Gold: Properties, History and Occurrence

• Gold has the symbol Au, atomic number 79
• It is a bright, slightly orange-yellow, soft, ductile, and malleable metal in its purest form
• Gold is relatively inert and one of the least reactive solids
• The metal is stable to air, water, alkali, and acids
• Gold reacts with aqua regia (nitric and hydrochloric acid mixture), cyanide, mercury, and halogens

History and occurrence

• Humans have known and used gold since the beginning of recorded history
• Gold artifacts have been found in caves dating back to 40,000 BC
• Gold was used for ornamentation and religious rituals
• Gold is found in hydrothermal veins and placer deposits
• Gold is often found with quartz and pyrite
• Major gold producers include South Africa, the United States, Australia, China, and Russia

Physical properties

• Gold has a density of 19.3 g/cm³
• Gold has a melting point of 1064 °C and a boiling point of 2856 °C
• Gold is a good conductor of heat and electricity
• Gold is the most malleable and ductile of all known metals; a single gram can be beaten into a sheet of one square meter
• Gold can be alloyed with other metals to increase its strength and resistance to wear

Chemical properties

• Gold has an electronegativity of 2.54 on the Pauling scale
• Gold has a standard reduction potential of +1.50 V
• Gold does not react with oxygen at ordinary temperatures
• Gold reacts with chlorine and fluorine at elevated temperatures, forming gold chlorides and fluorides
• Gold dissolves in aqua regia, a mixture of concentrated nitric acid and hydrochloric acid, to form chloroauric acid
• Gold forms a variety of complexes with ligands like cyanide, thiocyanate, and thiosulfate
• Gold exists in oxidation states of -1, +1, +2, +3, and +5, with +1 and +3 being the most common

Isotopes

• Gold has one stable isotope, gold-197
• Gold has several radioactive isotopes, including gold-198, which is used in medicine for cancer therapy

Compounds

• Gold(I) compounds are usually linear complexes
• Gold(III) compounds are usually square planar complexes
• Gold(I) chloride (AuCl) is an unstable compound that decomposes to gold and chlorine
• Gold(III) chloride (AuCl3) is a stable compound that forms complexes with ligands
• Chloroauric acid (HAuCl4) is an important compound, used in the purification of gold and in the preparation of other gold compounds

Applications

• Gold is used in jewelry, coinage, and other decorative items
• Gold is used in electronics for coating electrical contacts and connectors
• Gold is used in medicine for treating certain cancers and other diseases
• Gold is used in dentistry for fillings, crowns, and bridges
• Gold is used in aerospace for coating satellites and other spacecraft
• Gold is used as an investment

Extraction and refining

• Gold is extracted from ore by methods like cyanide leaching, amalgamation, and smelting
• Cyanide leaching is the most common method for extracting gold from low-grade ore
• Amalgamation extracts gold by dissolving it in mercury
• Smelting extracts gold by heating ore to high temperatures in a furnace
• Gold is refined by methods including electrolysis and the Miller process
• Electrolysis refines gold by passing an electric current through a gold solution
• The Miller process refines gold by bubbling chlorine gas through molten gold

Environmental considerations

• Gold mining can significantly impact the environment
• Cyanide leaching can contaminate water sources
• Mercury amalgamation can release mercury into the environment
• Smelting can release air pollutants
• Responsible gold mining practices are essential to minimize environmental impacts
• Recycling gold from electronic waste is becoming increasingly important

Gold is a chemical element, known for its bright yellow color and inertness. It has been used since the beginning of recorded history for ornamentation and religious rituals. Gold is found in hydrothermal veins and placer deposits and South Africa, the United States, Australia, China, and Russia are major gold producers.