An atom becomes unstable when the forces within its nucleus are unbalanced, specifically when there is an excess of protons or neutrons. This imbalance causes the nucleus to have excess internal energy. To reach stability, the unstable atom may emit particles such as protons, neutrons, or radiation (alpha, beta, and gamma rays), a process known as radioactive decay. The key factor behind instability is that the binding energy— the energy that holds the nucleus together via the strong nuclear force—is insufficient to permanently keep the nucleus intact. When the electromagnetic repulsion among protons overcomes the strong nuclear force due to an imbalance in the neutron- to-proton ratio, the nucleus becomes unstable and radioactive.

In summary, an atom is unstable because:

• It has too many or too few neutrons relative to protons.
• The nuclear binding energy is too low to hold the nucleus together.
• The strong nuclear force cannot fully counteract the repulsive electromagnetic force among excess protons.
• The nucleus undergoes radioactive decay emitting particles to reach a more stable state.