There are higher energy levels, but to promote the electrons to such a high energy levels requires an enormous voltage. Metals conduct electricity easily. In this case, the energy levels between the conduction and valence band are closely spaced and there are more levels than electrons so very little energy is required to find new energies for electrons to occupy. The resistivity of a material is measure of how difficult it is for a current to flow. The band theory of materials explains qualitatively the difference between these types of materials.
Electrons occupy energy levels from the lowest energies upwards.
However, some energy levels are forbidden because of the wave like properties of atoms in the material. The allowed energy levels tend to form bands. Electrons in the valence band do not participate in the conduction process. The first unfilled level above the valence band is known as the conduction band . Insulators have an energy gap that is far greater than the thermal energy of the electron, while semiconductor materials the energy gap is typically around 1eV. The diagram below summarises the energy band model of materials.