Electron Theory
Structure and distribution of electrical charges within atoms, molecules, ions and compounds; the molecular structure of conductors, semiconductors and insulators.
Electron theory — summary notes
Free- An atom has a nucleus of positively charged protons and uncharged neutrons, orbited by negatively charged electrons. A neutral atom has equal numbers of protons and electrons.
- Electrons occupy shells. Each shell n holds a maximum of 2n² electrons — so 2, 8, 18, 32 outward from the nucleus.
- The outermost occupied shell is the valence shell, and it alone determines electrical behaviour. The EASA rule: 1–3 valence electrons → conductor; exactly 4 → semiconductor; 5–8 → insulator.
- In a conductor such as copper (2, 8, 18, 1) the single valence electron is so loosely bound that thermal energy frees it at room temperature. These free electrons form the 'electron gas' that carries current.
- Current is the drift of free electrons toward the positive terminal. Conventional current is defined in the opposite direction (+ to −) — a historical convention predating the discovery of the electron.
- Free electrons drift extremely slowly — well under a millimetre per second — yet a circuit responds instantly, because it is the electric field (travelling at close to the speed of light) that propagates, not the electrons.
- ⚠ Exam trap: a good conductor has FEW valence electrons (1–3), not many. More valence electrons means a better insulator, not a better conductor.
- Shell capacity
- max electrons in shell n = 2n²
- Charge on an electron
- e = −1.602 × 10⁻¹⁹ C
- Current as charge flow
- I = Q / t (1 A = 1 coulomb per second)
- Drift velocity
- v = I / (n · A · e)
Copper has the configuration 2, 8, 18, 1. Classify it and explain why.
The valence shell holds 1 electron. With 1–3 valence electrons the material is a conductor — that lone electron is weakly held and breaks free at room temperature, giving copper its very low resistivity (1.7 × 10⁻⁸ Ω·m).
Silicon has 14 electrons. Give its shell configuration and classification.
Filling outward using 2n²: shell 1 takes 2, shell 2 takes 8, leaving 4 → 2, 8, 4. Exactly 4 valence electrons makes silicon a semiconductor — the basis of every diode and transistor in Module 4.
A lamp lights the instant its switch is closed, yet electrons drift at under 1 mm/s. Explain.
The conductor is already full of free electrons along its whole length. Closing the switch establishes an electric field that propagates at roughly two-thirds the speed of light, so electrons everywhere in the circuit begin drifting almost simultaneously. The signal is fast; the electrons are not.
Atomic structure & conduction (3D)
FreeElectron theory concept map
FreeElectron Theory
Electron theory quiz
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