--- title: "How are the electrons arranged?" subtitle: "Years 9 and 10 Chemistry · Atoms and the periodic table · Lesson 10" --- # How are the electrons arranged? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Neon glows in signs but never reacts with anything. Sodium, one proton heavier, catches fire in water. The difference is one electron. :::teacher-note **In one sentence:** Students arrange the electrons of elements 1 to 20 in shells (2, 8, 8, 2) and identify the outer-shell electrons, the variable the periodic table depends on (AC9S10U06, node 1, a hinge node). **Students should be able to say:** I can arrange the electrons of any element from 1 to 20 in shells. I can state the capacities of the first three shells (2, 8, 8). I can find the number of outer-shell electrons. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **Sodium has 11 electrons. How do you predict they are arranged around the nucleus?** ☐ All 11 in one shell ☐ 2, 8, 1 (three shells) ☐ 8, 3 ☐ 2, 9 (two shells) My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 2, 8, 1. The first shell holds 2 and the second 8, so the 11th electron starts a third shell. That single outer electron explains a lot about sodium. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: filling the shells :::keep-together ### Shell sizes **How many electrons do the first three shells hold, for the first 20 elements?** ☐ 8, 8, 8 ☐ 2, 4, 8 ☐ 2, 2, 2 ☐ 2, 8, 8 ::: :::teacher-note **Answer:** 2, 8, 8 ::: :::keep-together ### Chlorine **Chlorine has 17 electrons. How are they arranged?** ☐ 2, 8, 7 ☐ 2, 8, 8 ☐ 2, 15 ☐ 8, 8, 1 ::: :::teacher-note **Answer:** 2, 8, 7 ::: :::keep-together ### Magnesium **Magnesium has 12 electrons. How many are in its outer shell?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 2 2, 8, 2: two outer electrons. ::: :::keep-together ### Calcium **Calcium has 20 electrons. How many shells do they fill (or start)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 4 2, 8, 8, 2: four shells. ::: :::typical ## Elaborate: argon and potassium :::keep-together ### Potassium **Potassium has one more electron than argon. Where does it go?** ☐ It starts a fourth shell: 2, 8, 8, 1 ☐ Into the first shell ☐ Into the third shell: 2, 8, 9 ::: :::teacher-note **Answer:** It starts a fourth shell: 2, 8, 8, 1 A full shell, then a new one: that's why potassium starts a new row of the periodic table. ::: ::: :::wider ## Elaborate: coloured flames :::keep-together **Heated in a flame, sodium glows yellow and potassium lilac. Each element always gives the same colours. What does that suggest about electrons?** ☐ They sit at fixed energy levels, different in each element; jumping between them gives out light of particular colours ☐ The flame changes colour with temperature, not the element ☐ Electrons are coloured ::: :::teacher-note **Answer:** They sit at fixed energy levels, different in each element; jumping between them gives out light of particular colours Each element's emission spectrum is a fingerprint of its electron energy levels: evidence for Bohr's shells. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Electrons fill **shells** in order: **2**, then **8**, then **8** (then 2 more for potassium and calcium). - The **outer shell** is the outermost occupied one; its electrons decide how the element reacts. - A full shell is followed by a new shell, which is why rows of the periodic table end and begin. ## Exit ticket :::keep-together **Write the electron arrangement of aluminium (13 electrons). How many outer electrons does it have?** {{lines-3}} ::: :::teacher-note **Answer:** 2, 8, 3: three outer electrons. :::