Binding energy per atom

Hello! I am trying to understand where does the energy come from in fusion and fission reactions. Looking at the table, the more stable atoms are the ones with the highest binding energy- the atoms that fission and fusion reactions produce because both of those reactions move towards stable atoms. I also learned that finding the binding energy of an atom is possible through calculating the mass defect. The higher the mass defect, the higher the binding energy. That means that the stable atoms have a higher mass defect. Here's what I can't get figured out: if an atom with a smaller mass defect produces an atom with a bigger mass defect, it means that some mass turned into energy. Is the energy that came from the mass, released? Isn't this energy used to keep the atom together, meaning it turned into binding energy? Also, how does the mass "know" to turn into energy and what if it doesn't? Any help would be greatly appreciated since it's a lot of (con)fusion... (I know, bad joke) Thank you!

  • $\begingroup$ Look closely at the definition of binding energy. There is a sign convention at work here. $\endgroup$ Commented Jun 24, 2018 at 16:21
  • $\begingroup$ The binding energy is a negative potential energy. Higher negative actually is less than lower negative. Also, mass does not "turn" to energy; instead mass is energy stored in an object. For example, a heated object is heavier than a cold one, because molecules move faster. The kinetic energy of molecules does not "turn" to mass; it is mass. It is incorrect to think that mass and energy are two different concepts, they are the same. In relativity, the invariant ("rest") mass is the energy moving slower than light. $\endgroup$
    – safesphere
    Commented Jun 24, 2018 at 16:32

1 Answer 1


Mass of an atom can be calculated by adding the masses of its constituents, protons, neutrons, and electrons. The problem is that the real value is less. The difference is what is called the mass defect, correspondingly the more the mass defect, the stronger the atom bound, because to unbind it you have to supply the missing energy, and with fission and fusion you effectively turn some mass into energy.


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