Chlorine is the second element of the halogen group (group 17 of the periodic table) and is positioned between sulphur and argon in period 3 of this table. Its nucleus consists of 17 protons. It has two stable isotopes as discovered by Aston (Nature 104:393, 1919), 35Cl with 18 neutrons and 37Cl with 20 neutrons. According to the latest report from the IUPAC (Berglund and Wieser in Pure Appl Chem 83:397–410, 2011) 35Cl is the most common stable isotope with an abundance of 75.76 % of the chlorine, while 37Cl consists of the remaining 24.24 %.
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