Chlorine is a chemical element (atomic number 17) which at normal pressures and temperatures is a gas in it's pure molecular form.  It is very toxic to human life as it mixes easily with water and becomes very caustic.  

Chlorine in an aqueous solution is known as bleach and is commonly used as an antiseptic.

Chlorine compounds are common and often very safe, and ionic chlorine is necessary for life.  Chlorine together with sodium forms common table salt.

Chlorine at Wikipedia

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