Nuclear fission

The CED gives fission one sentence, at 15.7.A.7: the nucleus of an atom splits into two or more smaller nuclei, as well as subatomic particles. That final clause matters: a balanced reaction carries more on the product side than two nuclei.

Where the energy comes from is 15.7.A.4. Mass and energy may be exchanged in any nuclear reaction through $E = mc^2$, so if the total mass of the products is lower than the total mass going in, the shortfall leaves as energy, either as kinetic energy of the products or as photons (15.7.A.5). In binding energy terms, energy is released when the fragments are more tightly bound per nucleon than the nucleus they came from.

Nucleon number is conserved (15.7.A.2), and so are energy and momentum (15.7.A.3). Those rules are what a balancing question actually tests.

The statement students skip is 15.7.A.8: fission may occur spontaneously or may require an energy input, depending on the binding energy of the nucleus. Releasing energy and starting on its own are separate questions, and answering the second when you were asked the first loses a mark for true physics.

Chain reactions, critical mass and reactor design appear nowhere in the framework. Set this beside nuclear fusion, which runs the same energy argument in the other direction.