Question

when it is said to be the nucleus is unstable

Answer 1

The nucleus of this kind of atom is said to be stable. In some atoms the binding energy is not strong enough to hold the nucleus together, and the nuclei of these atoms are said to be unstable. Unstable atoms will lose neutrons and protons as they attempt to become stable.

Answer 2

In some atoms the binding energy is not strong enough to hold the nucleus together, the nuclei of this atom is said to be unstable.
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