--- title: "Band Theory, Defects and Why Some Solids Conduct" description: "Combining orbitals from a very large number of atoms produces bands rather than levels. Whether a band is full, empty or partly filled decides whether a solid is a metal, a semiconductor or an insulat" 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: 2026-09-12 --- > **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 Combining orbitals from a very large number of atoms produces bands rather than levels. Whether a band is full, empty or partly filled decides whether a solid is a metal, a semiconductor or an insulator, and small defects shift the answer. 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 One lonely atom holds its electrons on neat separate levels. Pack billions of atoms into a solid and each atom brings its own levels, packed so tightly that they blur into continuous bands of allowed energy. Between the bands sit gaps where no electron may go. **Example.** How full the bands are decides conduction. In a metal the top filled band is only partly full, so electrons have empty states next door and drift when pushed. In an insulator a full band meets a wide gap, and electrons cannot jump across, so nothing moves. A semiconductor has a small gap: heat or light kicks a few electrons across, leaving holes behind, and both carriers move. That is why warming silicon raises its conduction, while copper, already a metal, gains no new carriers from warmth. Tiny additions shift the answer a lot. Impurity levels just below the conduction band give an n-type solid, while levels just above the valence band give a p-type solid, and conduction changes sharply. Missing or extra atoms do similar work: a point defect or an off-ratio composition can open ionic conduction or paint the solid a new colour. **Recap.** Partly filled bands conduct, wide gaps block, and small gaps plus impurities tune the answer. ## 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) - [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)