Materials that have few electrons in the valence shell are usually considered good conductors, while materials with many electrons in the valence shell are considered good insulators. How many electrons in the valence shell qualify as few?

• A. 4 or less
• B. 2 or less
• C. 5 or more
• D. 3 or less

Answer: (D)

The main idea is that conductivity depends on how freely the outer (valence) electrons can move. In metals, valence electrons are not tightly bound to any one atom and can flow like a “sea” of electrons when a voltage is applied. When an atom has only a small number of valence electrons, those electrons are more readily available to participate in this movement, so the material tends to conduct well. As the number of valence electrons increases, electrons are more tightly bound in bonds, making it harder for them to move and the material more insulating.

Three valence electrons or fewer is the threshold typically described as “few,” so three or fewer qualify. Four valence electrons is more of a borderline case (think semiconductors like silicon), and five or more valence electrons generally lead to stronger bonding and less free movement of electrons, i.e., insulating behavior.