--- title: "Electron Shells and the Shape of the Periodic Table" description: "Fill electron shells for the first twenty elements and use the result to explain the table itself: the group number gives the outer-shell electrons, the period number gives how many shells are occupie" canonical: https://lightmysky.com/learn/science/electron-shells-and-the-shape-of-the-periodic-table-mt__Sdcw4RnZF source: https://lightmysky.com/learn/science/electron-shells-and-the-shape-of-the-periodic-table-mt__Sdcw4RnZF.md retrieved: 2026-09-02 --- > **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`. # Electron Shells and the Shape of the Periodic Table Fill electron shells for the first twenty elements and use the result to explain the table itself: the group number gives the outer-shell electrons, the period number gives how many shells are occupied. Subject: Science · Area: Matter & Materials · Ages 14 to 15 Page: https://lightmysky.com/learn/science/electron-shells-and-the-shape-of-the-periodic-table-mt__Sdcw4RnZF ## Ready when they can - Writes the electronic structure of any of the first twenty elements, such as 2,8,1 for sodium - Links group number to outer-shell electrons and period number to the number of occupied shells - Explains why group 0 elements take part in almost no reactions - Predicts from an element's position whether it will lose or gain electrons when it reacts ## Lesson: Electron shells and the shape of the periodic table Last week ended on a claim: chemistry is decided by electrons. If that is right, then counting an atom's electrons is not enough. Where they sit has to matter too. Electrons are arranged in shells around the nucleus, at set distances, and they fill from the inside out. The closest shell takes its electrons first, and the next one only starts once the shell inside it is full. The club's wall chart turns out to be a picture of exactly that. Each shell holds only so many. The first is full at 2. The second is full at 8. For the first twenty elements the third takes 8 as well, and only then does a fourth shell open. Write the arrangement as a list, innermost first. Sodium has 11 electrons: 2 fill the first shell, 8 fill the second, and 1 is left for the third. Write it 2,8,1. Chlorine has 17: 2, then 8, then 7 left over, so 2,8,7. *(drawing: Sodium's eleven electrons, grouped by shell: two, then eight, then the one left over. That is what 2,8,1 records.)* On this chart the columns are numbered 1 to 7, and the last one is group 0. Now count what you wrote. Sodium's last shell holds 1 electron, and sodium sits in group 1. It uses three shells, and it sits in period 3. That is the shape of the columns you have been reading. The last number in the arrangement gives the group, and how many numbers there are gives the period. Chlorine reads 2,8,7: seven outer electrons, so group 7, and three shells, so period 3. Group 0 is the exception, since its atoms have full outer shells and are numbered 0 anyway. *(drawing: Both facts you would have looked up on the chart fall straight out of the arrangement.)* **Example.** Neon is 2,8. Its outer shell is already complete, so it has nothing to gain by reacting and nothing worth giving away. That is why neon and the rest of group 0 take almost no part in chemistry. Every other atom is chasing that same full outer shell. Sodium, at 2,8,1, gets there fastest by losing one electron and leaving a full 2,8 behind. Chlorine, at 2,8,7, gets there fastest by gaining one. An atom that has lost or gained electrons is called an ion, and its charge counts what moved. Sodium becomes a 1+ ion, chlorine a 1- ion. That turns a position on the chart into a prediction. Metals sit on the left with few outer electrons, so they lose them and end up positive. Non-metals sit on the right with nearly full shells, so they gain electrons and end up negative. How far the outer shell is from full sets the size of the charge. Group 1 loses 1 for a 1+ ion, group 2 loses 2 for 2+, group 6 gains 2 for 2-, group 7 gains 1 for 1-. The 2,8,8 pattern holds as far as calcium. Past that the chart gets more complicated, and a later stop picks it up. **Recap.** Electrons fill shells from the inside out: 2, then 8, then 8 for the first twenty elements. The last number in the arrangement gives the group, and how many numbers there are gives the period. A full outer shell means no reaction, which is group 0. Every other atom loses or gains electrons by the shortest route to one. ## Practice 24 questions on this page, each with its working shown. ## Needs first - [Isotopes and Relative Atomic Mass](https://lightmysky.com/learn/science/isotopes-and-relative-atomic-mass-mt_NFoRZd6UN_) - [The Periodic Table](https://lightmysky.com/learn/science/the-periodic-table-mt_wfEfQuHOG-) ## Opens up - [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)