Confused about nuclear binding energy

[metalmaniac]

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 wouldn't 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

thanks

 

[ansgar]

binding energy counts as negative, just as potential energyless 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