Sodium sulfate (Na2SO4) is an ionic compound and does not have a band gap. The concept of a band gap is associated with semiconductors and insulators, which are materials with covalently bonded atoms and distinct energy bands for electrons. Electrolytes, such as Na2SO4, consist of dissolved ions, and therefore, exhibit ionic conductivity rather than electronic conductivity. In Na2SO4, the energy levels of the constituent ions are discrete and there is no well-defined band gap separating occupied and unoccupied electronic states.