Chlorine is named after the Greek word chlóros, meaning "pale green", and refers to the color of this gaseous element. It is a highly reactive gas that forms compounds, and does not exist in pure form in nature. The most common chlorine compound is sodium chloride, NaCl (in nature as the mineral halite, or rock salt). Chlorine compounds are important to the body and used by muscles and nerves. They are also present in sweat. Chlorine in its pure form is poisonous, and was used in World War I, as a weapon (soldiers had to wear masks to protect themselves from chlorine gas, Cl2). Today, chlorine is used in many other ways. For example, its compounds are present in many materials, from slippers to chloroform, CHCl3. It reacts with hydrogen to form hydrochloric acid, HCl, an industrial cleaner. This acid is a corrosive liquid that corrodes most metals, releasing hydrogen gas. Sodium hypochlorite is used to clean water in swimming pools, while bleaches and other cleaners use chlorine compounds to kill germs. One of the most widely used plastics, polyvinyl chloride (PVC), contains chlorine. It is a tough plastic, used to make many rigid objects.

Summary of properties (Cl)

Atomic weight[35.446, 35.457]
Discoverer (year)Scheele, Carl Wilhelm (1774)
Natural formgas (Cl2)
Electron configuration[Ne] 3s2 3p5
Melting point (ºC)-101
Boiling point (ºC)-35
Abundance in earth's crust (ppm)145
Isotope (abundance)35Cl (75.76), 37Cl (24.24)
Density g/cm33.21
Van der Waals radius (pm)175
Covalent radius (pm)100
Electronegativity (Pauling)3.16
Vaporization enthalpy (kJ/mol)20.4º
Enthalpy of fusion (kJ/mol)6.40
Specific heat capacity (J/g·K) at 25ºC and 1 atm0.48
Thermal conductivity (W/cm·K) at 25ºC and 1 atm<0.001 (Cl2)
Oxidation state+7, +5, +1, -1
Electron affinity (eV)3.61
1st Ionization potential (eV)12.9676

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