In theory anything lighter can fuse toward Iron. Anything heavier can fission the same direction. Splitting or fusing Iron requires input of energy even in theo

In theory anything lighter can fuse toward Iron. Anything heavier can fission the same direction. Splitting or fusing Iron requires input of energy even in theoretical scenarios at 100% efficiency. In practice a great many nuclear reactions are not chain reactions or don't yield enough energy to be useful. Really large atoms kinda want to fission due to the speed of light: forces can't propagate from one side to the other fast enough to completely balance out. So splitting Uranium is a lot easier to make useful because we have a head start as it were... if you leave it alone in a box some of it will go ahead and split in pieces for you for free. Fusion is just more difficult. If you leave hydrogen atoms confined in a box approximately none of them will spontaneously fuse to form helium.