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Gold has many practical applications, especially in industry and medicine. It owes all its usefulness to its unique chemical and physical properties.
Gold, of chemical symbol Au, is an element with an atomic number of 79, which means that each gold atom has 79 protons in its nucleus and 79 electrons orbiting the nucleus. The number of neutrons an differ from one gold atom to another, which would allow for different gold isotopes. The only non-radioactive isotope makes up all the gold that is found in nature. Gold has an atomic mass of 196.97 and has an atomic radius of 0.1442 nm. In theory, the gold atom should be much larger than this, but its electrochemical configuration allows it some unique properties as well.
It is the arrangement of the outermost electrons around the nucleus of the gold atom that lends gold its yellow color. The colors of metals are based on the transitions of electrons between energy bands, and it is these energy jumps that give off energy – each energy also produces a color. The conditions that are necessary to produce color for gold occur because an electron transits from the d band to an unoccupied position in the conduction band.
Metallic cold has a face-centered cubic crystal structure. This unique lattice allows free movement of the various atoms in the structure, which also allows gold to be durable and ductile. Gold is also dense and massive compared with other materials. Gold has a density of 19.3 grams per cubic centimeter, which is about 8 times greater than aluminum and over twice the density of steel.
Gold melts at 1064°C, but it can melt over a wide range of temperatures when it is mixed in an alloy with silver or copper. Gold transforms from liquid to gas at 2860°C. Gold conducts heat and electricity efficiently, with its efficiency bettered only by silver and copper. However, while silver and copper are prone to tarnishing, gold is not. It is this resistance to corrosion that makes gold extremely useful for manufacturing batteries, where other metals would corrode over time due to electric currents. The only way to corrode gold is through a mixture of nitric acid and hydrochloric acid.
Gold is also extremely malleable. It can be deformed and remolded without its strength failing. It can be hammered into wafers, beaten into sheets, and molded into nanoparticles without gold losing its strength. Gold is also extremely hard. It is biocompatible and resists oxidation. At nanoscale, gold can be quite different from its bulk form, which makes it a good tool for those working in nanotechnology.