Imagine a big chocolate bar 🍫 that you want to share. You break it into smaller pieces, each still tasty but easier to handle. In the nucleus, a heavy atom can be split into lighter nuclei, releasing a lot of energy in the process. This is nuclear fission.
| Reactant | Products |
|---|---|
| \$^{235}\text{U} + n\$ | \$^{141}\text{Ba} + ^{92}\text{Kr} + 3n\$ |
In the equation above, a uranium‑235 nucleus captures a neutron and splits into barium‑141, krypton‑92 and three free neutrons. The key point is that the total mass of the products is slightly less than the mass of the original nucleus plus the neutron. The missing mass, called the mass defect, is converted into energy:
\$ m{\text{initial}} > m{\text{final}} \quad \Rightarrow \quad E = \Delta m\,c^2 \$
Because the mass defect is tiny, the energy released is huge – that’s why nuclear reactors can produce a lot of electricity from a small amount of fuel.
Now think of building a Lego tower. You start with small blocks and stick them together to create something bigger and stronger. In the nucleus, light atoms can combine to form a heavier nucleus, releasing energy in the process. This is nuclear fusion.
| Reactants | Products |
|---|---|
| \$^{2}\text{H} + ^{3}\text{H}\$ | \$^{4}\text{He} + n\$ |
Here, a deuterium nucleus (\$^{2}\text{H}\$) and a tritium nucleus (\$^{3}\text{H}\$) fuse to form a helium‑4 nucleus and a free neutron. The mass of the helium‑4 nucleus is slightly less than the combined mass of deuterium and tritium. The missing mass again becomes energy:
\$ m{\text{initial}} > m{\text{final}} \quad \Rightarrow \quad E = \Delta m\,c^2 \$
Fusion powers the Sun and stars because the extreme temperatures and pressures allow light nuclei to overcome their repulsive forces and join together. On Earth, scientists are working to create controlled fusion reactions that could provide a clean, almost limitless energy source.