With atomic number Z = 17, chlorine belongs to the halogen family. It has 14 isotopes with masses ranging from 32 to 43. Only three of these isotopes exist naturally: chlorine 35, chlorine 37 and chlorine 36. With the exception of chlorine 35 and chlorine 37 , All are radioactive. The most widely used isotope of chlorine is chlorine 36, the other isotopes having radioactive periods less than one hour

Chlorine 36 (Cl-36) is the chlorine isotope whose nucleus consists of 17 protons and 19 neutrons. It is a cosmogenic radioisotope of chlorine with a trace half-life of 301,000 years in the environment, in a ratio of about 7 × 10^-13 to 1 with stable isotopes. Chlorine 36 disintegrates mainly by β- disintegration in Argon 36 (Ar-36) (98.1%), but also by disintegration ε in Sulfur 36 (S-36) (1.9%).

Chlorine 36 is readily measured by liquid scintillation.