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I understand why fission generates large amounts of energy when the nucleus is split, but then why does fusion generate such large amounts of energy. If fission releases energy when some mass is lost as energy, then shouldn't the fusion process absorb energy to fuse nuclei together?

I also am curious as to where the energy released from fusion comes from, while fission releases some of the energy of the strong nuclear force.

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Ok thanks, I thought it might fit here because it is nuclear chemistry, but I'll ask it on physics. –  Cameron Jun 10 '13 at 21:44
    
I'm afraid not. There is no such thing as nuclear chemistry. Chemistry almost exclusively deals with the outer shell of electrons of atoms. Anything going on in the nucleus or between nuclei is physics. –  Tanith Rosenbaum Jun 13 '13 at 19:41
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Nuclear chemistry is a bit of a misnomer - it means the chemistry of radioactive elements, techniques for handling radioactive materials safely, techniques for getting short-lived isotopes out of the reactor/cyclotron and into the right chemical form quickly before they decay etc. –  user1915639 Jun 13 '13 at 20:26
    
This wiki article may help you understand, en.wikipedia.org/wiki/Nuclear_binding_energy, particularly the binding energy per nucleon plot. –  anna v Jun 15 '13 at 16:17
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migrated from chemistry.stackexchange.com Jun 15 '13 at 15:20

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Fission is exothermic only for heavy elements, while fusion is exothermic only for light elements. Intermediate nuclei, in the iron/nickel range, are the most tightly bound, and so you generally release energy moving in that direction.

Fusing stable elements into uranium would consume energy, as would trying to break helium into hydrogen.

For a more thorough background, see for instance this post.

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