Here's why:

* Atomic Number: Fluorine has an atomic number of 9, which means it has 9 protons in its nucleus.

* Mass Number: The mass number is the sum of protons and neutrons in an atom's nucleus.

* Neutrons: Since the most common isotope of fluorine has 10 neutrons (19 - 9 = 10), its mass number is 19.

Important Note: While the most common isotope of fluorine has a mass number of 19, there are other isotopes with slightly different mass numbers. However, the most common isotope is the one that determines the element's standard atomic weight.