Isotopes of chlorine
Open-knowledge reference entry
Chlorine (17Cl) has two stable isotopes, 35Cl (75.8%) and 37Cl (24.2%), giving chlorine a standard atomic weight of 35.45. Artificical radioisotopes are known ranging from 28Cl to 52Cl, and there are also two isomers, 34mCl and 38mCl. The longest-lived radioactive isotope is 36Cl, which has a half-life of 301,000 years. All other isotopes and isomers have half-lives under an hour, and most under 10 seconds.
List of isotopes
Stable isotope analysis
The representative terrestrial abundance of chlorine contains about is 24.2% of 37Cl, with a normal range of 23.9–24.5% of chlorine atoms. When measuring deviations in isotopic composition, the usual reference point is "Standard Mean Ocean Chloride" (SMOC), although a NIST Standard Reference Material (975a) also exists. SMOC is known to have a 37Cl/35Cl ratio of 0.319627 ± 0.000199 (24.221% ± 0.0015% chlorine-37) and to have an atomic weight of 35.4525.
There is known variation in the isotopic abundance of chlorine. This heavier isotope tends to be more prevalent in chloride minerals than in aqueous solutions such as seawater, although the isotopic composition of organochlorine compounds can vary in either direction from the SMOC standard in the range of several parts per thousand.
Chlorine-36
Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about 7×10−13 to 1 with stable isotopes.
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