Which nucleus would be least likely to undergo radioactive decay?

Stable nuclei will not undergo in nuclear decay reactions. Stability of nucleus depends upon the force between nucleons.

If there is strong attraction force between the nucleons then the repulsion force between the protons will be neglected over this strong force and the nuclei is stable.

Also we can check the stability of nucleus by the ratio of neutrons and protons in the nucleus.

here the stability of nucleus is more it the ratio lie between 1 and 1.2 i.e.

so the neutron to proton ratio will decide the stability of nucleus.

So it the ratio lie for above range of neutrons and protons then it is least likely to go radioactive decay.

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