Chlorine is an element with atomic number 17 in the periodic table and the chemical symbol Cl. It is a greenish yellow gas with a boiling point of -29°F (-34°C) at normal atmospheric pressure and a freezing point of -151°F (-101.5°C). Naturally occurring chlorine consists almost entirely of the two stable isotopes, chlorine-35 and chlorine-37, which give it an atomic weight of 35.4527. Among the other important physical properties of chlorine is the fact that it is heavier than air and if released into the environment tends to collect in low-lying areas. Chlorine is a member of group 17 of the periodic table, which also contains fluorine, bromine, iodine and astatine, collectively known as the halogens.
Within the halogen group, in order of increasing atomic weight, this element lies between fluorine and bromine. The chemical properties of chlorine resemble those of the other halogen elements. It has seven electrons in its outermost shell and is very electronegative; it therefore has a strong tendency to attract electrons from other atoms, making it a powerful oxidizing agent. Electronegativity declines with increasing atomic weight within the halogen group. Chlorine is therefore less electronegative than fluorine, but more so than bromine — it will displace bromine from its compounds: 2NaBr + Cl2 -> 2NaCl + Br2.
Elemental chlorine exists as molecules consisting of two chlorine atoms held together by a single covalent bond. Most chlorine compounds feature the element in its -1 oxidation state, where it accepts an electron from another atom; however, when combined with the more electronegative elements oxygen and fluorine it has a positive oxidation state, which for oxides can be +1, +3, +4, +5, or +7. Oxygen and chlorine do not combine directly, but chlorine oxides, and compounds containing the hypochlorite (OCl-), chlorite (ClO2-), chlorate (ClO3-) and perchlorate (ClO4-) anions can be made by indirect means. Chlorine readily forms ionic bonds with metals, where it accepts an electron from the metal, resulting in a chloride ion (Cl-) with an oxidation state of -1, for example sodium chloride (NaCl), or common salt.
Hydrogen gas combines easily with chlorine to form hydrogen chloride (HCl); the reaction is explosive in the presence of sunlight. It is fairly soluble in water, dissolving to produce a mixture of hydrochloric acid, hypochlorous acid and dissolved chlorine. Ammonia combines with chlorine to form the toxic compounds monochloramine and dichloramine, or the highly shock sensitive explosive nitrogen trichloride, depending on the relative proportions. For this reason, household cleaning products containing chlorine should never be mixed with those containing ammonia. Chlorine reacts readily with many organic compounds and organic chlorides are important in the production of plastics such as polyvinyl chloride (PVC) and as non-flammable solvents.
Although it is a common element in the Earth’s crust, occurring in a number of minerals, such as halite (NaCl), the reactive properties of chlorine are such that it is not found naturally in the uncombined state. Chlorine is produced industrially mainly by the electrolysis of aqueous sodium chloride. This also produces sodium hypochlorite (NaOCl) in solution, better known as household bleach. The element is also a byproduct of the manufacture of sodium metal by the electrolysis of molten sodium chloride. In the laboratory, it can be produced in a number of ways, for example by the reaction of calcium hypochlorite with an acid.
In the form of the chloride ion, chlorine is essential to human life; however, elemental chlorine is toxic and corrosive. It has an unpleasant, choking smell and irritates the respiratory tract if inhaled. At high concentrations, it causes lung damage that may be fatal and the gas was used as a chemical weapon by the German army during World War I. The oxidizing properties of chlorine make it a useful disinfectant and bleaching agent. Compounds which release small amounts of chlorine are used to sterilize drinking water and swimming pools, and are included in various cleaning products.