system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why is energy produced from both nuclear fission and fusion even though they are fundamentally opposite processes?
gpt
Energy is released when something moves from a less stable state to a more stable state. For example, if you have something very high up, it is *unstable* because gravity is pulling it down. When it falls, it releases energy until it hits the ground and becomes stable, because at the ground it can't fall any more.

On the other hand, you have to put energy into something to move it out of a stable state. When something is sitting on the ground, you have to add energy into it to raise it up.

Atoms that are lighter than iron are in a less stable state than they would be with a larger nucleus. The strong nuclear force is, well...strong and like gravity it can pull *really* hard on protons and neutrons, much harder than the electromagnetic force is repelling the protons. When you fuse small atomic nuclei, they are becoming more stable, which releases energy.

Atoms that are heavier than iron are also in a less stable state. Although the strong nuclear force is much stronger than electromagnetism, it has a much smaller range. If the nucleus gets too big, the strong force isn't affecting all of the protons and neutrons in the nucleus, but the electromagnetic force *is*. The electromagnetic repulsion between protons is pushing them apart and the strong nuclear force is just barely holding them together. When the nucleus splits, it becomes small enough for the strong force to hold on better, so it becomes more stable and energy is released.

The opposite is true for fissioning small nuclei lighter than iron and fusing big nuclei heavier than iron. Fissioining small nuclei absorbs energy and fusing heavy nuclei absorbs energy. The lighter the atom is, the more energy you get from fusing it and the heavier the atom is the more energy you get from fission. You can't get lighter than hydrogen, and getting atoms heavier than uranium and plutonium is usually difficult because 1) they may not exist naturally and 2) they decay so fast that they don't last long enough to be useful. Iron is right in the middle and is in the most stable state.