system
Given the following post, respond as if you're commenting on Reddit:
human
What is it about radioactive materials that make them most suitable for fission? Why can't we split iron atoms?
gpt
You could split iron nuclei, it's just that the process could never be self-sustaining.

Elements less heavy than iron could fuse and release energy.

Elements more heavy than iron could split and release energy.

The process is self-sustaining only when energy is being released. So fusion "pushes" small nuclei towards the size of the iron nucleus, whereas fission pushes big nuclei towards the same size. Basically, iron is at the bottom of the mass defect valley, and things on either side tend to roll down towards it on their own.

But could you push things up from the bottom of the valley? Sure, if you spend some extra energy from outside. Just don't expect it to happen spontaneously.