Most particles passed through the foil as if through empty space. The gold foil was bombarded with positively charged alpha particles, having a mass four times that of a proton. The mass number of an atom is the number of neutrons plus protons in the atom. The neutron has nearly the same mass as the proton but zero charge. Two elements differ from one another by having a differing mass number.
Atomic number is the number of protons in the nucleus of an atom.
The mass of the electrons is a small fraction of the total mass of any atom. The element Fluorine has only one really stable isotope.