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# How are the electrons arranged?

Years 9 and 10 Chemistry · Atoms and the periodic table

<!-- Speaker note: 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). About 25 minutes (Engage and Explore 7 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 2 min). -->

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## Today

**We are learning to** electrons fill shells around the nucleus in order: 2 in the first, 8 in the second, 8 in the third (for the first 20 elements). The electrons in the outer shell decide how an element behaves.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it builds on Year 9's nuclear atom (What's inside an atom?). Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge is fluency: students who can't place outer electrons quickly can't use the periodic table later. The Explain checks and the book task are practice, not breadth. Board notes: the Bohr model, not to scale. Shells hold 2, 8, 8, then 2 (the simple rule for elements 1 to 20; the third shell holds more in heavier atoms, which Year 10 doesn't need). -->

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## Why it matters

> Neon glows in signs but never reacts with anything. Sodium, one proton heavier, catches fire in water. The difference is one electron.

<!-- Speaker note: In Year 9 you met the nuclear atom. This lesson arranges its electrons. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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)

<!-- Speaker 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. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the four options. '2, 9' means they know shell 1 but not the 8. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fshells%2Fstep-2&code=widget%3A+atom%0Atarget%3A+sodium-23%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/shells/?full=step-2#step-2)

<!-- Speaker note: Add protons, neutrons and electrons one at a time. Watch the electrons: when does a new shell start? -->

<!-- Speaker note: Teacher: stop the class at 3 and 11 electrons: what happened? -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* 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.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: 2, 8, 8

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: chlorine and the two number checks are the evidence. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Chlorine

**Chlorine has 17 electrons. How are they arranged?**

- 2, 8, 7
- 2, 8, 8
- 2, 15
- 8, 8, 1

* Answer: 2, 8, 7

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Magnesium

**Magnesium has 12 electrons. How many are in its outer shell?**

* Answer: 2

<!-- Speaker note: 2, 8, 2: two outer electrons. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Calcium

**Calcium has 20 electrons. How many shells do they fill (or start)?**

* Answer: 4

<!-- Speaker note: 2, 8, 8, 2: four shells. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: argon and potassium

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fshells%2Fstep-4&code=widget%3A+atom%0Astart%3A+18%2C+22%2C+17%0Atarget%3A+argon-40%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/shells/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Completing argon's third shell, then seeing potassium start a fourth, links full shells to the end of a row in the periodic table. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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

* Answer: It starts a fourth shell: 2, 8, 8, 1

<!-- Speaker note: A full shell, then a new one: that's why potassium starts a new row of the periodic table. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: coloured flames

**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

* Answer: They sit at fixed energy levels, different in each element; jumping between them gives out light of particular colours

<!-- Speaker note: Each element's emission spectrum is a fingerprint of its electron energy levels: evidence for Bohr's shells. What this step is for: Flame colours are evidence for fixed energy levels: the observation that pushed the atomic model to shells. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Write the electron arrangement of aluminium (13 electrons). How many outer electrons does it have?**

* 2, 8, 3: three outer electrons.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: Why do elements in a group behave alike? (node 2), then periodic trends (node 3); predicting properties from position is a class task (node 4). In class: flame tests (sodium, potassium, lithium, copper chlorides) with a hand spectroscope if available. -->