At this point, I understand that at least one fundamental difference between conductors and semiconductors is that in conductors, there is typically no band gap because the valence band and conductance band overlap one another; in semiconductors, there is no such overlap meaning there is an energy hurdle to be overcome before an electron can break free from the valence band and move about in the conduction band.
What I still don't get is WHY is that the case? Is this just a simple fact of electron configuration such that I should be asking the phys-chemists at Chemistry Stackexchange about this? Is there something going on at the fundamental particle level that accounts for the differences in behaviors? Do we even know the "why" of this yet?
Also, this may be related: as you pare down the size of a sample of conductive material and get into the range of perhaps a couple dozen atoms, you achieve quantum confinement which means that you've created a band gap between valence and conductance bands. So what property of matter is it that accounts for this?
This is all a single question, I'm just trying to approach it from a few different angles to underscore my confusion. Thanks for your input!