Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: An atom's mass number is its mass in atomic mass units (amu), which is about equal to the total number of protons and neutrons in the atom. Different isotopes of an element have different mass numbers because they have different numbers of neutrons. Answer: neutrons
A:
Different isotopes of an element have different mass numbers because they have different numbers of what?