Answer: Energy and momentum conservation (and lepton number).
- A Charge conservation, which is already satisfied by the electron alone
- B Conservation of total rest mass before and after the decay
- C Energy and momentum conservation (and lepton number)
- D Baryon number conservation, which is unaffected by lepton emission
Correct answer: C. Energy and momentum conservation (and lepton number)
Explanation: Antineutrino is needed to conserve energy (continuous electron spectrum), momentum, angular momentum, and lepton number in beta decay. Pauli proposed it 1930.
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.