Opposites attract, so electrons and protons are attracted to each other, making it somewhat difficult to understand why an electron wouldn’t just get as close as possible to the nucleus and crash into it. The key to answering this question has to do with the fact that electrons are very, very small particles, so they are governed by rules that don’t apply to larger objects. As we’ve talked about a little already, electrons are best thought of as clouds of negative charge surrounding the nucleus. Their properties are governed by rules that describe the cloud as a whole, rather than as a single particle. It turns out that there is something favorable about the electron being spread out, or delocalized, around the nucleus. For reasons we won’t go into in detail, when the electron’s cloud gets packed closer to the nucleus, the energy associated with its motion (its kinetic energy) begins to rise, which makes the situation unstable. There’s a balance between the stability associated with placing the electron close to the nucleus (the favorable positive-negative charge attraction) and that associated with spreading out the electron’s cloud (to keep its kinetic energy low). This prevents the electron’s cloud from getting too close to the nucleus or the electron just crashing into the nucleus.