Nuclear fission

From Citizendium, the Citizens' Compendium

Nuclear fission takes place when the nucleus of a "heavy" (i.e., high atomic number) element absorbs a neutron and splits into two or more nuclei of lighter elements, with the release of substantial energy. Not all heavy element nuclei will split under neutron bombardment; it is specific isotopes of heavy elements that are fissionable.

The actual bombardment is considered part of physics, but the new nuclei, which attract electrons and form new atoms of the lighter elements, are detectable by methods of chemistry.