Fusion and Fission of different atoms have different energy responses
Basically, if an atom has a low atomic number, combing them (doing Nuclear Fusion) releases more energy than it takes. You’re overcoming the strong force. If an atom has a high atomic number, they have a lot of energy already stored inside them. Hitting them with a particle release more energy than it takes. You’re overcoming the weak force.
Apparently Fe (iron) is the Critical Point in the fusion/fission curve, where Fusion begins taking energy and Fission begins releasing releasing energy.
