Isotopes of chlorine
Chlorine (Cl) has 25 isotopes with mass numbers ranging from Cl to Cl and 2 isomers (Cl and Cl). There are two stable isotopes, Cl (75.77%) and Cl (24.23%), giving chlorine a standard atomic weight of 35.45. The longest-lived radioactive isotope is Cl, which has a half-life of 301,000 years. All other isotopes have half-lives under 1 hour, many less than one second. The shortest-lived are Cl and Cl, with half-lives less than 10 picoseconds and 30 nanoseconds, respectively—the half-life of Cl is unknown. Chlorine-36. Trace amounts of radioactive Cl  exist in the environment, in a ratio of about 7×10 to 1 with stable isotopes. Cl is produced in the atmosphere by spallation of Ar by interactions with cosmic ray protons. In the subsurface environment, Cl is generated primarily as a result of neutron capture by Cl or muon capture by Ca. Cl decays to either S (1.9%) or to Ar (98.1%), with a combined half-life of 308,000 years. 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 Cl were produced  by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, Cl is also useful for dating waters less than 50 years before the present. Cl has seen use in other areas of the geological sciences, forecasts, and elements. Chlorine-37. Stable chlorine-37 makes up about 24.23% of the naturally occurring chlorine on earth. Variation occurs as chloride mineral deposits have a slightly elevated chlorine-37 balance over the average found in sea water  and halite deposits.