Question

when is the nulecus said to be 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

When the number of protons is equal to the number of neutrons