I have seen the question about why the electrons don't fall into the nucleus, and I understand that completely. However, the converse of this question is very interesting as well: "What prevents the positively charged nucleus from wandering into the electron cloud?" You might postulate that the electrons would just shift around their orbits to accommodate the wandering nucleus, but this would not be a correct answer at all, especially for atoms in crystalline structures. When a wandering nucleus hits the inside radius of the electron cloud, what happens? Does it bounce off? If so, why? Or would electron capture occur? Electron capture might occur, but then why does C9 exhibit the behavior of electron capture, but C14 does not? They both have the same electric potential, of 6 proton charges and 6 electron charges. Any thoughts?
An S electron does have a probability of being at/in the nucleus. This is referred to as "Fermi Contact Interaction" in the literature. The effects of this interaction can be observed through NMR and EPR (ESR) spectroscopy.