Given the question: Answer the following question given this paragraph: 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. Q: Different isotopes of an element have different mass numbers because they have different numbers of what? Choices: - protons - electrons - nuclei - neutrons A:
The answer is:
neutrons