What makes up the mass number of an atom?
molecules and electrons
electrons and neutrons
atoms and neutrons
protons and neutrons
The mass number is the number of protons plus the number of neutrons in an atom. For example, most atoms of helium have 2 neutrons, so their mass number is 2 + 2 = 4. This mass number means that an atom of helium has a mass of about 4 amu.