Xenon has an atomic number of 54. A particular isotope of xenon has a mass number of 131. How many protons and how many neutrons does each atom of that isotope have?
The mass of an atom depends on the number of massive particles. These include (i) the protons, fundamental nuclear particles with unit positive charge, and (ii), the neutrons, fundamental neutral particles with ZERO electronic charge.
Now we have elemental xenon, and thus we KNOW that the nucleus contains
Because electrons have to a first approximation negligible mass, we don't really have to consider electronic mass for this problem.