Why is energy released during decay? 
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*Why is energy released when an atom decays into another atom, even though no energy is added?

*What does the mass defect mean?


Is it because a nucleus which decays is unstable (proton/neutron = 1)? For example, in large nucleons the electric force becomes dominant, therefore, it wants to get a better binding energy and decays, I think. Where does the required energy come from? It can go over the potential well, so it should tunnel, I suppose. 
Is the energy release (mass defect) given by the relationship between the change in binding energy and mass via $E=mc^2$ where the daughter neuclus has a lower potential energy?
How is it even possible that an atom decays when no energy is added? Must it tunnel? How does it get over the barrier of the potential well? 
I read this : How to explain $E=mc^2$ mass defect in fission/fusion
but i still don't understand it.
 A: 
Why is energy released when an atom decays into another atom, when no energy is added?

Atoms/nuclei are already created when we study them and organize them into the periodic table of elements. 
At the level of nucleons and elementary particles in general, special relativity holds. When we look at the periodic table of elements and count the number of neutrons and protons in a nucleus, adding up their masses ends up in a higher number than the mass of that nucleus. This is due to special relativity that binds the nucleons into a nucleus  at the creation of that element, by the nuclear force , into bound energy levels.  This is the nuclear binding energy curve:

What does a decay of a nucleus mean? It means that the binding energy level holding the nucleons together is unstable, has a lifetime for decay into smaller nuclei: the energy balance of before and after, adding masses, leaves energy over which goes to kinetic energy of the fragments and is observed.
No energy is added because it already exists from the creation of the nucleus.

What does the mass defect mean?

It means that  masses  are considered energy according to special relativity, and the energy budget has to take it into account as described above.
Now on how the original atoms were formed the dominant model is the Big Bang nucleosyntesis part, but that is another story.
