of gold. Nevada produces the majority of the gold produced in all of the United States. The remaining are from placer deposits in Alaska, and other gold deposits in western states. Most of the gold recovered in the United States is recovered by only about 30 mines. The largest gold mines in the United States today are located in northern Nevada.

Brazil and Canada export significant amounts of gold to the United States.

Uses

Most gold is used to make jewelry and other art items. Because it is chemically stable and conducts electricity so well, it is very important in electronics. Electronic applications represent a significant amount of the United States’ annual gold consumption, followed by dentistry and a variety of other applications.

Gold has been very important as the standard for currency. In 1792, the United States Congress established gold and silver as the standard for the nation’s money. The U.S. Department of the Treasury holds a major stockpile of gold.

Substitutes and Alternative Sources

Palladium, platinum and silver have been substituted for gold. Alloys (mixtures) of gold and other base metals are extensively used in jewelry and electronics to reduce the amount of gold used while assuring the positive features for which gold is desired.

Traditional mining of lode and placer deposits and by-product production of gold from copper and silver mining will continue to be the main sources of gold. There is gold in seawater, but recovering this gold is unlikely to ever be profitable. Recycling gold from used electronics will provide only a very small amount of gold, though some do find this venture profitable.

Further Reading

Glossary

Citation

Institute, M. (2011). Gold. Retrieved from http://www.eoearth.org/view/article/153052

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