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
- 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)
Electron theory concept map
Electron Theory
Electron theory quiz
Electron Theory, quiz
1. The maximum number of electrons that any shell n can hold is given by:
2n²n²2n2. A material with exactly 4 valence electrons is classified as a:
SemiconductorConductorInsulator3. Copper has the electron configuration 2, 8, 18, 1. This makes it a:
ConductorSemiconductorInsulator4. A material with 6 valence electrons would be expected to be a:
InsulatorConductorSemiconductor5. In a neutral atom, the number of protons is:
Equal to the number of electronsEqual to the number of neutronsAlways greater than the number of electrons6. Which particle in the nucleus carries no electrical charge?
The neutronThe protonThe electron7. Electric current in a metal conductor is the movement of:
Free electronsProtonsNeutrons8. Conventional current is defined as flowing:
From positive to negativeFrom negative to positiveIn the same direction as electron drift9. When a switch closes, a lamp lights almost instantly because:
The electric field propagates at close to the speed of lightElectrons travel from the switch to the lamp at the speed of lightElectrons are created at the switch10. The outermost occupied electron shell of an atom is called the:
Valence shellNucleusConduction band