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# Why does the periodic table repeat?

Years 9 and 10 Chemistry · Atoms and the periodic table

<!-- Speaker note: Students explain that a new period begins when a new shell starts filling, that properties repeat because outer-electron counts repeat, and read an element's shells from its period and group (AC9S10U06, node 3). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 4 min). -->

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## Today

**We are learning to** across a period the outer electrons go up from 1 to 8; then a new shell starts and the count goes back to 1, so a new period begins and the properties repeat. The period number is the number of shells. Across a period, elements change from metals to non-metals.

- I can explain why a new period starts.
- I can explain why properties repeat across the periods.
- I can use an element's period and group to give its electron arrangement.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It builds on Shells and Groups (nodes 1 and 2). Not a hinge node: it pulls shells and groups together into the table's shape. The prediction catches students who expect outer electrons to keep counting up (sodium with 9). Board notes: the table shows elements 1 to 20 only, in groups 1, 2 and 13 to 18 (transition metals start in period 4, after calcium). Colour by outer electrons counts helium's 2 as a full shell (with the noble gases). The metalloids shown are boron and silicon. Shells fill 2, 8, 8, 2 for these elements; the 8 in the third shell is a simplification that stops working after calcium. Teacher: In your own words: the period 2 row with sodium under lithium is the evidence. -->

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## Why it matters

> Mendeleev arranged the elements in 1869 and left gaps for ones nobody had found. Years later they were discovered, just as he had predicted. Electrons explain why his pattern worked, but nobody knew about electrons yet.

<!-- Speaker note: Elements in a group have the same outer electrons. This lesson asks why the table goes in rows, and why the pattern repeats. -->

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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

**Lithium has 1 outer electron. Each element after it has one more, up to neon with 8 (a full shell). Sodium comes next. How many outer electrons does sodium have?**

- 0
- 1
- 3
- 9

<!-- Speaker note: What happens: 1. Neon's second shell is full, so sodium's eleventh electron starts a third shell: 2, 8, 1. With 1 outer electron, sodium starts a new row and behaves like lithium. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally 1 / 9 / 0 / 3. The 9s are the misconception the board fixes. -->

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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%2Ftrends%2Fstep-2&code=widget%3A+atom%0Amode%3A+table%0Acolour%3A+outer%0Afind%3A+Li%2C+Be%2C+B%2C+C%2C+N%2C+O%2C+F%2C+Ne%2C+Na%0A&fresh=1}}

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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask where the colour resets, and what the shell picture shows at that moment. -->

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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?

* 1. Neon's second shell is full, so sodium's eleventh electron starts a third shell: 2, 8, 1. With 1 outer electron, sodium starts a new row and behaves like lithium.

<!-- 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>

## New period

**Why does a new period (row) start at sodium?**

- Each row holds exactly 10 elements
- Sodium is the first metal
- Its electrons start filling a new shell

* Answer: Its electrons start filling a new shell

<!-- Speaker note: Each period fills one shell. When a shell is full, the next electron starts a new one, and a new period begins. -->

<!-- Speaker note: Teacher: the period, group and shells questions are the node's success criteria. -->

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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>

## Period from shells

**An element has electrons in shells 2, 8, 8, 2. Which period is it in?**

* Answer: 4

<!-- Speaker note: Four shells: period 4. (It is calcium, in group 2 with 2 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>

## Group from shells

**Which group? Use the outer electrons.**

Groups: Group 1 · Group 17 · Group 18

2, 1 · 2, 8, 8, 1 · 2, 7 · 2, 8, 7 · 2, 8 · 2, 8, 8

* Answer: 2, 1 → Group 1; 2, 8, 8, 1 → Group 1; 2, 7 → Group 17; 2, 8, 7 → Group 17; 2, 8 → Group 18; 2, 8, 8 → Group 18

<!-- 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>

## Shells from position

**Sulfur is in period 3, group 16. What is its electron arrangement?**

- 2, 8, 8, 6
- 2, 8, 6
- 2, 6

* Answer: 2, 8, 6

<!-- Speaker note: Period 3: three shells. Group 16: six outer electrons. So 2, 8, 6 (16 electrons, atomic number 16). -->

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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>

## Why it repeats

**Why do properties repeat from one period to the next?**

- The number of outer electrons repeats, and outer electrons decide how an element reacts
- The atoms get heavier in a repeating way
- Elements were placed to make the pattern look neat

* Answer: The number of outer electrons repeats, and outer electrons decide how an element reacts

<!-- Speaker note: Every period runs from 1 outer electron to a full shell. Elements with the same number of outer electrons, in the same column, react in similar ways. -->

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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: metals to non-metals

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Ftrends%2Fstep-4&code=widget%3A+atom%0Amode%3A+table%0Acolour%3A+metal%0Afind%3A+Na%2C+Mg%2C+Al%2C+Si%2C+P%2C+S%2C+Cl%2C+Ar%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: Following period 3 from sodium to argon links metallic character to outer electrons: few to lose on the left, nearly full shells to fill on the right. -->

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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>

## Metal or non-metal

**Across period 3, the elements change from metals to non-metals. How do outer electrons explain this?**

- Atoms on the right are bigger
- On the left, atoms have 1 to 3 outer electrons that are easily lost (metals); on the right, nearly full shells gain or share electrons (non-metals)
- Metals have no outer electrons

* Answer: On the left, atoms have 1 to 3 outer electrons that are easily lost (metals); on the right, nearly full shells gain or share electrons (non-metals)

<!-- Speaker note: Metals lose outer electrons easily. With more outer electrons, losing them all gets harder, and gaining or sharing to fill the shell takes over. -->

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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: Mendeleev's gaps

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Ftrends%2Fstep-5&code=widget%3A+atom%0Amode%3A+table%0Acolour%3A+metal%0Afind%3A+B%2C+Si%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/trends/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Mendeleev's table worked as a pattern before electrons explained it: a pattern can be used to predict before it is understood. -->

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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>

## Prediction before explanation

**Mendeleev ordered elements by mass and grouped them by how they reacted, leaving gaps. He predicted the properties of a missing element below silicon; germanium was found 17 years later (1886) and matched. What does this show?**

- Mendeleev knew about electron shells
- The table is just a convenient list with no meaning
- A pattern can predict new things before anyone can explain it; electrons later explained why it works

* Answer: A pattern can predict new things before anyone can explain it; electrons later explained why it works

<!-- Speaker note: Good scientific patterns predict. The explanation (outer electrons) came decades later and showed why the pattern works, and why the table is ordered by atomic number, not mass. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- Each **period** fills one electron shell. The period number is the number of shells an element uses.
- Across a period, the outer electrons go up by 1 each time, from 1 to 8 (a full shell). Then a new shell starts and the count goes back to 1.
- Because the outer electrons repeat, properties repeat: elements in the same **group** behave alike.
- Across a period, elements change from metals (few outer electrons, easily lost) to non-metals (nearly full shells).

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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

**Element X has electrons 2, 8, 3. Which period and group is it in, and is it more likely a metal or a non-metal?**

* Period 3 (three shells), group 13 (three outer electrons): aluminium, a metal.

<!-- 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: node 4 (predicting and explaining an element's properties from its structure) is a class task: give students unfamiliar elements in groups 1, 2, 17 and 18 and ask them to predict reactions. In class: element cards sorted into a table without labels; Mendeleev's predictions for eka-silicon against germanium's measured properties. -->