An isotope of the element fluorine has 9 protons and 10 neutrons. what is the name of this isotope? fluorine-

The name is fluorine-19.

Usually, in the names of isotopes, we say the name of the element followed the mass number (eg. Carbon-12). The mass number is the sum of the number of protons and neutrons of that atom. Electrons are not included since they're too light compared to protons and neutrons. 

Therefore, to find out the name of the isotope, we have to find the mass number. Add up 9 and 10, which makes 19, so, the answer is fluorine-19.