Properties of Gold

Gold (symbol Au) has an atomic number of 79, i.e., each gold atom has 79 protons in its nucleus. The atomic mass of the gold atom is 196.967, and the atomic radius is 0.1442 nanometers. Interestingly this is smaller than would be predicted by theory. The arrangement of outer electrons around the gold nucleus is related to gold’s characteristic yellow color. The color of a metal is based on transitions of electrons between energy bands. The conditions for the intense absorption of light at the wavelengths necessary to produce the typical gold color are fulfilled by a transition from the d band to unoccupied positions in the conduction band.

Gold’s attractive warm color has led to its widespread use in decoration. While the number of protons in a gold nucleus is fi xed at 79, the number of neutrons can vary from one atom to another, giving a number of isotopes of gold. However, there is only one stable nonradioactive isotope accounting for all naturally found gold. The crystal structure for metallic gold is face-centered cubic (FCC).

This crystal structure contributes to gold’s very high ductility since FCC lattices are particularly suitable for allowing the movement
of dislocations in the lattice. Such dislocation movement is essential for achieving high ductility. The density of gold (19.3 grams per cubic centimeter) depends on both its atomic mass and the crystal structure. This makes gold rather heavy compared to some other common materials. For example, aluminum has a density of 2.7 grams per cubic centimeter, and even steel’s density is only 7.87 grams per cubic centimeter.

The melting point of pure gold is 1,064 degrees centigrade, although when alloyed with other elements such as silver or copper,
the gold alloy will melt over a range of temperatures. The boiling point of gold, when gold transforms from the liquid to gaseous state,
is 2,860 degrees centigrade. The ability of gold to effi ciently transfer heat and electricity is bettered only by copper and silver, but unlike these metals, gold does not tarnish, making it indispensable in electronics.

The electrical resistivity of gold is 0.022 micro-ohm per meter at 20 degrees centigrade. Th e thermal conductivity is 310 watts per meter per kelvin at the same temperature. Th e corrosion resistance of gold is perhaps one of its most useful properties. Electrode potentials are a useful method for representing the tendency of a metal to corrode.

Electrode potentials are measured with reference to hydrogen, and an electrochemical series can be prepared for metals. Not surprisingly, gold is at the top of the series, indicating its high corrosion resistance. In practice, it is corroded only by a mixture of nitric and hydrochloric acid (aqua regia). In everyday use gold does not tarnish.

The metal gold is extremely malleable (the extent to which a material can undergo deformation in compression before failure). In the annealed state, it can be hammered cold into a translucent wafer 0.000013 centimeters thick. One ounce of gold can be beaten into a
sheet covering over 9 square meters and 0.000018 centimeters thick. Gold is also ductile (degree of extension that takes place before
failure of a material in tension), and one ounce can be drawn into 80 kilometers (50 miles) of thin gold wire (5 microns diameter) to make electrical contacts and bonding wire.

The Young’s modulus of elasticity of a material is related to rigidity or stiff ness and is defined as the ratio between the stress applied and the elastic strain it produces. Gold has a Young’s modulus of 79 giga pascals, which is very similar to silver, but signifi cantly lower than iron or steel.

Hardness is defi ned as the ability of a material to resist surface abrasion. The relative hardness of materials was historically assessed
using a list of materials arranged in such order that any material in the list will scratch any one below it. Th us, diamond, the hardest
substance known, heads the list with a hardness index of 10 while talc is at the bottom with a hardness index of 1. On this scale, gold has a value of 2.5 to 3, i.e., it is a soft metal. For more accurate measurements the Vickers hardness measurement (Hv) is used, and gold has a value of approximately 25 Hv in the annealed condition.

Gold demonstrates excellent biocompatibility within the human body (the main reason for its use as a dental alloy), and as a result there are a number of direct applications of gold as a medical material. Gold also possesses a high degree of resistance to bacterial colonization, and because of this it is the material of choice for implants that are at risk of infection, such as those in the inner ear. Gold forms a number of interesting compounds based on the familiar oxidation states 11 and 13. Gold-based chemicals include
halides, cyanides, and sulfides.

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