An insulator is normally an atom with ___ electrons in the valence shell.

• A. two
• B. five or more
• C. four
• D. six

Answer: (B)

An insulator resists current because its outermost electrons are tightly bound and not easily freed to move as charge carriers. The number of electrons in the valence shell matters: when there are five or more valence electrons, those outer electrons tend to be held more tightly, making it harder for them to become free to conduct electricity. That means fewer electrons can move under an applied voltage, giving the material high resistance. In common insulating materials such as many plastics and ceramics, this higher count in the valence shell is typical, which is why they don’t conduct well. In contrast, metals and many materials with fewer valence electrons allow electrons to move more freely, so they conduct better.