Here's a breakdown:

* Fissionable Nucleus: The process starts with a specific type of atom, usually a heavy atom like Uranium-235 or Plutonium-239.

* Neutron Absorption: A neutron strikes the nucleus of the fissionable atom.

* Instability and Splitting: This absorption causes the nucleus to become unstable and split into two or more lighter nuclei, called fission products.

* Energy Release: Along with the fission products, a tremendous amount of energy is released in the form of kinetic energy of the fission products and neutrons, as well as gamma rays.

So, while the process is initiated by a neutron striking the atom, the actual splitting and energy release happen within the nucleus itself.