Gold, symbol Au, with atomic number 79 and atomic weight 197.2, is a yellow metal. It is in the group I, subgroup B of the periodic table that contains copper and silver. The main oxidation states of gold are +1 and +3. The principal physical properties are showed in the next lines:
- Color is golden "butter" yellow.
- Luster is metallic.
- Transparency is opaque.
- Crystal System is isometric; 4/m bar 3 2/m
- Crystal Habits include massive nuggets and disseminated grains. Also wires, dendritic and arborescent crystal clusters.
- Cleavage is absent.
- Fracture is jagged.
- Streak is golden yellow.
- Hardness is 2.5 - 3
- Specific Gravity is 19.3 (extremely heavy even for metallic minerals)
- Associated Minerals include quartz, nagyagite, calaverite, sylvanite, krennerite, pyrite and other sulfides.
- Other Characteristics: ductile, malleable and sectile, meaning it can be pounded into other shapes, stretched into a wire and cut into slices.
- Notable Occurrences include California and South Dakota, USA; Siberia, Russia; South Africa; Canada and other localities around the world.
- Best Field Indicators are color, density, hardness, sectility, malleability and ductility
About the constant physical must be mentioned that its melting point is 1063 oC, boiling point 3081 oC, specific heat at 298 oK is 1.288 x 10-1 J/g.K.
Chemical Reactions: Gold is one of the metals less active. Thus, do not suffer oxidation neither in air nor oxygen, for this reason is called noble metal. The gold can resist the attack by HCl, HNO3, and H2SO4 in environment temperature and 100 oC. Also, the gold resists the attack by HBr, HI, and FH. Aqua Regia (HNO3 and HCl, 1/3) dissolves the gold easily. The gold can be attacked by a hot solution prepared with HNO3 and H2SO4.
All the halogens react with the gold. Bromine is the most active dissolving something of metal in environment temperature. Chlorine is less active when is dry, but when is wet, and more even in high temperature, can attack easily to the gold. Iodine reacts almost like chlorine. Fluorine only attacks the gold in very high temperature.
Sulfur does not attack the gold, but alkaline sulfides can react with the gold. The carbon does not react with the gold. HCN and alkaline cyanides dissolves the gold, and the presence of oxidant agents can enhance the attack.
Alkaline hydroxides in solution do not dissolve gold but if they are smelted, can exist a reaction. Alkaline peroxides attack the gold in high temperature.
Mercury reacts with the gold producing amalgams. This property is used in the extraction of gold from gravel and black sands.
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