1) Well, they're not usually purporting to give a true explanation of electronic motion, but to illustrate the basic principle that electrons in an atom cannot move freely, but have specific patterns of motion connected to specific, discrete, energy states. (Also perhaps that these states are related to angular momentum, although the specific angular momenta of the Bohr model are wrong). That's the only valid lesson that can be drawn from it, apart from its historical significance. The alternative here is simply quantum theory. "SPDF" is actually slightly misleading, since those are spectorscopic designations originally, which the more sophisticated Bohr-Sommerfeld models did take into account by introducing more quantum numbers. But none of that was really adequately justified theoretically. (Which is basically what everyone was busy trying to do up until Schrödinger, 1926)
2, 4) I
think I know, but I'm not a solid-state guy so I'll pass on these, since there are folks posting here who I'm certain will give better answers than I could.
3) I'd have to say 'quantum-mechanically'. The electrons have no definite location, and correspondingly, no definite trajectory. They tunnel, and have no problems moving from one location to another without passing intermediate points (e.g. consider a p-orbital; it has a nodal plane perpendicular to the two lobes, where the probability of finding the electron is exactly zero.) It's tempting to think of them as not moving, since their probability density is static. But they do move; they have kinetic energy. They exhibit many-body effects and relativistic effects due to motion, etc. But fundamentally this boils down to the unresolved issue of how to interpret QM.
5) That's basically a way of looking at it that comes from basic chemistry, or for a somewhat more advanced theoretical justification, valence-bond theory. Basically you can look at the chemical bond in terms of two idealized extremes: A covalent bond, where the two electrons that form the bond are shared exactly equally between the two atoms, and an ionic bond, where one electron from the Gallium atom is entirely on the Arsenic atom.
In chemistry they look at that as 'resonance', you have two resonance forms:
Ga-As (covalent) <--> Ga
+-As
- (ionic)
In the latter case you have more electrons on the arsenic. In reality, bonds between two different nuclei are never purely covalent or purely ionic. So what they're saying here is that the covalent form dominates, but there's a slight ionic contribution, and conclude that there's a 'little bit' more of the bonding electrons on the arsenic. I recently got
http://i.imgur.com/tow7V.jpg" to a cute explanation from an organic chem textbook. As it explains, these two resonance forms don't actually exist, they're just a theoretical way of looking at it. A more rigorous justification for that way of looking at it is valence-bond theory, in which the covalent and ionic forms can be viewed as two different quantum states with two different wave functions, and the real ground state is a superposition of the two. In this case, a superposition where the covalent contribution is largest.
So don't read too much into it, it's really just the result of a particular theoretical way of looking at it. Physically it doesn't amount to more than the observation that arsenic is slightly more http://www.thecatalyst.org/electabl.html" than gallium.
Now left question 2 and 4. But maybe you can try to explain as well, I think you really understand it.