Facts about Gold

Gold, like no other metal, has a fascinating history and a special place in the world.  For thousands of years it has been used as an ornament of kings, a currency and standard for global currencies, and more recently, in a wide range of electronic devices and medical applications. 

Gold's many unique properties have secured it a central role in history and human development. 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, arum. In the Periodic Table of Elements, gold is classified as a transitional metal with the following characteristics:

• Symbol: Au
• 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
• oxidation states: +1, +3
   
  Like other transition metals gold's valence electrons, or the electrons it uses to combine with other elements, are present in more than one shell. Unusually for a metal, gold can also form compounds (aurides) in which its oxidation number is negative (-1). For example, gold can combine with cesium to form cesium auride, CsAu, and rubidium to form rubidium auride, RbAu. These are ionic compounds with non-metallic properties in which the Cs or Rb ions are charged +1 while the Au atoms are charged 1-.
  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.
  
  
  
  Gold is found in nature in quartz veins and secondary alluvial deposits as a free metal or in a combined state. It is widely distributed although it is rare, being 75th in order of abundance of the elements in the crust of the Earth. It is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold occurs, in chemical combination with tellurium, in the minerals calaverite and sylvanite along with silver, and in the mineral nagyagite along with lead, antimony, and sulfur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulfide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in seawater to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in seawater is more than 9 billion metric tons, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.