Well, since electrons have approximately zero mass an atom or an ion will have approximately the same mass. Mass of an atom is determined by the protons and neutrons in the nucleus, which each have a mass of 1 atomic mass unit (or 1 gram per mole as the mole is defined). Like I said the mass of an electron is approximately zero, however in reality it is 0.00055 atomic mass units at rest. So, if we’re being super nitpicky an ion with more electrons (so a negative ion) will weight slightly more than a neutral ion, while a positive ion (less electrons) will weigh slightly less.