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Confused about nuclear binding energy

  1. May 27, 2010 #1
    hi there,

    now I've been googling around for ages now trying to find this answer but i cant

    i have read time and time again that a nucleus always has less mass than the sum of the masses of the nucleons in it. This is put down to the binding energy, this is where i get confused

    surely if it takes energy (binding energy) to hold the nucleus together and energy = mass then the nucleus would have more mass than the sum of the masses of the nucleons wouldnt it?
    because its mass would be the mass of each of the nucleons plus the mass of the binding energy given by e=mc^2

    can someone please explain how this works?

    and another thing....

    i know that the energy released from fission and fusion is because the total binding energy of the resultant nuclei is less than the binding energy of the original nuclei, but why is it less? and why is iron/nickel the 'threshhold'

    any help greatly apreciated

  2. jcsd
  3. May 27, 2010 #2
    binding energy counts as negative, just as potential energy

    less means more negative thus strongly bound (here you have the hint for the answer for the first question!)

    why Iron/Nickel : some nucleus has to be the most stable one
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