Large atomic nuclei are unstable because they lose neutrons and protons over time. A nucleus is comprised of uncharged neutrons and positively charged protons that, in large atoms, cannot be stably held together by the strong atomic force.
Particles with like charges, such as protons, repel one another. The strong atomic force counteracts the repulsive effects that protons have on one another within an atomic nucleus. Atomic nuclei with a low number of protons exhibit a correspondingly small repulsive force, while atomic nuclei with a high number of protons exhibit a much greater force. Scientists refer to the balance between repulsive electrostatic forces and binding atomic forces as the binding energy of a nucleus. The binding energy of large, unstable atomic nuclei is insufficient to keep the atom from radiating protons and neutrons over time.
Scientists refer to the particles ejected by unstable atomic nuclei as radiation. Radiation occurs at a predictable rate in some atoms, allowing scientists to use unstable atomic nuclei to determine the age of materials that contain them. Atoms with unstable nuclei continue to radiate particles until they reach a more stable state, in which the binding energy of the nucleus has enough power to hold the atom together.