How can the atomic number and mass number be used to determine the amount of each subatomic particle present in an atom of a given element?

Because of the environment that

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.

Hairy thick gross thing? idk