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Gold - Physical, Mechanical, Thermal and Electrical Properties of Gold - World Gold Council

Topics Covered

Atomic Structure of Gold
Origin of Colour
Isotopes of Gold
Crystal Structure of Gold
Density of Gold
Melting and Boiling Points of Gold
Conductivity of Heat and Electricity
Malleability
Ductility
Young's Modulus
Hardness of Gold
Biocompatibility
Oxidation States
Gold’s Properties at the Nanoscale

Atomic Structure of Gold

Understand more about the unique chemical and physical properties of gold that enable it to be utilised in a diverse range of practical applications. 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 0.1442nm.

Interestingly this is smaller than would be predicted by theory.

Origin of Colour

The arrangement of outer electrons around the gold nucleus is related to gold's characteristic yellow colour. The colour 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 colour are fulfilled by a transition from the d band to unoccupied positions in the conduction band. Gold's attractive warm colour has led to its widespread use in decoration

Isotopes of Gold

Whilst the number of protons in a gold nucleus is fixed 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 non-radioactive isotope accounting for all naturally found gold.

Crystal Structure of Gold

The crystal structure for metallic gold is face centred cubic FCC (see image). 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.

Density of Gold

The density of gold (19.3 gcm-3) depends on both its atomic mass and the crystal structure. This makes gold rather heavy compared to some other common materials. For example, aluminium has a density of 2.7 gcm-3 and even steel's density is only 7.87 gcm-3.

Melting and Boiling Points of Gold

The melting point of pure gold is 1064°C, 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 2860°C.

Conductivity of Heat and Electricity

The ability of gold to efficiently 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 m at 20 °C. The thermal conductivity is 310 W m-1 K-1at the same temperature. The corrosion resistance of gold is perhaps one it’s 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 as indicated below. 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.

Table 1. Electrode potential of gold and related elements.

Electrode Potential (V) Element
+1.5 Gold
+0.8 Silver
-0.4 Iron
-0.8 Zinc
-1.66 Aluminium

Malleability

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 cm thick. One ounce of gold can be beaten into a sheet covering over 9 square metres and 0.000018 cm thick.

Ductility

Gold is also ductile (degree of extension which takes place before failure of a material in tension) and one ounce can be drawn into 80 km (50 miles) of thin gold wire (5 microns diameter) to make electrical contacts and bonding wire.

Young's Modulus

The Young's modulus of elasticity of a material is related to rigidity or stiffness and is defined as the ratio between the stress applied and the elastic strain it produces. Gold has a Young's modulus of 79 GPa which is very similar to silver, but significantly lower than iron or steel.

Hardness of Gold

Hardness is defined as the ability of a material to resist surface abrasion. The relative hardness of materials were historically assessed using a list of materials arranged in such order that any material in the list will scratch any one below it. Thus, diamond the hardest substance known, heads the list with a hardness index of 10 whilst 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 is used and gold has a value of approximately 25Hv in the annealed condition.

Biocompatibility

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 colonisation and because of this it is the material of choice for implants that are at risk of infection, such as the inner ear.

Oxidation States

Gold forms a number of interesting compounds based on the familiar oxidation states +1 and +3. Gold-based chemicals include halides, cyanides, and sulfides.

Table 2. Summary of properties of gold.

Property
Atomic weight
196.97
Atomic number
79
Number of naturally occuring isotopes
1
Melting point °C
1064
Crystal structure
FCC
Density gcm-3
19.3
Thermal conductivity W m-1 K-1
310
Electrical resistivity micro-ohm m at 20°C
0.022
Young's modulus E GPa
79
Hardness Hv
25
Tensile stress MPa
124
0.2% proof stress MPa
30
Poissons ratio
0.42

Gold’s Properties at the Nanoscale

It is important to draw a distinction between the properties of gold in the bulk form and those properties it exhibits when present in the form of tiny nanoparticles. At the nanoscale, gold's properties can be markedly different These differences are explained in a paper from Professor Mike Cortie of the University of Technology in Sydney explains. The unique properties of gold at the nanoscale lead to its use in a growing number of applications including colloids for biomedical marking and catalysts in chemical processing and pollution control.

Source: World Gold Council

For more information on this source please visit World Gold Council

Date Added: Mar 9, 2010 | Updated: Jun 11, 2013
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