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Element Fact
The liquid helium-4 possesses the most unusual physical properties that defy common sense. Below 2.2 K it becomes a superfluid that can flow, against gravity, up the wall of a container.

Gold, Au

gold sample

Atomic number: 79
Atomic mass: 196.96655(2)
Natural abundance: 0.004 ppm
Isotopes:
197Au (196.96655) - 100%

History and Production

From Anglo-Saxon gold. The symbol Au is derived from Latin aurum for gold. It has been used as coins and ornaments by Egyptians since 3400 BC. The important commercial source is obtained by mining gold-containing rock. The rock is crushed and treated with mercury to extract gold from rocks (amalgamation). Gold can also be leached from the crushed rocks with a cyanide solution. The metal can be further refined by means of electrolysis. Its main use is in settling international debts and in the manufacture of jewellery. It is also used in dentistry, electronic industry and aerospac industry as heat reflection and brazing alloys. Recently, it has also found its increasing use as value backing for digital currencies circulating in the Internet.

Physical Data

Gold is soft and is the most dutile and malleable of all metals. It has a beautiful metallic yellow in appearance. When finely divided it may look black, ruby or purple. It is a very good conductor of both heat and electricity. It is stable in air and does not attack by most acids except aqua regia. It is widely but sparsely distributed in nature which consists of 0.004 ppm of the earth's crust. It is usually found in elemental form but also in tellurides.

Interatomic distance: 288.4 pm
Melting point: 1064.18C
Boiling point: 2856C
Thermal conductivity/Wm-1K-1: 317 (27C)
Density/kgm-3: 19300 (20C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 366.1 kJ/mol
Gibbs free energy of formation: 326.3 kJ/mol
Entropy: 180.5 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Xe] 4f14 5d10 6s1
Term symbol: 2S1/2
Electron affinity: 222.7491 kJ/mol   Electronegativity (Pauline): 2.40
Ionization energy (first, second, third): 890.126, 1977.95, - kJ/mol

Chemical Properties

God in general is resistance to chemical attacks, but with a few exception. In the presence of air, it reacts with potassium cyanide solution, to give a gold(I) complex, [Au(CN)2]-. Gold also dissolves in a mixture of concentrated hydrochloric acid and nitric acids (aqua regia) to give chlorauric acid, HAuCl4. Gold is also dissolved by bromine trifluoride to form gold(III) fluoride. This is one of the few 'non-complex' gold compound, in the trivalent form.

Test for gold:
Gold compounds are easily reduced in alkaline solution to metallic gold which may occur in colloidal form which can be red, blue or intermediate colors. This can be dried and weighed in quantitative analysis.

References

D. Thompson, 'A golden future for catalysis', Chemistry in Britain, Vol. 37, November 2001.


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