A nucleus with a larger number of protons than neutrons (or vice versa) can become unstable due to the imbalance in the forces within the nucleus. The nucleus is composed of protons and neutrons held together by the strong nuclear force, which must counteract the electrostatic repulsion between the positively charged protons. Neutrons provide a stabilizing effect by reducing the repulsion among protons without adding charge. If the ratio of protons to neutrons deviates far from the optimal range, the nuclear strong force cannot adequately bind the nucleons together because:

• Too many protons: electrostatic repulsion overwhelms the strong force.
• Too few neutrons: insufficient neutron "glue" to stabilize the repulsive forces.

This imbalance leads to nuclear instability, causing the nucleus to undergo radioactive decay to reach a more stable configuration.

Thus, the reason a nucleus with an unusual proton-to-neutron ratio is unstable is that the nuclear forces can no longer effectively counterbalance the repulsive electric forces, leading the nucleus to seek stability by changing its composition through decay processes.