Only fluorine-19 is stable and naturally occurring in more than trace quantities; therefore, fluorine is a monoisotopic and mononuclidic element.
The least stable known isotope is 14F, whose half-life is 500(60) yoctoseconds,[4] corresponding to a resonance width of 910(100) keV.
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.
[4] 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.