Final answer:
When a neutron hits a U-235 atom, it causes nuclear fission, splitting the atom into two smaller nuclei, emitting extra neutrons and energy, which may cause a nuclear chain reaction if conditions allow.
Step-by-step explanation:
When a neutron collides with a U-235 atom, a process called nuclear fission begins. During this reaction, the nucleus of the U-235 atom absorbs the neutron, becoming U-236 which is highly unstable and quickly splits into two smaller nuclei, also known as fission fragments, while releasing additional neutrons and a significant amount of binding energy. This released energy can be harnessed for generating electricity or other applications. For example, if U-235 is bombarded with a neutron, a fission reaction might occur resulting in daughter nuclei like Barium (Ba) and Krypton (Kr), as well as additional neutrons that can perpetuate the reaction in other U-235 atoms if they don't escape from the material, leading to a nuclear chain reaction. If too many neutrons are lost and do not interact with more U-235 atoms, the chain reaction will not be sustained.