Isotopes of chlorine 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 principal stable isotopes, Cl (75.78%) and Cl (24.22%), 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 20 and 30 nanoseconds, respectively—the half-life of Cl is unknown. Bromine is the only other halogen with more than one stable isotope. Trace
Fluorine other elements from carbon to magnesium are twenty or more times as common. This is because stellar nucleosynthesis processes bypass fluorine, and any fluorine atoms otherwise created have high nuclear cross sections, allowing further fusion with hydrogen or helium to generate oxygen or neon respectively. Beyond this transient existence, three explanations have been proposed for the presence of fluorine: Fluorine is the thirteenth most common element in Earth's crust at 600–700 ppm (parts per million) by mass. Elemental fluorine in Earth's atmosphere would easily react with atmospheric water vapor, precluding its natural occurrence; it is found only in combined mineral