EASA Module 3.1 · Electron Theory
Electrons occupy shells around the nucleus. Only the outermost — the valence shell — decides whether a material conducts. Compare three materials and apply an EMF to see why.
An atom is neutral because protons (+) equal electrons (−). Electrons fill shells outward, each holding at most 2n² — 2, 8, 18, 32. The valence shell is what matters electrically: 1–3 valence electrons → conductor, 4 → semiconductor, 5–8 → insulator. Copper's single valence electron is so loosely held it breaks free at room temperature, creating the "electron gas" that carries current. Sulfur's six are tightly bound, so no current flows.
Notice the drift speed when you apply an EMF. Free electrons drift astonishingly slowly — under a millimetre per second — yet a lamp lights the instant you close the switch. The electrons were already everywhere in the conductor; what travels near the speed of light is the electric field, not the electrons themselves. This is a favourite exam point.
Model note: this is the Bohr shell model — the one the Part-66 syllabus uses. Real electrons do not orbit in neat planes; they occupy three-dimensional probability clouds (orbitals). The Bohr picture predicts shell capacity and valence behaviour correctly, which is all that is examined here.