Nuclear Binding Energy
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The actual mass of an atomic nucleus is always a little smaller than the sum of the rest masses of all its nucleons (protons and neutrons).
This is because some of the mass of the nucleons were changed into energy needed to form the nucleus.
This energy is called binding energy.
The higher the binding energy, the more stable the nucleus is.
A graph of binding energies vs. mass numbers would show that the binding energy increases as the mass number gets higher until around mass number 60, then it starts to decline from that point on.
A region of greatest stability is on the peak of the curve, around a mass number of 55 to 80.
Unstable elements with a mass number less than the region tend to undergo fusion (the combination of nuclei) to reach the region.
Unstable elements with a mass number greater than the region tend to undergo fission (the splitting of nuclei) to reach the region.