Thomson determined the charge to mass ratio of electrons in his cathode ray tube experiments. The difference between the mass number of an atom and the atomic number is always equal to the number of protons. The mass number of an atom is the number of neutrons in the atom. The number of electrons in a neutral atom of an element is always equal to the atomic number of the element.
Atomic number is the number of protons in the nucleus of an atom.
The mass of a proton and a neutron are virtually identical. The charge of an electron and a proton are equal but opposite. The mass of the electrons is a small fraction of the total mass of any atom. All are correct. I and III are correct but II is false.