Physicists love simplifying idealizations , and this is especially true in the physics of materials.  The simplest decent model for metals, for example, is the ideal Fermi gas , where we neglect the existence of atoms entirely and just model the electrons as noninteracting particles in some box.  One step up from there, the Sommerfeld model , assumes that the electrons are in a perfectly periodic crystal lattice.  In both cases, the standard semiclassical approach treats the electrons as waves b