Final answer:
The isotope of fluorine with a mass number of 19 and atomic number of 9 has 9 protons.
Step-by-step explanation:
The atomic number of an element represents the number of protons in the nucleus of an atom. In the case of fluorine, since its atomic number is 9, it means that there are 9 protons in its nucleus. The mass number, on the other hand, is the sum of the protons and neutrons in the nucleus.
So in this isotope of fluorine, since the mass number is 19, there must be 10 neutrons (19 - 9) in addition to the 9 protons.
So therefore the isotope of fluorine with a mass number of 19 and an atomic number of 9 has 9 protons. This is because the atomic number defines the element and the number of protons determines the element's identity. In the case of fluorine, the atomic number of 9 means it has 9 protons.