Chlorine (IPAEng: ˈklɔəriːn, from the Greek word 'χλωρóς' (khlôros)(meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its common elemental form (Cl₂ or "dichlorine") under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable, suffocating odor that is detectable in concentrations as low as 3.5 ppm,Merck Index of Chemicals and Drugs, 9th ed., monograph 2065 and is choking and poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary. In the upper atmosphere, chlorine-containing molecules have been implicated in the destruction of the ozone layer.

Characteristics

At standard temperature and pressure, two chlorine atoms form the diatomic molecule chem: 2. This is a pale yellow-green gas that has its specific strong smell. The bonding between the two atoms is relatively weak (only of 242.580 ±0.004 kJ/mol) which makes the chem: 2 molecule highly reactive.

Along with fluorine, bromine, iodine and astatine, chlorine is a member of the halogen series that forms the group 17 of the periodic table - most reactive group of elements. It combines readily with nearly all elements.

Compounds with oxygen, nitrogen, xenon, and krypton are known, but do not form by direct reaction of the elements. Chlorine, though very reactive, is not as extremely reactive as fluorine. Pure chlorine gas does, however, support combustion of organic compounds such as hydrocarbons, although the carbon component tends to burn incompletely, with much of it remaining as soot. At 10 °C and atmospheric pressure, one liter of water dissolves 3.10 L of gaseous chlorine, and at 30°C, 1 L of water dissolves only 1.77 liters of chlorine.

Chlorine is a member of the salt-forming halogen series and is extracted from chlorides through oxidation often by electrolysis. With metals, it forms salts called chlorides. As the chloride ion, Cl⁻, it is also the most abundant dissolved ion in ocean water.

Isotopes

main: Isotopes of chlorine Chlorine has isotopes with mass numbers ranging from 32 to 40. There are two principal stable isotopes, ³⁵Cl (75.77%) and ³⁷Cl (24.23%), giving chlorine atoms in bulk an apparent atomic weight of 35.4527 g/mol.