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So I'm currently trying to wrap my head around nuclear fusion, but there is something that keeps bugging me: I've read in many places that when two hydrogen atoms combine, the resulting helium atom has a mass inferior to the sum of the mass of the two original atoms, because this mass has been given off as energy. But why? Why does some of the mass become energy? Is it something that we don't know yet? I've read about binding energy as well, but I cannot understand how it relates to energy being released during a fusion, or what it even is.

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  • $\begingroup$ $E = mc^{2}$ - nuclear binding energy is often expressed as mass, because we can measure the mass of nuclei quite well. (Chemical bonds also change the mass of the molecule from that of the separated atoms, just to a small extent). $\endgroup$ – Jon Custer Aug 31 '16 at 13:41
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    $\begingroup$ Possible duplicate of How to explain $E=mc^2$ mass defect in fission/fusion $\endgroup$ – Crimson Aug 31 '16 at 13:49
  • $\begingroup$ minor nitpick, you need 4 hydrogen atoms to produce 1 helium atom. 2 hydrogen atoms would produce helium-2, which I'd assume would be extremely unstable $\endgroup$ – Jim Aug 31 '16 at 13:58