After scientists discovered electrons and protons, what led them to believe there was a third subatomic particle? a. elements have molar masses that are numerically equal to their atomic mass. b. the mass of an atom's nucleus is greater than the combined masses of an atom's protons. c. the mass of an atom is greater than the combined masses of an atom's protons and electrons. d. elements have isotopes that have the same atomic number but different atomic mass numbers.