How do I decide whether a nuclear reaction releases energy using atomic masses?
Calculate Q = (total initial mass - total final mass)c^2.
Calculate Q = (total initial mass - total final mass)c^2. With neutral atomic masses, electron counts must balance on both sides or be handled explicitly. Positive Q means products have lower rest mass and the reaction releases energy; negative Q is the minimum energy deficit before momentum constraints. Never compare just one nucleus or ignore emitted particles.
Key point
Q > 0 releases energy; include every reactant and product with balanced electron accounting.
Common mistake
reversing final and initial masses.
Memory tip
mass lost from rest mass appears as released energy.
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