Add to the rule “Atomic Structure”

All Atoms have a Mass equal to the sum of their Protons and Neutrons.

Add a new rule “Fusion”

As a daily action, an Atom may Fuse with a Reagent. Fusion is only possible if the Atom has a Mass greater than or equal to the Protons of the Reagent. When an Atom and Reagent Fuse, the Atom gains all of the Protons, Neutrons, and Electrons of the Reagent and the Reagent ceases to exist.

Add a new rule “Fission”

An Atom has a Nuclear Instability equal to their Protons minus their Neutrons, or 0 if the result is negative. If an Atom’s Nuclear Instability is greater than 0, the Atom is Fissionable. If it has been at least one day since any Atom has checked to see if the Fissionable Atom Decays, any Atom may check to see if the Fissionable Atom Decays. To do so, the Atom rolls DICEX where X is equal to the Fissionable Atom’s Mass. If the Result is less than or equal to the Fissionable Atom’s Nuclear Instability, they Decay. When an Atom Decays, they lose half their Protons rounded down, half their Neutrons rounded down, and half their Electrons rounded down. A new Reagent is then immediately Added with the number of Protons, Neutrons, and Electrons lost by the Decaying Atom.

In the rule “Reagents” replace

If it has been at least one day since any Atom has Added a Reagent, an Atom may Add a Reagent.

with

If it has been at least one day since any Atom has Added a Reagent, or there are fewer Reagents than there are non-Reagent Atoms, an Atom may Add a Reagent.