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# Why do elements in a group behave alike?

Years 9 and 10 Chemistry · Atoms and the periodic table

<!-- Speaker note: Students explain why elements in the same group have similar chemical properties using outer-shell electrons, contrasting group 1, group 17 and the noble gases (AC9S10U06, node 2, a hinge node). About 26 minutes (Engage and Explore 7 min · Explain 11 min · Evaluate 3 min · Typical adds 4 min, Wider about 2 min). -->

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## Today

**We are learning to** elements in the same group have the same number of outer-shell electrons, so they react in similar ways. Group 1 metals lose their one outer electron easily; group 17 non-metals gain one easily; group 18 has full shells and hardly reacts.

- I can explain that elements in the same group have the same number of outer-shell electrons.
- I can use outer electrons to explain the properties of a group.
- I can contrast a reactive group with the noble gases.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea, after How are the electrons arranged? (node 1). Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge: students learn group properties as lists ("alkali metals are reactive") without the reason. Every question here asks for the outer-electron explanation. Board notes: the first 20 elements only, by group (1, 2, 13 to 18) and period. Each element's nucleus shows its commonest isotope. Hydrogen sits above group 1 with a note that it is a non-metal. -->

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## Why it matters

> Lithium, sodium and potassium look nothing alike in a jar, yet drop any of them in water and they fizz across the surface. Why do they behave the same?

<!-- Speaker note: Last lesson: electrons fill shells. This lesson uses the outer shell to read the periodic table. -->

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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, sodium and potassium all react with water in the same way. What do you predict their atoms have in common?**

- The same number of outer-shell electrons
- The same total number of electrons
- The same mass
- Full outer shells

<!-- Speaker note: What happens: Each has one outer-shell electron: lithium 2, 1; sodium 2, 8, 1; potassium 2, 8, 8, 1. Elements in the same group have the same number of outer electrons, so they react in similar ways. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the options; 'same total electrons' is the misconception to address. -->

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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%2Fgroups%2Fstep-2&code=widget%3A+atom%0Amode%3A+table%0Afind%3A+Li%2C+Na%2C+K%0A&fresh=1}}

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

<!-- Speaker note: Click an element to see its shells. The outer electrons are orange. -->

<!-- Speaker note: Teacher: ask a student to read out the shell arrangements of Li, Na and K: the last number is the same. -->

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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?

* Each has one outer-shell electron: lithium 2, 1; sodium 2, 8, 1; potassium 2, 8, 8, 1. Elements in the same group have the same number of outer electrons, so they react in similar ways.

<!-- 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>

## Sort by group

**Which group does each belong to?**

Groups: Group 1 · Group 2 · Group 17 · Group 18

Lithium (2, 1) · Magnesium (2, 8, 2) · Fluorine (2, 7) · Chlorine (2, 8, 7) · Neon (2, 8) · Argon (2, 8, 8) · Potassium (2, 8, 8, 1)

* Answer: Lithium (2, 1) → Group 1; Magnesium (2, 8, 2) → Group 2; Fluorine (2, 7) → Group 17; Chlorine (2, 8, 7) → Group 17; Neon (2, 8) → Group 18; Argon (2, 8, 8) → Group 18; Potassium (2, 8, 8, 1) → Group 1

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the group 17 prediction is the node's checkpoint: position to outer electrons to behaviour. -->

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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>

## Noble gases

**Why do the noble gases (group 18) hardly react?**

- They have no electrons
- They are gases
- Their outer shells are full, so they don't tend to lose, gain or share electrons

* Answer: Their outer shells are full, so they don't tend to lose, gain or share electrons

<!-- 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>

## Predict X

**Element X is in group 17. What do you predict about it?**

- Seven outer electrons; it gains one easily, so it is a reactive non-metal
- A full outer shell; unreactive
- One outer electron; a reactive metal

* Answer: Seven outer electrons; it gains one easily, so it is a reactive non-metal

<!-- 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>

## Sodium and chlorine

**Why do sodium (group 1) and chlorine (group 17) react together so readily?**

- Sodium loses one electron easily and chlorine gains one easily
- They have the same number of outer electrons
- They are both metals

* Answer: Sodium loses one electron easily and chlorine gains one easily

<!-- Speaker note: Each ends up with a full outer shell: sodium by losing one electron, chlorine by gaining one. They form sodium chloride (salt). -->

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## Elaborate: halogens and noble gases

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fgroups%2Fstep-4&code=widget%3A+atom%0Amode%3A+table%0Afind%3A+F%2C+Cl%2C+He%2C+Ne%2C+Ar%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: A second and third group on the same board, one reactive and one not, show the principle working across the table, not just in group 1. -->

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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>

## Helium

**Helium has only 2 electrons, not 8. Why is it in group 18?**

- Because it is a gas, like the other noble gases
- Its only shell holds 2, so its outer shell is full, like the other noble gases
- It's a mistake in the table

* Answer: Its only shell holds 2, so its outer shell is full, like the other noble gases

<!-- 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>

## Elaborate: down the group

**Potassium reacts with water much more violently than lithium. Why, when both have one outer electron?**

- Potassium's outer electron is further from the nucleus, so it is held less tightly and lost more easily
- Potassium has more outer electrons
- Potassium is heavier

* Answer: Potassium's outer electron is further from the nucleus, so it is held less tightly and lost more easily

<!-- Speaker note: More shells means the outer electron is further out and more shielded by the inner electrons: easier to lose, so more reactive. What this step is for: A trend within a group (reactivity rising down group 1) extends the model: the outer electron's distance from the nucleus matters too. -->

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## Evaluate: look back

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

**Revision summary**

- Elements in the same **group** have the same number of **outer-shell electrons**, so they react in similar ways.
- **Group 1** (alkali metals): one outer electron, easily lost: very reactive metals.
- **Group 17** (halogens): seven outer electrons, one short of full: very reactive non-metals.
- **Group 18** (noble gases): full outer shells: hardly react.
- The **period** (row) tells you how many shells an atom has.

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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

**Bromine is in group 17. Predict how many outer electrons it has and how it reacts. Explain.**

* Seven outer electrons; it gains one easily, so it is a reactive non-metal, like chlorine.

<!-- 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: periodic trends (node 3); predicting the properties of an unfamiliar element from its position is a class task (node 4). In class: a teacher demonstration of lithium, sodium and potassium in water (video if the school does not hold them). -->