Electrons aren't really particles, they aren't really waves they're both.. so some whacky shit happens
It doesn't ever crash into the nucleus because it doesn't really orbit the nucleus in the traditional sense. You cant talk about where it is or isnt, you can talk about where it has the highest probability of it being found.
It is most likely that the electron be right on top of the nucleus based on the probability density alone, but there is also a volume element to where the electron will be found. The surface area of the sphere that makes up the nuclues is so tiny that there is basically a 0% chance that the electron would be in that volume, even though it is the most probable distance.
This probably explains it better than i can (its got pikturs)
http://www.chem1.com/acad/webtut/atomic/WhyTheElectron.html