There are two stable isotopes, 35Cl (75.8%) and 37Cl (24.2%), giving chlorine a standard atomic weight of 35.45.
Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about 7×10−13 to 1 with stable isotopes.
The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years.
Additionally, large amounts of 36Cl were produced by neutron irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958.
In chloride-based molten salt reactors the production of 36Cl by neutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing 35Cl).