Which material tends to resist current and capture freed electrons to fill valence shells?

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

Answer: (A)

Electrical resistance is driven by how many charge carriers can move freely. Insulators keep electrons tightly bound in their atoms, so there are very few free carriers to carry a current. The energy needed to push an electron into a mobile state is large, and any freed electron tends to be captured again by nearby atoms, filling a valence shell and stopping it from contributing to conduction. That recombination and lack of mobile electrons is why insulators resist current. By contrast, metals have plenty of free electrons that move readily, and semiconductors can be made to conduct more or less depending on conditions and doping, which is why they don’t behave like insulators.