Why do heavy atomic nuclei have a tendency to become more stable by fission?

Heavy atomic nuclei tend to become more stable through nuclear fission because they can achieve a higher binding energy per nucleon by splitting into smaller fragments.12

This process is best understood using the liquid-drop model, which treats the nucleus as a charged drop of liquid. In this model, stability is determined by a balance between two competing forces:34
* Short-range nuclear forces: These attractive forces between nucleons promote stability and surface tension, which favors a spherical shape.24
* Coulomb repulsion: The long-range electrostatic repulsion between positively charged protons acts to destabilize the nucleus.24

As the number of nucleons increases, the repulsive force between protons grows. For heavy nuclei, this repulsion becomes significant enough that the system can reach a more stable, lower-energy state by deforming and eventually splitting into two smaller, more tightly bound nuclei. The difference in binding energy between the original heavy nucleus and the resulting fragments is released as energy during the fission process.21