Materials whose atoms feature tightly bound electrons are described as:

• A. Insulators
• B. Conductors
• C. Semiconductors
• D. Superconductors

Answer: (A)

When electrons are tightly bound to atoms, they can’t move freely to carry electric current. Electrical conduction depends on electrons being able to populate a conduction pathway; if the valence electrons are held tightly with a large energy gap to the conduction band, very few electrons can be promoted to move, so the material resists current flow. That describes insulators.

In contrast, conductors have electrons that are readily delocalized and free to move, which is why they conduct electricity well. Semiconductors have an intermediate situation: a smaller energy gap and the ability to control conduction through temperature or impurities. Superconductors can conduct with zero resistance, but that occurs under special conditions and involves quantum pairing of electrons, not simply tight binding.

So the description of tightly bound electrons aligns with insulators, explaining their poor electrical conductivity.