Electron Theory
Atoms and charge
All electrical behaviour stems from atomic structure: a nucleus of protons (positive) and neutrons (neutral) surrounded by electrons (negative) arranged in shells. The number of protons defines the element; the electron arrangement defines electrical behaviour. The outermost shell holds valence electrons; in conductors (copper, silver, aluminium) these are loosely held and become free electrons able to drift under an applied field. Insulators hold electrons tightly; semiconductors sit between and conduct only when excited by heat, light or doping.
Quantities
Charge is measured in coulombs (C). One electron carries 1.602 x 10⁻¹⁹ C, so one coulomb is about 6.24 x 10¹⁸ electrons. The energy gained by one electron accelerated through one volt is the electron volt (1 eV = 1.602 x 10⁻¹⁹ J). Ionisation is the removal of an electron from an atom, leaving a positive ion.
Traps
- Electron flow is from negative to positive; conventional current is the opposite. Questions often ask which way electrons move.
- The electron charge is negative - a common distractor states positive.
- A coulomb is a large quantity; candidates confuse coulombs (charge) with amperes (rate of charge flow).
- Free-electron density, not atomic size, decides conductivity.
Worked example
How many electrons in 2 C? N = Q/e = 2 / (1.602 x 10⁻¹⁹) ≈ 1.25 x 10¹⁹ electrons. Rule of thumb: divide coulombs by 1.6 x 10⁻¹⁹.