We all know the story. Electrons and protons are attracted to each other. That's why a balloon rubbed on hair clings to clothes. The electrons it gained are crying out for protons and dragging the rest of the balloon along with them. But electrons and protons are right next to each other in the atom. Why don't they just smoosh together?
Learning science is a lot like learning history; when you get to one class what you learn is that the things you learned in the last class, or in the last four years of high school, was wrong. Often, the teachers of the previous class get terribly resentful about this, and slip in little previews of what you'll be learning a few years from now. This adds some confusion for students and not a little crankiness for the later professors, but it is somewhat less surprising to learn, for example, that the model of an atom that has served so faithfully when describing bonds and electric flow and such simply doesn't hold up when you want to learn why the electron and the proton, which apparently are so enamored of each other that they'll pull together your laundry every time you take it out of the dryer, don't just rush at each other when they're staring at each other over the radius of, say, a hydrogen atom. A hydrogen atom has one central proton, which apparently attracts electrons, and one electron, which attracts protons, orbiting planet-like, around it. Despite their desire for each other, they don't just cross that tiny distance and come together in a torrid subatomic night of passion, and that makes no sense (in many ways, I suspect).
The only explanation for this, according to physics teachers, is that an electron, of course, does not hang outside a nucleus like a planet in a star system. How quaint it is that you believed that for all those years! It makes them chuckle, sympathetically and decorously, into their copy of Surely You're Joking Mr. Feynman (It's signed!). The problem is that an electron doesn't exist as a planet-like blob and it doesn't orbit anything. Instead it's something that kind of 'might' exist over a range of area and at a range of velocities.
The overall combination forms an amorphous cloud of potential electron. And this cloud has an equilibrium. When it can spread out over a large space, it can have a pretty low range of velocities. When it's packed into a smaller space, its various velocities go up, and it pushes away again. (Yes, it's that Heisenberg Uncertainty Principle that you last saw making a nuisance of itself in the second Jurassic Park movie.) The 'orbit' of the electron is the happy medium between the lovestruck electron rushing in towards the proton and collapsing the cloud, and the electron spreading away from the proton and growing listless and still.
For those who find this romance too sad, take heart. A bit of that cloud actually does pass into the nucleus, so they can be united, although they rarely interact even when they buzz through each other. Only unstable atoms, with a lot of protons in their center, will occasionally snag an electron. This wild night will leave the world with a thoroughly satisfied neutron - just a little bit heavier than a proton - and a little baby electron-sized neutrino being shot out of the atom to make its way in the world.
Orbital Image: Ulrich Mohrhoff