Here's why:

* Atomic Number: The atomic number of an element defines the number of protons in its nucleus. Fluorine's atomic number is 9.

* Isotopes: The number "19" in "Fluorine-19" refers to its mass number, which is the total number of protons and neutrons in the nucleus. While the number of neutrons can vary (creating isotopes), the number of protons always remains the same for a given element.