Answer: Number of protons (atomic number).
- A Mass number, which actually differs between isotopes
- B Number of neutrons, which actually differs between isotopes
- C Number of protons (atomic number)
- D Mass number, neutron number, and proton number all at once
Correct answer: C. Number of protons (atomic number)
Explanation: Isotopes of an element have the same atomic number Z (same protons) but different mass numbers (different neutrons).
Binding energy per nucleon rises sharply for light nuclei, peaks around iron-56 (the most stable nucleus), then slowly declines for heavier nuclei - which is exactly why fusing light nuclei or splitting heavy nuclei both release energy: each moves the products toward this stability peak.
Concept context
Radioactivity, nuclear reactions, fission, fusion, and binding energy.