What is gold?
Gold is a chemical element or yellow precious d-block metal of Group 11 or 1B of the periodic table with symbol Au and atomic number 79. It is used widely from ancient history for jewelry or coinage making. The name gold is derived from the old English word geolu means yellow and from the Latin word aurum. Copper, silver, and gold constitute the family of coinage or currency metals. These metals occur in nature in a native form with very similar chemical and physical properties in chemistry. The purity of gold is expressed in carats, pure gold is 24 carat and common alloyed of copper is 22 carat.
Properties of gold
It is chemically unreactive, extremely malleable, ductile, high density (19.32 gm cm-3) golden-yellow metal.
It is a good conductor of heat and electricity. In learning chemistry, the characteristic golden yellow color arises from the absorption in the UV light from blue regions of the electromagnetic spectrum corresponding to the excitation of electrons from filled d-band to s-p conduction band. The metal from fcc crystal lattice in the solid form.
|Properties of gold|
|Electronic configuration||[Kr] 4f14 5d10 6s1|
|Meting point||1064 °C|
|Boiling point||2808 °C|
|Molar heat capacity||25.418 J mol-1 K-1|
|Electronegativity||2.54 (Pauling scale)|
|Ionization energy||1st – 890.1 kJ/mol
2nd – 1980 kJ/mol
|Crystal structure||face-centered cubic (fcc)|
|Electrical resistivity||22.14 nΩ⋅m at 20 °C|
Gold on the periodic table
Gold is placed in group-11 on the periodic table with d-block or transition metal. The valence shell electronic configuration of Au [Xe] 4f14 5d10 6s1 with filled d-orbital. It is placed with transition metal due to the presence of vacant d-orbital in +3 oxidation number or state.
Where is gold found?
Native gold is found in nature in two forms, reef-Au (deposit or dispersed fine particles in quartz veins in deep mines), alluvial-Au (primary deposit by geological action and redeposited in alluvial sands gravel as fine grains). The combined form of gold is found in a few minerals like calaverite, AuTe2, and sylvanite, (Ag, Au)Te2. It is also found in sea water to the extent of 10-3 ppm but no extraction methods were developed to gain the metal from sea water.
Gold production by country
The top 10 commercial gold producer countries in World according to the 2019 report given below table,
|Country||Gold production (tonnes)|
Gold processing steps
Gold is processed mainly by the cyanide and amalgamation process.
Cyanide gold extraction process
The cyanide process is going through the following steps,
- The finely powdered rocks were ignited with dilute sodium cyanide (NaCN) solution and lime in presence of air for oxidation. 4Au + 8NaCN + 2H2O + O2 → 4Na[Au(CN)2] + 4NaOH
- The solution is filtered and Au deposited from the filtrate by zinc shavings. Zn + 2Na[Au(CN)2] → 2Au + Na2[Zn(CN)4]
- Zinc is dissolved out by dilute sulfuric acid and the dried residue is melted with borax.
- The crude gold contains copper, silver, and lead. Lead is removed by cupellation. Copper is removed by oxidative fusion of borax and silver may be removed by boiling with concentrated sulfuric acid. Electrolysis is the best method for refining Au.
The amalgamation process was also used to separate native gold from alluvial sand and gravel deposits. The finely crusted rocks amalgamated with mercury with the stream of water over copper plates. The amalgam is scared and mercury is distilled out in iron retorts.
The first ionization energy of Ag is the lowest among the noble metals family, the sum of the first and second ionization energy of Cu is the lowest, the sum of the first second, and the third ionization energy of Au is lowest. The fact suggests the common oxidation state of Cu, Ag, and Au. For gold, atomization, ionization, hydration energy favors the formation of Au(III) in aqueous solutions.
The chemically unreactive gold is the noblest transition metal of the periodic table. It does not attract oxygen, sulfur, or readily react with halogens like fluorine, chlorine, bromine, and iodine molecule. It will be dissolved in cyanide solution in presence of hydrogen peroxide or aqua regia (a mixture of concentrated nitric acid and hydrochloric acid).
Although the small size but no simple monovalent cation of gold can exist, most of the compounds are formed by covalent bonding or by the form of complexes. The simple compound of gold(I) is restricted to chlorine, bromine, and iodine only. AuCl and AuBr are prepared by controlled thermal decomposition of respective halides. AuI is prepared by heating the metal with iodine solution or adding iodine solution to AuCl3 solution.
The orange crystalline solid of Au(III) fluoride is made by the action of fluorine on Au2Cl6 at 300 °C. AuF7 and AuF5 are examples of Au compounds that show a higher than +3 oxidation state formed by the covalent chemical bonding.
The oxide, Au2O obtained by dehydrating AuOH with alkali metals and sulfur dioxide. Au(III) is the most common oxidation state of gold offering oxide like Au2O3. Amorphous brown Au2O3, 2H2O is precipitated by alkali from the solution containing AuCl4–. The anhydrous oxide is obtained by drying the hydrated compounds over P4O10 followed by careful heating.
It is an olive green explosive powder obtained by careful digestion of Au2O3 or its hydrate with ammonia. The dry powder explodes with a flash on striking or heating. The probable formula of fulminating Au is HN=Au-NH2, 1.5H2O.
+2 oxidation state is unfavorable in comparison to Au(I) and Au(II) and few complexes of Au(II) have been claimed. For example, tetra n-butylammonium salt of the bis (maleonitriledithiolato) aurate(II). Au2S3 is prepared by passing H2S over dry LiAuCl4, 2H2O at 10 °C. The lithium chloride is separated by extracting with alcohol and dried at 70 °C.
Uses of Gold
- Gold and its compounds are extensively used in medicine in old age Ayurveda as well as in modern Allopathy. It is used particularly for the treatment of arthritis.
- Because of its bright color and low chemical reactivity, gold was the first metal that attracts the human eye to prepare coins, different types of craft jewelry, and decorative objects.
- Due to the unique qualities, the materials are universally accepted in goods and services in the form of coins or bullion to affect the currency of the world. The modern currency is paper-based currency but gold paly a key role dominate the currency of the country. It is accepted by all nations as a medium of international payments technique.
- A purple powder containing colloid of Au is absorbed on the hydrated tin(II) oxide. It is used in making ruby glass. The purple powder is prepared by reducing Au(II) chloride with SnCl2.
- Due to electrical conductivity, thermal conductivity, and unreactive properties, we use a large amount of gold for industrial purposes in electric or electronics engineering. It is used for making terminals, plating contacts, printed circuits, and semiconductors.
What is 24k gold?
Pure gold is 24 carats containing 24 parts of Au by weight and 2 parts of other metals in weight. It is alloyed with other metals due to its increased softness and brightness for easy use for jewelry, goldware, and coin making. The alloyed metals are silver, copper, zinc, platinum, and palladium. The silver alloyed gold is used to make coins or goldware and the platinum or palladium alloyed metal is commonly used for making craft jewelry.