What is the nature of nuclear energy? This is closely related to the correct explanation of mass defect.
I did some research of that topic and cannot come to a single comprehensive and consistent description.
Below are related statements I gathered or can think of, describing the problem area. To me, each of them seems to make sense, but some of them are contradictory, so obviously - wrong. Please kindly point out and explain these errors.
The more nucleons in a nucleus, the bigger the nucleus is, so the average distance of a nucleon to each another is higher, hence the long-distance electromagnetic repulsion tends to overcome short-distance strong nuclear force, up to the point of occasional alpha decay in elements of transuranic end of the spectrum.
The less nucleons in a nucleus, the closer in average they are, so the strong force per each is higher and easily overcomes electromagnetic repulsion.
Given (1) and (2), the smaller the nucleus, the stronger it is bound.
The stronger the nucleus is bound, the higher its binding energy.
The higher the binding energy of nucleus, the more energy is stored per nucleon in the system.
The higher the binding energy per nucleon, the more difficult it is to split the atom.
The more difficult it is to split the atom, the more stable the atom is.
The more energetic a binding, the more difficult it is to break the binding, the more stable the atom made out of such bindings.
Natural systems tend to evolve to lower, not higher energy states.
The literature presents the binding energy chart per element, with its peak at iron (~56 nucleons). Both the lighter and heavier elements tend to have smaller binding energy.
Iron is the most stable element. It is abundant in the universe, as natural atomic evolution tends to get close to it from both ends of atomic number spectrum.
The surplus energy in the nuclear reaction is achieved when heavy elements are split (fission), or light element are fused (fusion).
The surplus energy is the energy taken out of the system, i.e. average energy per nucleus is higher before the reaction and lower after the reaction.
After the reaction - the resulting elements are closer to the iron atomic number
Mass defect is directly proportional to the binding energy. The stronger the binding energy per nucleon, the less mass per nucleon.
Example for neutron, proton and them bound together in deuterium:
$$
\begin{array}{c}
\begin{alignat}{7}
m_\text{n}&=1.008665 \, \mathrm{u} & \hspace{50px} & m_\text{p}=1.007276 \, \mathrm{u} \\
m_{\text{n}+\text{p}}&=2.015941 \, \mathrm{u} & & m_\text{d}=2.013553 \, \mathrm{u}
\end{alignat} \\[5px]
\Delta_m =0.002388 \, \mathrm{u} = 2.224\ \frac{\mathrm{MeV}}{c^2}
\end{array}
$$
The explanation of below (seemingly) contradictions somehow eludes me. Hope it is apparent to you:
why the lightest elements are not the ones which are most strongly bound? Possibly the peak at iron might be caused the geometry factor - i.e. when accounting for nucleons 3D positions, the average forces between them are no longer proportional to just the number of nucleons.
why does iron seem to need in its properties: the highest energy per nucleon as the most strongly bound, most stable element; and on the other hand: the lowest energy per nucleon, for the surplus energy be given off when approaching iron in fission/fusion reactions from either side of atomic number spectrum?
Also - what is the definition and explanation of the mass defect:
mass defect is the surplus energy given off from fusion/fission and hence is the difference between total mass-energy of the system before and after reaction
mass defect comes from different proportion of mass vs energy in an atom depending on its binding energy. When the binding energy is higher in an atom, the more of the total mass-energy of the system is stored in the binding of the nucleons and less in their mass - and the total stays the same. If so then why does the total mass-energy change after the split/fuse reaction?
