Heheheheh...

Neils Bohr first proposed the orbital electron theory - that electrons orbit the nucleus in pre-defined orbits. There were two big glaring errors with this - if the electron was excited to a higher orbit, how did it travel from one to the other? Also that the charged particle oscillating round the nucleus would induce an electromagnetic wave - it would be releasing energy, and therefore its orbit would decay and it would collapse on itself.

OK, so we pretty much trash orbital theory...

So in the quantum description of things, you cannot define position and momentum at the same time - the more precise the position you have, the greater the momentum uncertainty and vice versa....

The state of all matter (in this case, electrons) can be described in terms of a wavefunction - that is, an equation which describes the total energy of a particle in terms of its kinetic and potential energy components with respect to all space. In the electron in a nucleus case, however, we can simplify the environment down to simply the interaction with the nucleus (think of just a hydrogen atom - only 1 proton, one electron).

The upshot of the wavefunction is that the square of the wavefunction gives a relative probability for the existence of your particle at any position in all space - relative in that the integral of the wavefunction squared over all space equals 1.

From this, and the emission/absorption spectra of hydrogen (electrons associated to the nucleus occupy well-defined energy levels) we can create a density "cloud" round the nucleus for each energy level, and for the ease of definition (i.e. to define a solid "orbital" shape), the density is arbitrarily set to 95%. Don't ask me why, thats just what I was taught...

So anyway, that how electron clouds (or orbitals) are described using basic quantum physics.

There was a question about alternate states for electrons? What I think is really, really exciting about the world of particle physics is the wave-particle duality in itself...take Young's slit experiment for example...

(Groans from everyone who has seen this before)

If you fire an electron beam at an extremely fine grating, you see a diffraction pattern, yes? This is because the wave properties of the particles interfere to create lines of constructive or destructive interference.

Now, if you slow down the electron beam until it fire discrete electrons, one at a time, with significant delays between shots (i.e. each electron travels through the grating alone) and let it run to build up a picture of the impacts that occur, then what do you see?

A diffraction pattern. Fantastic. Try describing that to a classroom of fifteen year olds and you get the pure joy of seeing everyone's mind drop into neutral.

"Honestly officer, I never interfered with myself"

Bourne