Fluorine-19 is a stable isotope of the element fluorine, which has an atomic number of 9. It is the only naturally occurring isotope of fluorine, making up nearly 100% of all fluorine found in nature. Fluorine-19 has 10 neutrons and 9 protons in its nucleus, giving it an atomic mass of approximately 19 atomic mass units. This isotope is significant in various applications, particularly in nuclear magnetic resonance (NMR) spectroscopy, where it is used to study the structure of organic compounds. Additionally, fluorine-19 plays a role in medical imaging, especially in positron emission tomography (PET) scans, where it can be used as a tracer.