Forbidden band gap is the group of energy levels that cannot be occupied by the electrons, these energies lies in between conduction band and valence band. In pure conductors, electrons are loosely bound to the nucleus hence, can detach easily at room temperature also. A large number of free electrons thus, available are conduction electrons. The energy level of these electrons corresponds to the conduction level, hence valence level and conduction level are overlapped. Due to this, there is no forbidden band gap present in between conduction band and valence band hence, for metals the forbidden band gap is 0 eV.