--- title: "Ionisation Energy and the Evidence for Sub-Shells" description: "Successive ionisation energies jump wherever the next electron has to come from closer in. Those jumps are the direct evidence for shells, and the smaller dips across a period are the evidence for sub" canonical: https://lightmysky.com/learn/science/ionisation-energy-and-the-evidence-for-sub-shells-mt__suwzd1kVQ source: https://lightmysky.com/learn/science/ionisation-energy-and-the-evidence-for-sub-shells-mt__suwzd1kVQ.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`. # Ionisation Energy and the Evidence for Sub-Shells Successive ionisation energies jump wherever the next electron has to come from closer in. Those jumps are the direct evidence for shells, and the smaller dips across a period are the evidence for sub-shells. Subject: Science · Area: Chemistry · Ages 16 to 17 Page: https://lightmysky.com/learn/science/ionisation-energy-and-the-evidence-for-sub-shells-mt__suwzd1kVQ ## Ready when they can - Defines first ionisation energy with the state symbols that make the definition unambiguous - Reads an element's group off a graph of its successive ionisation energies - Explains the general rise across a period and the drop from the end of one period to the start of the next - Explains the small dips at aluminium and at sulfur and says what they reveal about sub-shells ## Lesson: Pulling electrons off, step by step First ionisation energy is the energy needed to remove one electron from each atom in a mole of gaseous atoms. In symbols: X(g) becomes X+(g) plus one electron. The state symbols matter, since they pin the definition to lone gaseous atoms. Knock electrons off one by one and the cost climbs in small steps, then suddenly leaps. The leap comes when the next electron must leave a shell closer to the nucleus, held far more tightly. That cost shapes how easily a metal gives up electrons in a reaction, and counting the small steps before the leap tells you the group: three easy removals means group 3. **Example.** Across a period the cost generally climbs, since the same shell feels a growing nuclear charge. But aluminium dips below magnesium because its outer electron sits in a higher 3p orbital, further out than the 3s pair. Sulfur dips below phosphorus because its new paired electron repels its partner and leaves more easily. The cost then falls from the end of one period to the start of the next, as a new shell opens further out. Jumps mark shells, dips mark sub-shells. **Tip.** Spectra back this up: electrons sharing a sub-shell cluster at one binding energy, while deeper shells need clearly more. Read a graph of successive values the same way: each big jump crosses a shell. **Recap.** Jumps mark new shells, dips mark new sub-shells, and the trend follows the pull of the nucleus. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Electron Shells and the Shape of the Periodic Table](https://lightmysky.com/learn/science/electron-shells-and-the-shape-of-the-periodic-table-mt__Sdcw4RnZF) - [Group 1, Group 7 and Group 0: Trends You Can Predict](https://lightmysky.com/learn/science/group-1-group-7-and-group-0-trends-you-can-predict-mt_cy1OrVAjIo) ## Opens up - [Lattice Enthalpy and Born-Haber Cycles](https://lightmysky.com/learn/science/lattice-enthalpy-and-born-haber-cycles-mt_pUku4iUjYa) - [Orbitals, Sub-Shells and Electron Configuration](https://lightmysky.com/learn/science/orbitals-sub-shells-and-electron-configuration-mt_sRMo1f5PfJ) - [The Hydrogen Atom: Quantum Numbers and Orbital Shapes](https://lightmysky.com/learn/science/the-hydrogen-atom-quantum-numbers-and-orbital-shapes-mt_zt9_eR1884)