Answer: Number of protons.
- A Number of neutrons
- B Number of protons
- C Mass number
- D Number of electrons in outer shell
Correct answer: B. Number of protons
Explanation: Atomic number Z = number of protons in the nucleus. It uniquely identifies an element.
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.