High Quality Content by WIKIPEDIA articles! Chlorine (Cl) has 24 isotopes with mass numbers ranging from 28Cl to 51Cl and 2 isomers (34mCl and 38mCl). There are two principal stable isotopes, 35Cl (75.78%) and 37Cl (24.22%), found in the relative proportions of 37.89:12.11, not 3:1, respectively, giving chlorine a standard atomic mass of 35.453, not 35.5. The longest-lived radioactive isotope is 36Cl 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 29Cl and 30Cl, wiith half-lives less than 20 and 30 nanoseconds, respectively—the half-life of 28Cl is unknown.