How do energy bands arise in a solid? Use labelled band diagrams to compare a conductor, a semiconductor and an insulator.

1. 1 · Define an energy band. A band is a group of very closely spaced energy levels in a solid.
2. 2 · Explain how bands form. Close atoms cannot share the same levels (Pauli's principle), so each level splits; N atoms give 2N levels about 10⁻²³ eV apart.
3. 3 · Name the two bands. Filled valence levels form the valence band; empty higher levels form the conduction band, where electrons move freely.
4. 4 · Classify the solids.
   • Conductor: bands overlap, ΔE = 0.
   • Semiconductor: VB full and CB empty at 0 K, ΔE ≤ 3 eV (Si 1.1 eV).
   • Insulator: ΔE > 3 eV, so electrons cannot cross.
5. 5 · Draw the three band diagrams. [Diagram: three panels with electron energy upward; overlapping VB and CB; small gap ΔE ≤ 3 eV; wide gap ΔE > 3 eV; every band and gap labelled]
6. 6 · Real life. Silicon's small gap lets sunlight free electrons, which is why solar panels use it.
7. 7 · Close in one line. So, the size of the energy gap decides how well a solid conducts.

Scoring tip: Draw the three panels to the same scale and write ΔE beside every gap. The 3 eV dividing value earns a mark.