To move an electron up to the next state, or band in the crystal, we have to go to the state of (Na+)*, and excited state, and that takes several eV. In between there are no states, and we say that there is a band gap or forbidden band. We draw the two energy level pictures:

• To move an electron up to the next state, or band in the crystal, we have to go to the state of (Na+)*, and excited state, and that takes several eV. In between there are no states, and we say that there is a band gap or forbidden band. We draw the two energy level pictures:

Na (conductor) NaCl (insulator)

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