Answer: Mass of nucleus - sum of component nucleon masses.
- A Mass of nucleus - sum of component nucleon masses
- B Usually exactly zero for any stable or unstable nucleus
- C Usually negative in magnitude for every known nucleus
- D The mass of the constituent protons mainly, ignoring neutrons
Correct answer: A. Mass of nucleus - sum of component nucleon masses
Explanation: Mass defect: delta_m = Z m<sub>p</sub> + N m<sub>n</sub> - M<sub>nucleus</sub>. This is positive for stable nuclei. Binding energy = delta_m × c².
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.