An insulator typically has valence electrons in the valence shell of at least how many?

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

Answer: (B)

Electrical conduction depends on how easily electrons can be excited into a conduction path. In insulators, the electrons are tightly bound in the outer shell and the energy gap between the valence band and the conduction band is large. When an atom has five or more valence electrons, those outer electrons are strongly bound in covalent or ionic bonds, which tends to produce a wide band gap. A wide band gap means it takes a lot of energy to free an electron to conduct, so the material resists current flow and acts as an insulator. That’s why five or more valence electrons in the outer shell is the typical rule of thumb for insulators. (There are exceptions, such as some materials with four valence electrons that still form insulators due to their network structure, but the general pattern used here centers on five or more.)