Gold is a remarkable, rare metal, with an unparalleled combination of chemical and physical properties. It is the only yellow metal and bears its name from the Old English word for yellow, 'geolu'. It is also the only metal that forms no oxide film on it's surface in air at normal temperatures, meaning that it will never rust or tarnish.
Gold's chemical symbol, Au, comes from the latin word for gold, aurum. In the Periodic Table of Elements, gold is classified as a transitional metal with the following characteristics;
Atomic number: 79
Atomic mass: 196.96655 amu
Number of protons/electrons: 79
Number of neutrons: 118
Melting point: 1,064.43°C (1,337.58°K, 1,947.97°F)
Boiling point: 2,807.0°C (3,80.15°K, 5,084.6°F)
Density @ 293°K: 19.32 grams per cubic centimeter
Crystal structure: cubic
Gold may be alloyed with various other metals to give it special properties. In its pure form, gold has a metallic luster and is sun yellow, but when mixed or alloyed with other metals, such as silver (Ag), copper (Cu), zinc (Zn), nickel (Ni), platinum (Pt), palladium (Pd), tellurium (Te), and iron (Fe), creates various color hues ranging from silver-white to green and orange-red. Usually, red, yellow and green golds are made by adding varying amounts of copper (Cu) and silver (Ag) to produce alloys of 10 to 14 carats. White golds have traditionally been made by alloying nickel (Ni), zinc (Zn) and copper (Cu) with gold, but more recently silver (Ag) and palladium (Pd) have replaced the zinc. These color variation treatments to gold are mostly used in jewelry.
Fineness is a metallurgical term indicating the purity of gold and is expressed in parts per 1,000. Minted coins and bars have a fineness of 999.9 parts per 1,000. The purity of gold in an item or the amount of gold in an alloy is measured with a unit called a carat. An item of fine or pure gold is 24 carat, with lesser carat values indicating the proportion of fine gold content as a fraction of 24. For example, an 18 carat gold item contains 18/24 (or 3/4) fine gold and 6/24 (or 1/4) alloy by weight.
Properties of Gold
An unparalleled combination of chemical and physical properties make gold invaluable to a wide range of everyday applications. One of the most important of these properties is gold’s virtual indestructibility.
Gold is the most non-reactive of all metals. It is called a "noble" metal (an alchemistic term) because it does not oxidize under ordinary conditions, meaning that it will never rust and never tarnish.
Gold's physical properties of high electrical conductivity and chemical inertness make it an excellent and reliable conductor, particularly in harsh environments, where temperatures can range from -55°C to 200°C. The use of gold in circuitry ensures reliability of equipment operation, particularly in the vital activation of safety airbag mechanisms in motor vehicles or deployment of satellites and spacecraft.
No other metal is as ductile or as malleable as gold. A single ounce of the metal can be drawn into a wire five miles long. Gold can be hammered into sheets so thin that light can pass through.
High purity gold reflects infrared (heat) energy almost completely, making it ideal for heat and radiation reflection. Gold-coated visors protected astronauts’ eyes from searing sunlight on the Apollo 11 moon landing.
Gold is also an excellent conductor of thermal energy. It is used in many electronic processes to draw heat away from delicate instruments. For example, the main engine nozzle of the space shuttle uses a 35% gold alloy.