Occurrence
The representative terrestrial abundance of chlorine-37 is 24.22(4)% of chlorine atoms, with a normal range of 24.14–24.36% 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 be around 24.219% chlorine-37 and to have an atomic weight of around 35.4525
There is a known variation in the isotopic abundance of chlorine-37. This heavier isotope tends to be more prevalent in chloride minerals than in aqueous solutions such as sea water, although the isotopic composition of organochlorine compounds can vary in either direction from the SMOC standard in the range of several parts per thousand.
Read more about this topic: Chlorine-37
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