Halogens in space - Chlorine

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Chlorine (from Ancient Greek: χλωρóς khlôros "pale green") is a chemical element with symbol Cl and atomic number 17. It is the second lightest halogen after fluorine. Chlorine is in the halogen group (17). The element forms diatomic molecules under standard conditions, called dichlorine. It has the highest electron affinity and the third highest electronegativity of all the elements; for this reason, chlorine is a strong oxidizing agent.

In the interstellar medium, chlorine is produced in supernovae via the r-process.

In nature, chlorine is found primarily as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans — about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the Dead Sea and in underground brine deposits.

Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. In the Earth's crust, chlorine is present at average concentrations of about 126 parts per million, predominantly in such minerals as halite (sodium chloride), sylvite (potassium chloride), and carnallite (potassium magnesium chloride hexahydrate). Over 2000 naturally occurring organic chlorine compounds are known.
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