2.1 Electron Theory & Atomic Structure
Key Takeaways
- Atoms contain a positively charged nucleus (protons and neutrons) surrounded by negatively charged electrons in shells
- Valence electrons occupy the outermost shell and largely decide whether a material conducts, semi-conducts, or insulates
- Free electrons can leave their parent atoms and drift under an electric field—this drift is the basis of current in metals
- Ionization is the gain or loss of electrons that turns a neutral atom into a charged ion (cation or anion)
- For CAAS SAR-66 Module 3 (topics 3.1–3.2), B1/B2 technicians need Level 1–2 fluency in charge distribution inside atoms, molecules, ions, and compounds
Why Electron Theory Matters on Module 3
CAAS AC 66-13 Module 3 opens with electron theory (syllabus 3.1) because every aircraft electrical fault—open circuits, shorted wiring, arcing, ESD damage to avionics—starts with how charge sits inside atoms and how it moves between them. At B1/B2 Level 1–2 depth you are not expected to solve quantum equations, but you must describe the structure and distribution of electrical charges within atoms, molecules, ions, and compounds, and you must use that language correctly in exam scenarios.
Think of electron theory as the “parts catalogue” for electricity. Before you can talk about amperes, ohms, or generators, you need a clear picture of what is moving, what is fixed, and what can be stripped away by heat, friction, or high voltage.
Atomic Building Blocks
An atom is the smallest unit of an element that still shows that element’s chemical identity. Nearly all of an atom’s mass sits in a tiny central nucleus. Orbiting (or, more accurately, occupying probability regions around) that nucleus are electrons.
| Particle | Charge | Relative mass | Location | Technician note |
|---|---|---|---|---|
| Proton | Positive (+1 elementary charge) | ≈ 1 atomic mass unit | Nucleus | Number of protons = atomic number; fixes the element |
| Neutron | Neutral (0) | ≈ 1 atomic mass unit | Nucleus | Adds mass; isotopes differ only in neutron count |
| Electron | Negative (−1 elementary charge) | ≈ 1/1836 of a proton | Shells around nucleus | Moves easily; responsible for current and bonding |
A neutral atom has equal numbers of protons and electrons, so the net charge is zero. If the atom loses one or more electrons, it becomes a positive ion (cation). If it gains electrons, it becomes a negative ion (anion). Ions are central to battery chemistry, electrolysis, and lightning—topics that reappear later in Module 3 and in airframe/avionics practice.
Worked identity check: Copper has atomic number 29, so a neutral copper atom has 29 protons and 29 electrons. Strip one valence electron and you have a Cu⁺ ion with 29 protons and 28 electrons—net charge +1. That free electron is what can join the sea of mobile charge in a copper wire.
Electron Shells, Valence Electrons, and Free Electrons
Electrons occupy energy shells (also called orbits or levels). The innermost shell holds up to 2 electrons; the next up to 8; then 18, and so on under the usual capacity rules. The outermost occupied shell is the valence shell, and the electrons in it are valence electrons.
- Materials with 1–3 valence electrons (typical metals: copper, aluminium, silver) tend to release those electrons easily → good conductors.
- Materials with a full or nearly full valence shell (many plastics, glass, rubber, ceramics) hold electrons tightly → insulators.
- Materials with 4 valence electrons (silicon, germanium) sit in between → semiconductors.
A free electron is a valence electron that has enough energy to leave its parent atom and move through the material. In metals, a large population of free electrons forms an “electron gas” or sea. When you connect a battery, the electric field nudges this sea: electrons drift from negative toward positive. That organised drift is electric current. Conventional current is drawn the opposite way (positive to negative)—a historical convention Module 3 expects you to recognise—but the physical carriers in metal wires are electrons.
Trap to avoid: Do not confuse “free electron in a conductor” with “static charge on an insulator.” Free electrons in copper move through the bulk metal under a field. Static charge on a plastic panel is stuck until a path or discharge appears. Both involve electrons; only the mobility differs.
Molecules, Compounds, and Charge Distribution
When atoms share or transfer valence electrons, they form molecules and compounds:
- Covalent bonding (shared electrons) is common in organic insulation, epoxy resins, and many sealing compounds used on aircraft.
- Ionic bonding (transferred electrons) produces lattices of positive and negative ions—classic salt-like solids. In solution or molten form those ions can carry current; in dry solid form they usually do not.
- Metallic bonding leaves valence electrons delocalised across a lattice of positive ion cores—exactly why aircraft wiring metals conduct so well.
Charge is therefore not evenly “smeared” everywhere. In a polar molecule, one end may be slightly negative and the other slightly positive even though the whole molecule is electrically neutral. That polarity affects how materials attract water, dust, or static charge—relevant when you wipe composite fairings or handle plastic packaging near open avionics bays.
Ionization Basics for Aircraft Technicians
Ionization is any process that removes or adds electrons so a neutral particle becomes an ion. In hangar and line work you meet ionization in several practical forms:
- Thermal ionization / thermionic emission — hot filaments in older valves or sensors liberate electrons.
- Photoelectric effect — light knocks electrons free (photocells and some sensors).
- High-voltage ionization of air — corona, spark gaps, and lightning channels turn insulating air into a conductive plasma of ions and free electrons.
- Chemical ionization — battery plates and electrolytes exchange electrons and ions during charge/discharge.
Scenario: During a night line check, a technician sees a faint purple glow near a high-voltage lead on ground-support equipment. That glow is ionized air (corona). It warns that the electric field is intense enough to strip electrons from air molecules. Left unchecked, corona wastes energy, generates ozone, and can escalate into arcing that damages insulation.
Exam trap: Ionization does not create or destroy charge overall; it separates charge. The total number of positive and negative elementary charges in a closed system stays balanced. Lightning looks dramatic because charge that was separated over kilometres suddenly reunites through a conductive channel.
Numbers Worth Memorising
| Quantity | Typical value / fact |
|---|---|
| Elementary charge e | ≈ 1.6 × 10⁻¹⁹ coulomb |
| Electron mass vs proton | Electron ≈ 1/1836 proton mass |
| Copper valence electrons | 1 (excellent conductor) |
| Silicon valence electrons | 4 (semiconductor) |
| Neutral atom rule | Protons = electrons |
If a question asks how many free electrons participate in current, remember: in a good metallic conductor, roughly one valence electron per atom is available to the free-electron sea—enough that a millimetre of wire still contains an enormous number of carriers.
Linking Back to SAR-66 Study Strategy
Topic 3.1 is Level 1–2 knowledge: define, identify, and apply. When you later meet Ohm’s law, capacitance, or magnetism, map each phenomenon back to electrons and ions. If you can answer “what charge moved, and through what medium?” you will rarely be blindsided by Module 3 wording.
A neutral copper atom has 29 protons. After it loses one valence electron and becomes a Cu⁺ ion, how many electrons remain and what is the net charge?
Which statement best describes valence electrons for Module 3 electron theory?
During corona discharge near high-voltage ground equipment, air becomes faintly conductive primarily because:
In metallic bonding inside aircraft copper wiring, electrical current is carried mainly by: