... Optional: Quantum Physics . ... Energy Levels . ... Consider a proton inside the nucleus. ... Such nuclei, and heavy nuclei close to them in mass, are all on the brink of instability, and even without externally added energy, they find a way to get rid of some of their disruptive positive charge The forces on protons inside these nuclei resemble those on a bunch of marbles inside the "crater" of a volcano-shaped surface--smooth sloping sides outside, but a moderately deep crater on top ( drawing ). ...