Atomic bomb. Colored image of an atomic bomb explosion. Atomic or nuclear weapons work by releasing the energy within the nucleus of atoms. The simplest atomic bombs use plutonium or a specific type (isotope) of uranium. These are fissile materials, meaning that if they absorb a neutron they will split into smaller atoms, releasing energy and more neutrons. These neutrons can cause more fissions in what is called a 'chain reaction'. More sophisticated atomic bombs also contain certain isotopes of hydrogen. These hydrogen bombs (H-bombs) use the heat created by a chain reaction to fuse together the hydrogen atoms, releasing even more energy.