--- title: "Orbitals, Sub-Shells and Electron Configuration" description: "Electrons fill orbitals that hold two each, grouped into s, p and d sub-shells. Writing configurations this way explains the block structure of the table and says exactly what an ion has lost." canonical: https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ source: https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ.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`. # Orbitals, Sub-Shells and Electron Configuration Electrons fill orbitals that hold two each, grouped into s, p and d sub-shells. Writing configurations this way explains the block structure of the table and says exactly what an ion has lost. Subject: Science · Area: Chemistry · Ages 16 to 17 Page: https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ ## Ready when they can - Writes the full electron configuration of any element up to krypton - Explains why the 4s sub-shell fills first and why its electrons also leave first when a transition metal forms an ion - Draws the arrows-in-boxes picture for a named element, including the half-filled cases - Links the s, p and d blocks of the table to the sub-shell being filled across them ## Lesson: Addresses for every electron Electrons live in orbitals, and each orbital holds at most two. Orbitals group into sub-shells: s with one orbital, p with three, d with five. Filling follows the Aufbau order, lowest energy first: 1s, then 2s, 2p, 3s, 3p, and onward. Iron, with 26 electrons, ends as 1s2 2s2 2p6 3s2 3p6 4s2 3d6. The 4s sub-shell fills before 3d because at that point it sits lower in energy. Potassium and calcium place outer electrons in 4s, and the 3d row only begins at scandium. Ions flip the story: a transition metal losing electrons drops its 4s first, even though they arrived first. Iron(II) is therefore 3d6 with an empty 4s. Chromium (4s1 3d5) and copper (4s1 3d10) break ranks for the extra stability of a half-full or full d set. **Example.** The arrows-in-boxes picture shows what the writing hides. Each box is one orbital and each arrow one electron, with arrows pairing up only after every box in the set holds one. Oxygen 2p, for example, is three boxes holding four arrows: one pair plus two singles. **Tip.** The periodic table is this picture laid flat. The two left columns are the s block, the six right columns the p block, and the middle ten the d block. Read across a row and you are filling the sub-shell the block is named for. **Recap.** Fill lowest first, park 4s before 3d, remove 4s first, and read the blocks as the filling order. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Ionisation Energy and the Evidence for Sub-Shells](https://lightmysky.com/learn/science/ionisation-energy-and-the-evidence-for-sub-shells-mt__suwzd1kVQ) - [Inside the Atom: Protons, Neutrons and Electrons](https://lightmysky.com/learn/science/inside-the-atom-protons-neutrons-and-electrons-mt_WqDXAr2IkA) ## Opens up - [The Exclusion Principle and Multi-Electron Atoms](https://lightmysky.com/learn/science/the-exclusion-principle-and-multi-electron-atoms-mt_4tLa-3Av0A) - [Hybridisation, Conjugation and Delocalisation in Organic Molecules](https://lightmysky.com/learn/science/hybridisation-conjugation-and-delocalisation-in-organic-molecules-mt_Anv0MSQ4hq) - [Trends Across Period 3: Radius, Ionisation Energy and Melting Point](https://lightmysky.com/learn/science/trends-across-period-3-radius-ionisation-energy-and-melting-point-mt_stFFQ6GPIh) - [Transition Metals: Variable Oxidation State, Colour and Catalysis](https://lightmysky.com/learn/science/transition-metals-variable-oxidation-state-colour-and-catalysis-mt_UIVC6d7wak) - [The Schrodinger Equation and What a Wavefunction Says](https://lightmysky.com/learn/science/the-schrodinger-equation-and-what-a-wavefunction-says-mt_WCwbaO9rpR) - [Molecular Orbital Theory for Homonuclear Diatomics](https://lightmysky.com/learn/science/molecular-orbital-theory-for-homonuclear-diatomics-mt_wRBDDmRYka)