Question 9 · Complements de química URV Exam Bridging Courses in Chemistry

Using band theory and electronic structure principles, briefly explain why metallic chromium (Cr) is an electrical conductor, germanium (Ge) acts as a semiconductor, and silicon dioxide (SiO2) behaves as an electrical insulator.

Solución: Cr: Overlapping or partially filled bands ($E_g = 0$), free electron movement. Ge: Small band gap ($E_g \approx 0.67\text{ eV}$), thermal excitation promotes electrons. SiO2: Very large band gap ($E_g > 8\text{ eV}$), thermal energy is insufficient to promote electrons across the forbidden gap.