What is an atom called that has no free electrons?

An atom that has no free electrons is characterized as an insulator. Insulators are materials that do not allow the free flow of electric charge because their electrons are tightly bound to their nuclei. This lack of free electrons means that insulators resist electrical conductivity, making them essential in applications where preventing the flow of electricity is crucial.

In contrast, conductors have free electrons that can move easily through the material, facilitating the flow of electricity. Semiconductors have properties that fall between conductors and insulators; their conductivity can be manipulated by the presence of impurities or by external factors like temperature. Electrolytes, on the other hand, are substances that produce ions when dissolved in a solvent, allowing them to conduct electricity. Thus, the defining characteristic of an insulator is its inability to have free electrons, making it the correct answer in this context.