Atomic Structure
Part of Electrical Theory
The atomic basis of electrical conductivity—why some materials conduct and others insulate, explained from first principles.
Why This Matters
Electrical behavior—why copper wire conducts while rubber insulates, why graphite has intermediate conductivity, why silicon can switch between conducting and insulating—cannot be understood without some knowledge of atomic structure. The electrical properties of materials are determined by how electrons are arranged around atomic nuclei and how tightly those electrons are bound.
This knowledge is not merely theoretical. Understanding why different materials conduct allows informed selection of conductors, insulators, and semiconductors from available materials. It explains why certain alloys are better for specific applications, why temperature affects resistance differently in metals and carbon, and why contact between dissimilar metals can create voltage (the basis of thermocouples and the first batteries).
The level of atomic physics needed for practical electrical work is modest—the Bohr model and the concepts of electron shells, valence electrons, and band theory provide sufficient foundation for all practical work.
Atomic Structure: The Basics
Every atom consists of:
- Nucleus: Protons (positive charge) and neutrons (no charge) packed tightly together. The nucleus contains nearly all the atom’s mass.
- Electrons: Negatively charged particles orbiting the nucleus. Much less massive than protons, but the electrons determine chemical and electrical behavior.
Atomic number: The number of protons in the nucleus. Also equals the number of electrons in a neutral atom. Determines which element the atom is.
Key elements for electrical work:
| Element | Atomic number | Common role |
|---|---|---|
| Carbon (C) | 6 | Resistors, arc contacts, dry cells |
| Copper (Cu) | 29 | Primary conductor |
| Iron (Fe) | 26 | Magnetic cores, cheap conductors |
| Silver (Ag) | 47 | Best conductor, relay contacts |
| Aluminum (Al) | 13 | Conductor, antenna elements |
| Silicon (Si) | 14 | Semiconductor |
| Lead (Pb) | 82 | Battery plates, solder |
Electron Shells and Energy Levels
Electrons do not orbit randomly. They occupy specific energy levels (shells) around the nucleus, with each shell having a maximum capacity:
- Shell 1 (K shell): maximum 2 electrons
- Shell 2 (L shell): maximum 8 electrons
- Shell 3 (M shell): maximum 18 electrons
- Shell 4 (N shell): maximum 32 electrons
Valence electrons: The electrons in the outermost occupied shell. These determine chemical bonding and electrical behavior.
Copper: 29 electrons. Configuration: 2-8-18-1. One electron in the outermost shell—loosely bound and easily freed to become a conduction electron.
Carbon (graphite): 6 electrons. Configuration: 2-4. Four valence electrons, with complex bonding that leaves some electrons free to move in the plane of the graphite layers.
Silicon: 14 electrons. Configuration: 2-8-4. Four tightly bound valence electrons in pure form—essentially an insulator, but small amounts of impurities (dopants) dramatically alter conductivity.
Band Theory: Why Materials Conduct
For bulk materials (not isolated atoms), the energy levels of individual atoms broaden into bands due to interactions between neighboring atoms.
Valence band: The energy band that contains the electrons used in chemical bonding. Electrons here are localized to specific atoms and cannot contribute to conduction.
Conduction band: An energy band above the valence band where electrons are not bound to specific atoms and can move freely through the material. These are the conduction electrons.
Band gap: The energy difference between the top of the valence band and the bottom of the conduction band.
Three categories of materials:
Conductors (metals): The conduction band overlaps the valence band, or the conduction band is partially filled. Electrons at the top of the occupied levels can move freely with virtually no energy barrier. At room temperature, plenty of electrons are already in the conduction band.
Insulators: The band gap is large (5–10 eV). The conduction band is empty; the valence band is full. Electrons cannot acquire enough energy from room temperature thermal motion to cross the gap. Very little current flows.
Semiconductors: The band gap is small (0.1–2 eV). At absolute zero temperature, semiconductors are insulators—the conduction band is empty. But at room temperature, thermal energy excites some electrons across the gap. Conductivity is intermediate and strongly temperature-dependent.
Band gap examples:
| Material | Band gap (eV) | Classification |
|---|---|---|
| Copper | 0 (overlap) | Metal conductor |
| Silicon | 1.1 | Semiconductor |
| Germanium | 0.67 | Semiconductor |
| Carbon (diamond) | 5.5 | Insulator |
| Glass | 5–10 | Insulator |
| Rubber | 8–12 | Insulator |
Why Temperature Affects Different Materials Differently
Metals: As temperature rises, lattice vibrations increase, scattering conduction electrons more frequently. Resistance increases. The number of conduction electrons barely changes (there are already many).
Semiconductors: As temperature rises, more electrons gain enough energy to cross the band gap. The number of conduction electrons increases dramatically. This effect exceeds the scattering increase, so resistance decreases. Semiconductor resistance has a negative temperature coefficient.
Carbon (graphite): Behaves similarly to a semiconductor—resistance decreases with increasing temperature. This is why early incandescent lamps with carbon filaments had the unusual property of having lower resistance when hot. The carbon arc lamp exploits this: the arc heats the carbon rods, lowering their resistance, drawing more current, making the arc hotter—a positive feedback that requires a ballast resistor to stabilize.
Conductivity of Alloys
Alloys (mixtures of metals) typically have higher resistivity than either pure component. The mixed atomic structure creates more scattering sites for electrons.
Why this is useful:
- Nichrome (nickel-chromium alloy): resistivity 100× copper. Used deliberately for heating elements and resistors where high resistance per length is needed.
- Manganin (copper-manganese-nickel): temperature coefficient near zero. Used for precision resistors that must not change value with temperature.
- Steel (iron-carbon alloy): resistivity 10× pure iron. Cheap, magnetic, used for transformer laminations and relay cores.
Why pure copper is used for conductors: Pure copper has lower resistivity than copper alloys. Even small amounts of impurities (1% silver, for example) increase copper’s resistivity by 10–20%. For power wiring where low resistance is essential, metal purity matters.
Ions and Electrolytic Conduction
In solution, salts dissociate into ions—atoms that have gained or lost electrons and thus carry a net charge. These ions can move through the liquid, carrying charge. This is electrolytic conduction—fundamentally different from electronic conduction in metals.
Implications:
- Batteries work through controlled electrolytic reactions at electrode surfaces
- Electrolysis (electroplating, aluminum smelting) uses electrolytic conduction deliberately
- Water contamination of insulation creates ionic conduction paths that bypass the insulator—why moisture resistance matters for all electrical installations
- Corrosion at dissimilar metal junctions (galvanic corrosion) occurs because the metals are at different electrical potentials, driving ion flow through any electrolytic path
The contact potential between dissimilar metals—measured in millivolts to volts—is what makes thermocouples work and what makes mixed-metal connections degrade over time. When copper wire meets aluminum terminal, both the potential difference and the electrolytic corrosion process must be managed to maintain a reliable connection.