Fluorine-19
Although fluorine (F) has 18 known isotopes from 14F to 31F and one isomer (18mF), only one of these isotopes is stable, that is, fluorine-19; as such, it is a monoisotopic element. The longest-lived radioisotope is 18F with a half-life of 109.771 minutes. All other isotopes have half-lives under a minute, the majority under a second, making fluorine a mononuclidic element as well. The least stable isotope is 15F, whose half-life is 4.1 x 10−22 seconds, corresponding to a spectral linewidth of about 1 MeV. Only 14F has an unknown half-life.
Relative atomic mass: 18.9984032(5)
The nuclide 18F is the radionuclide of fluorine with the longest half-life, 109.771 minutes, allowing it to serve commercially as an important source of positrons. Its major use is for the production of the radiopharmaceutical fludeoxyglucose for positron emission tomography scanning in medicine.
Like all positron-emitting radioisotopes, 18F also has a probability to decay by electron capture. In this case, 18F decays into 18O, 96.86 (19)% of the time by beta plus (positron) emission and 3.14 (19)% by electron capture.
It is the lightest unstable nuclide with equal odd numbers of protons and neutrons, 9 of each. (See also the "magic numbers" discussion of nuclide stability.)
Fluorine-19, 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, so it is used in fluorine-19 NMR spectroscopy.
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