Add a proton.
Change the element.
One shared nucleus. Fuse $PROTON and the element changes. Every swap adds binding energy. Below iron nothing can split it; above iron, anyone can. How it works
The mass decides the element.
Proton is a single nucleus, shared by everyone. Fusing $PROTON adds mass; the mass sets the proton count, and the proton count is the element. Hydrogen, carbon, iron, gold: each one is reached together.
Iron is the wall.
Protons enter, and they leave when they want to. Nothing can split the nucleus.
Fusing burns a share, and no proton leaves. The nucleus can be split.
Anyone can fire it. Its price falls as the nucleus grows heavier, and the longer it goes unfed.
Every swap feeds the nucleus.
A share of each trade in the pool is set aside as binding energy. It accrues until the nucleus is split, then goes to the protons that held it, in proportion to their mass and to how long they stayed. The atom starts again from zero.