What is true about insulators in the context of band theory?

In the context of band theory, the correct aspect of insulators is that they possess filled bands with a large energy gap separating them from the conduction band. This significant energy differential is crucial because it indicates that electrons within an insulator cannot easily gain enough energy to move to the conduction band, where they would be free to contribute to electrical conductivity. As a result, insulators do not conduct electricity under normal conditions.

The reasoning behind this is tied to the electronic structure of insulators. In an insulator, the valence band, which is filled with electrons, is completely occupied, while the conduction band is empty. The large band gap (energy differential) between these two bands means that a substantial amount of energy is required to promote an electron from the valence band to the conduction band, preventing electrical conduction at room temperature. This distinction is fundamental to understanding why materials are classified as insulators in band theory.

The other choices do not accurately describe insulators in this context. Insulators do indeed have a valence band; they do not feature overlapping orbitals as in metals; and they are characterized by their inability to conduct electricity easily, which is the opposite of what is stated in the last option.