has 17 known isotopes, with atomic masses ranging from 14F to 31F, and two isomers. Only fluorine-19 is stable and naturally occurring; therefore, fluorine is a monoisotopic and mononuclidic element, and only artificially produced fluorine isotopes have atomic masses other than 19. The longest-lived radioisotope is 18F; it has a half-life of 109.739 minutes. 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 14F, whose half-life is 500 × 10−24 seconds, corresponding to a spectral linewidth of about 1 MeV.
Of the unstable nuclides of fluorine, 18F has the longest half-life, 109.739 minutes. It has two decay modes, of which the main one is positron emission. For this reason 18F is a commercially important source of positrons. Its major value is in the production of the radiopharmaceuticalfludeoxyglucose, used in positron emission tomography in medicine. Like all positron-emitting radioisotopes, 18F also may decay by electron capture. In either case, 18F decays into 18O. The two decay modes do not happen equally frequently however; 96.86% of the decays are by beta plus emission and 3.14% by electron capture. Fluorine-18 is the lightest unstable nuclide with equal odd numbers of protons and neutrons, having 9 of each.
Fluorine-19
Fluorine-19 is the only stable isotope of fluorine. Its abundance is 100%; no other isotopes of fluorine exist in significant quantities. Its binding energy is 147801 keV. Fluorine-19 is NMR-active with spin of 1/2, so it is used in fluorine-19 NMR spectroscopy.
Fluorine-20
Fluorine-20 is one of the more unstable isotopes of fluorine. It has a half-life of 11.07 seconds and undergoes beta decay, transforming into its daughter nuclide20Ne. Its specific radioactivity is 1.885 × 109 TBq/g and has a lifetime of 15.87 seconds.
Fluorine-21
Fluorine-21, as with fluorine-20, is also one of unstable isotopes of this element. It has a half-life of 4.158 seconds. It undergoes beta decay as well, which leaves behind a daughter nuclei of 21Ne. Its specific radioactivity is 4.78 × 109 TBq/g.
Isomers
Only two nuclear isomers, fluorine-18m and fluorine-26m, have been characterized. The half-life of 18mF before gamma ray emission is 162 nanoseconds. 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 26mF is 2.2 milliseconds; it decays mainly to the ground state of 26F or to one of high excited states of 26Ne with delayed neutron emission.
