The standard explanation, for why some materials are conductors, and some are insulators, is that they have different band structure. An insulator has only the valence band filled, and no electrons on the conduction band. An conductor instead has electrons also on the conduction band. The explanation looks like an explanation for a while, but I'm forced to ask a dumb question here: Why precisely is the lowest band a kind of band where electrons are not spatially mobile, and why are the electrons on the second band spatially mobile instead? Does the electrons ability to move in the momentum space have something to do with their ability to move in the spatial space? In fact this entire picture of conduction being explained by energy bands seems very strange. If we assume that the electrons are on some bands, we are already assuming that they are localized in the momentum space, and thus we are assuming the they are not localized in the spatial space. The conduction of electricity means the electrons move. How can phenomena like this be explained with a model, where electrons are assumed to be not spatially localized? The location of an electron cannot really move, if the electron is badly delocalized, since then the location doesn't exist.