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Element Fact
Iridium is the most corrosion-resistant metal known. Together with platinum, it is used to make the international prototype of the kilogram, which is the standard definition for the unit of mass.

Chlorine, Cl

chlorine sample

Atomic number: 17
Atomic mass: 35.453(2)
Natural abundance: 126 ppm
35Cl (34.96885271) - 75.77%
37Cl (36.96590260) - 24.23%

History and Production

Derived from Greek, chloros, meaning greenish yellow. It was the first of the halogens to be isolated, in 1774, by C.W. Scheele, by oxidizing hrdrochloric acid and manganese(IV) dioxide. However, he did not recognized it as a compound. However, H. Davy insisted that it was an element and was given the name chloros in 1811 which relates to the color of the gas. The bleaching power of chlorine has long been recognized by Scheele since 1774. To date, it is still used as such in industry. The industrial production of chlorine gas is achieved by the electrolytic process of chloride ions of natural brine. It is also produced from electrolysis of fused sodium chloride, of which sodium metal is the byproduct. It is also used as a disinfectant, germicide and chlorination of domestic water supplies. It is also used in paper productions, textiles, plastics and many other consumer products.

Physical Data

It is a greenish-yellow, gas, which readily dissolves in water, especially at a lower temperature. It is poisonous and has a characteristic pungent smell that irritates the mucous membrane. It is easily liquified by a pressure of a few atmospheres at ordinary temperature and stored as a liquid in cylinders. The majority of chlorine is contained in the sea water reserves as chloride ions.

Interatomic distance: 198.8 pm
Melting point: -101.5C
Boiling point: -34.04C
Thermal conductivity/Wm-1K-1: 0.0089 (27C)
Density/kgm-3: 1507 (b.p.), 3.214 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 121.3 kJ/mol
Gibbs free energy of formation: 105.3 kJ/mol
Entropy: 165.2 J/mol K
Heat capacity: 21.8 J/mol K

Electronic data

Electronic configuration: [Ne] 3s2 3p5
Term symbol: 2P3/2
Electron affinity: 348.5751 kJ/mol   Electronegativity (Pauline): 3.16
Ionization energy (first, second, third): 1251.19, 2297.70, 3821.79 kJ/mol

Chemical properties

Chlorine reacts with hydrogen to give hydrogen chloride, HCl. The reaction is slow in the dark but proceeds violently in light. chlorine has a strong affinity for hydrogen and will react with many compounds containing hydrogen. Chlorine can also react with most elements, both metals and non-metals, forming chlorides.

Test for chlorine (as chloride ion):
(1) White precipitate of silver chloride forms when silver nitrate is added to a chloride solution containing nitric acid. The precipitate is soluble in ammonia solution.
(2) If a chloride is heated with manganese dioxide and concentrated sulfuric acid, chlorine gas is evolved.
(3) If the chloride is heated with sodium or potassium dichromate chromyl chloride, CrO2Cl2, is evolved. If this is passed into water, a yellow solution of a chromate is formed.

Other chlorine-containing compounds such as perchlorates can be converted to chloride prior to testing by strong heating. In the case of chlorates, it can be reduced to chloride by sulfurous acid, H2SO3.

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