Does this graph represent energy states in a single atom which makes it into the semiconductor or is this a representation of sum total of energy levels occupied by different electrons in the lattice as a whole? Also, this graph gives the impression that the conduction band somehow 'emerges' from the higher energy states. Why should the energy at which conduction can take place have anything to do with the higher energy state in an atom? Final question: (For example, in the case of Silicon) During the formation of the lattice, an electron gets excited from 3s to 3p orbital. At what point in this graph does that transition happen and how is it represented?