The zone of highest stability lies between Fe-56 and Br-66.

The curve passes through a maximum at iron-56. The nuclei become less stable as the mass numbers become higher therefore we can expect atoms with higher atomic numbers to fission and break into smaller more stable isotopes, On the other hand, it would be the fusion of the lower atomic number nuclei which would result in more stable isotopes. For this reason it is H and He with which fusion attempts are being made. 

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