Which type of substance has no free electrons in the valence shell?

Insulators are materials that do not have free electrons in their valence shells, which is a defining characteristic. In insulators, the electrons are tightly bound to their atoms and do not move freely, preventing the flow of electric current. This lack of free electrons means that insulators resist electrical conductivity, making them suitable for applications where preventing the flow of electricity is crucial.

In contrast, conductors have free electrons that can move easily under the influence of an electric field, allowing them to conduct electricity efficiently. Semiconductors, while having properties between conductors and insulators, also have some free electrons, especially when doped or under certain conditions. Therefore, the unique property of insulators—having no free electrons—makes them the correct choice in this context.