--- title: "Band Theory, Defects and Why Some Solids Conduct" description: "Interacting atomic orbitals produce closely spaced electronic states called bands. Band filling and gaps help explain electronic conduction. Temperature changes both carrier populations and scattering" canonical: https://lightmysky.com/learn/science/band-theory-defects-and-why-some-solids-conduct-mt_RuPWyBzGOp source: https://lightmysky.com/learn/science/band-theory-defects-and-why-some-solids-conduct-mt_RuPWyBzGOp.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Band Theory, Defects and Why Some Solids Conduct Interacting atomic orbitals produce closely spaced electronic states called bands. Band filling and gaps help explain electronic conduction. Temperature changes both carrier populations and scattering, while dopants and defects can change electronic or ionic transport. Subject: Science · Area: Chemistry · Ages 21 to 22 Page: https://lightmysky.com/learn/science/band-theory-defects-and-why-some-solids-conduct-mt_RuPWyBzGOp ## Ready when they can - Explains how discrete orbitals become a band as the number of atoms grows - Uses band filling and gap size to classify a metal, a semiconductor and an insulator - Explains n-type and p-type doping in terms of where the extra levels appear - Links a point defect or a non-stoichiometric composition to ionic conductivity or to colour ## Lesson: Bands decide who conducts An isolated atom has discrete electron energy levels. When atoms interact, their orbitals combine into states spread through the solid. N interacting atomic orbitals produce N states. For very large N, their energies are closely spaced and form a band. They remain discrete states, but are often treated as nearly continuous. Gaps separate ranges of allowed electronic energy. Use the simple independent-electron band model, neglecting localisation and strong electron correlations. In a metal, partly filled or overlapping bands provide occupied and nearby empty states. An electric field can change electron occupation and produce current. An ideal band insulator at zero temperature has full occupied bands separated from empty bands by a gap. At ordinary temperatures, a wide gap strongly suppresses electronic conduction; it does not mean electrons never move. *(drawing: This counts states from one orbital per atom. It is not an energy-axis diagram.)* Heat or suitable light can excite electrons across a semiconductor gap, leaving holes. Both carriers conduct. Warming intrinsic silicon usually increases carrier numbers and conductivity. In copper near room temperature, carrier numbers change little; stronger lattice vibrations increase scattering and lower electron mobility and conductivity. In lightly doped silicon, shallow donors just below the conduction band release electrons, giving n-type material. Shallow acceptors just above the valence band take electrons from it and leave holes, giving p-type material. Conductivity depends on carrier numbers and mobility; doping need not increase it in every regime. A vacancy is a missing particle at a normal site. Mobile ions can hop into vacancies and conduct without crossing an electronic band gap. An electron trapped at an anion vacancy can absorb selected visible wavelengths and change colour. This localised optical process need not create a mobile carrier or change the bulk band gap. For classification, compare band filling and the gap. A pure semiconductor has full valence bands at zero temperature and a relatively small gap allowing thermal electron-hole generation. A wide-gap insulator strongly suppresses that generation. A typical metal has a partly filled band or overlapping bands. **Recap.** Bands describe electronic states. Carrier numbers and mobility set electronic conductivity; mobile defects can also support ionic conduction. *Band explanations adapted from OpenStax University Physics Volume 3, section 9.5. Additional temperature, doping and defect teaching and questions were written for this project.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Close Packing, Unit Cells and Ionic Lattice Types](https://lightmysky.com/learn/science/close-packing-unit-cells-and-ionic-lattice-types-mt_LzpVzwrDm3) ## Optional connections - [Molecular Orbital Theory for Homonuclear Diatomics](https://lightmysky.com/learn/science/molecular-orbital-theory-for-homonuclear-diatomics-mt_wRBDDmRYka) - [Metallic Bonding and Alloys](https://lightmysky.com/learn/science/metallic-bonding-and-alloys-mt_yXOGOwHmZ4) ## Opens up - [Solid-State Synthesis: Controlling Phase, Defect and Dopant](https://lightmysky.com/learn/science/solid-state-synthesis-controlling-phase-defect-and-dopant-mt_5gri5iZ0lV)