A nucleus becomes unstable mainly due to an imbalance between the strong nuclear force and electrostatic repulsion among protons, and an improper neutron-to-proton ratio. Specifically, nuclei are unstable when:

• The electrostatic repulsion between protons becomes too great for the strong nuclear force to hold the nucleus together, especially in large nuclei with high atomic numbers.
• The neutron-to-proton ratio is not within a stable range. For lighter elements, a roughly 1:1 ratio is stable, but larger nuclei require more neutrons than protons to offset the increasing electrostatic repulsion.
• An excess of neutrons can also cause instability because neutrons fill higher energy levels, which can make it energetically favorable for the nucleus to undergo beta decay to convert neutrons into protons.
• As the nucleus becomes larger, the strong nuclear force becomes less uniformly attractive and can become repulsive if nucleons are too close, causing further instability.

In summary, nuclear instability arises from an imbalance of forces inside the nucleus and an unfavorable neutron-to-proton ratio, leading to radioactive decay as the nucleus attempts to reach a more stable state.