Fluorine (9F) has 18 known isotopes ranging from to (with the exception of) and two isomers (and). Only fluorine-19 is stable and naturally occurring in more than trace quantities; therefore, fluorine is a monoisotopic and mononuclidic element.
The longest-lived radioisotope is ; it has a half-life of . All other fluorine isotopes have half-lives of less than a minute, and most of those less than a second. The least stable known isotope is, whose half-life is, corresponding to a resonance width of .
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See main article: Fluorine-18. Of the unstable nuclides of fluorine, has the longest half-life, . It decays to via β+ decay. For this reason is a commercially important source of positrons. Its major value is in the production of the radiopharmaceutical fludeoxyglucose, used in positron emission tomography in medicine.
Fluorine-18 is the lightest unstable nuclide with equal odd numbers of protons and neutrons, having 9 of each. (See also the "magic numbers" discussion of nuclide stability.)
Fluorine-19 is the only stable isotope of fluorine. Its abundance is ; no other isotopes of fluorine exist in significant quantities. Its binding energy is . Fluorine-19 is NMR-active with a spin of 1/2+, so it is used in fluorine-19 NMR spectroscopy.
Fluorine-20 is an unstable isotope of fluorine. It has a half-life of and decays via beta decay to the stable nuclide . Its specific radioactivity is and has a mean lifetime of .
Fluorine-21, as with fluorine-20, is also an unstable isotope of fluorine. It has a half-life of . It undergoes beta decay as well, decaying to, which is a stable nuclide. Its specific activity is .
Only two nuclear isomers (long-lived excited nuclear states), fluorine-18m and fluorine-26m, have been characterized. The half-life of before it undergoes isomeric transition is . This is less than the decay half-life of any of the fluorine radioisotope nuclear ground states except for mass numbers 14–16, 28, and 31. The half-life of is ; it decays mainly to its ground state of or (rarely, via beta-minus decay) to one of high excited states of with delayed neutron emission.