Fission and fusion sound like opposites — one breaks a nucleus apart, the other jams two together — and mechanically they are. But both release energy for the same underlying reason: a nucleus's binding energy per nucleon (each proton or neutron) varies depending on how large the nucleus is, and both processes move nuclei toward the size where binding energy per nucleon is highest, releasing the difference as energy.
One curve explains both reactions
Plot binding energy per nucleon against atomic mass, and the result is a curve that rises steeply for the lightest elements, peaks around iron and nickel, then declines slowly for heavier elements. That peak is the most stable configuration nuclear matter can be in. Very heavy nuclei, like uranium, sit on the declining side of the curve — splitting one into two mid-sized nuclei moves both fragments toward the peak, releasing energy. Very light nuclei, like hydrogen, sit on the steeply rising side — fusing two of them into a slightly heavier nucleus also moves toward the peak, also releasing energy. Elements near the peak, like iron, release energy from neither process, which is also why iron is a practical dead end for stars: fusing it further absorbs energy instead of releasing it.
Why one holds up cities and the other holds up stars
The two reactions have almost opposite practical requirements, which is why they show up in such different places. Fission happens relatively easily — a heavy, unstable nucleus like uranium-235 can be split just by hitting it with a slow-moving neutron, and each split releases more neutrons that can split further nuclei, making a self-sustaining chain reaction achievable at scales small enough to fit in a power plant. Fusion requires forcing two positively charged nuclei close enough together to fuse, against their mutual electric repulsion, which needs temperatures and pressures found naturally only in the cores of stars — the Sun fuses hydrogen into helium at roughly fifteen million degrees Celsius and under gravitational pressure no human-built vessel can match, which is exactly why controlled fusion power on Earth has stayed a difficult engineering problem for over half a century, even though the reaction itself is a proven source of enormous stellar energy.
What we're still unsure about
The physics of both reactions is well established; the open questions are almost entirely engineering ones. Fission's practical problems — long-lived radioactive waste, the risk of runaway chain reactions, proliferation of weapons-capable material — are well understood but not fully solved. Fusion's practical problem is the reverse: reliably confining plasma hot enough to fuse for long enough, and extracting more energy than it takes to sustain the reaction, is a milestone several major experimental reactors have only recently approached, and nobody in the field claims to know exactly how many more years of engineering stand between current results and a commercially viable fusion power plant.
This sits inside Nuclear Fission & Fusion, one of seven topics in Modern Physics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.