The core of an atom exists precariously:  massive repulsive electromagnetic forces between closely-assembled protons in the nucleus must be counterbalanced.  "Stability" thus reflects the balance of power between strong nuclear force, weak nuclear force, and electromagnetic force.  The nuclear binding energy quantifies the energy necessary to maintain coherence.

Isotopes are atoms with the same atomic number (proton count) but different atomic masses (number of neutrons). Heavier elements are more likely to have binding energies insufficient to maintain a stable nuclear configuration.  Such radioisotopes may undergo decay by emission of energetic quanta.  The following is a brief overview of both the photons and particles encountered in diagnostic and therapeutic Radiation Biology.