1. The problem statement, all variables and given/known data Why are protons in the nucleus and electrons aren't?? 3. The attempt at a solution So this is the question I was asked. I understand that protons are held together by nuclear (or strong) force. I understand that an atom is stable because there's an equilibrium between this nuclear force and the electrostatic force caused by the repulsion between the protons. I also understand that for electrons to exist they must keep moving to keep a certain amount of energy, and that that is related to Heisenberg's uncertainty principle. But I don't really know what my professor expects me to get at with this question, and reading introductions to quantum mechanics have just made me more confused. Is there a property or principle I'm missing??