Diamond is an insulator but silicon a semiconductor, though both are group 14. Why? Step 1: Both have filled valence bands; difference is the band gap size. Step 2: Diamond: gap ~5.5 eV — thermal energy (~0.025 eV) can't bridge it → insulator. Step 3: Silicon: gap ~1.1 eV — heating/doping pushes electrons across → conducts. Step 4: Doping P (5 valence e⁻) into Si adds donor level near conduction band → n-type. Band gap size, not chemistry, decides.