Nuclear Fission and Fusion

Atoms have tiny centers called nuclei. Nuclear fission and nuclear fusion are two different ways nuclei can release large amounts of energy.

Nuclide notation

We write nuclei as $^{A}_{Z}\!X$, where $A$ is the nucleon (mass) number and $Z$ is the proton (atomic) number.

Nuclear fission: splitting heavy nuclei

Fission is when a heavy nucleus (like uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing 2–3 more neutrons and energy.

Example fission equation:

$$^{235}_{92}\!U + \,^{1}_{0}\!n \;\to\; ^{141}_{56}\!Ba + \,^{92}_{36}\!Kr + 3\,^{1}_{0}\!n + \text{energy}$$

• Chain reaction: the new neutrons can cause more fissions.
• Control in reactors: control rods absorb extra neutrons; a moderator slows neutrons so fission happens steadily; shielding protects people.
• Use: nuclear power stations convert the released energy to heat, then to electricity.

Nuclear fusion: joining light nuclei

Fusion is when two light nuclei join to make a heavier nucleus. This happens in the Sun, where hydrogen nuclei fuse into helium.

Common fusion example (deuterium + tritium):

$$^{2}_{1}\!H + \,^{3}_{1}\!H \;\to\; ^{4}_{2}\!He + \,^{1}_{0}\!n + \text{energy}$$

• Conditions: extremely high temperature and pressure are needed to overcome electrostatic repulsion between positive nuclei.
• Use: powers stars; scientists are researching reactors for future electricity generation.

Why energy is released

In both fission and fusion, the total mass of the products is slightly less than the mass of the reactants. The “missing mass” becomes energy: $E = mc^2$.

Comparing fission and fusion

• Fission: heavy nucleus splits; used in power stations; produces long-lived radioactive waste; easier to do on Earth.
• Fusion: light nuclei fuse; powers the Sun; produces very high energy with less long-lived waste; hard to achieve on Earth.

Common misconceptions

• Nuclear reactions change the nucleus, not electrons; they are different from chemical reactions.
• The Sun’s energy comes from fusion, not fission.
• “Mass disappears” is not correct; it is converted to energy according to $E = mc^2$.