[...] mass convert to energy
That's a common formulation but it's misleading. A better one could look like "every energy has a mass".
Kinetic energy for instance increases the mass of a moving object. Well, some Relativity specialists call "mass" only the rest mass and use some longer wording for mass+energy. I don't care.
The strong interaction that keeps the protons and neutrons together has a mass. Because it's attractive, the energy of the composite nucleus is less than its separate constituents, and so is the mass of the nucleus. If
4He is formed from two protons and two neutrons (over a complicated reaction path), the energy of the helium is less and the difference gets released during its formation.
The electrostatic interaction between the protons has a mass too. This one is repulsive, so it increases the nucleus' mass. Nuclei exist only because the strong interaction overwhelms the electrostatic repulsion, but as bigger nuclei have more and more protons, the electrostatic repulsion increases more quickly, so nuclei can't be too big. You observe that the nucleus' mass defect per nucleon decreases beyond iron.
Beyond lead, well before the size where nuclei would explode into protons and neutrons, they find ways to split into more favourable rearrangements. The most common one is to expel an alpha particle, that's alpha radioactivity. Sometimes heavy nuclei split into two or three smaller ones, that's fission.
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Topic: Relative isotopic masses are not whole numbers (Read 3175 times)