Hi all A conductor is a conductor, because it has an unfilled energy band. Likewise, insulators have filled bands, hence they do not conduct. Now, these two statements say nothing about the mobility of the electrons. How is it that an unfilled band is equivalent of mobile electrons, while a filled band is equivalent of non-mobile electrons? I think it has something to do with a continous change of the momentum of the electrons by applying a field; am I correct?