Chlorine is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. 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 elemental form (Cl2 or "dichlorine") under standard conditions, chlorine is a powerful oxidant and is used in bleaching and disinfectants, as well as an essential reagent in the chemical industry. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary. In the upper atmosphere, chlorine-containing molecules such as chlorofluorocarbons have been implicated in ozone depletion. At standard temperature and pressure, two chlorine atoms form the diatomic molecule Cl2. This is a yellow-green gas that has its distinctive strong smell, the smell of bleach. The bonding between the two atoms is relatively weak (only 242.580 ±0.004 kJ/mol), which makes the Cl2 molecule highly reactive. The boiling point at regular atmosphere is around −34 ˚C, but it can be liquefied at room temperature with pressures above 8 atmospheres.
