---
marp: true
theme: default
html: true
---

# Symbols, formulas or models?

Year 8 Chemistry · Elements and compounds

<!-- Speaker note: Students compare symbols, formulas, structural formulas, models and particle diagrams, say what each shows, read formulas, and choose a representation for a purpose (AC9S8U06, node 5). About 24 minutes (Engage and Explore 7 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 3 min). -->

---

## Today

**We are learning to** chemists show the same substance in different ways. A symbol names an element; a formula (H₂O) gives the types and numbers of atoms; a structural formula shows which atoms are joined; a 3D model shows the shape. Each shows something the others don't, so you pick the one that suits the job.

- I can read a chemical formula and count the atoms.
- I can say what symbols, formulas and models each show.
- I can choose a representation that suits the job.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; model kits in class do this best. Set the width with `?width=minimum`, `typical` (default) or `wider`. It closes the Elements and compounds unit, after Mixed, not joined? Not a hinge node, but formula-reading slips carry into Year 9 equations: the small number applied to both symbols, and big and small numbers confused. The prediction and the 3CO₂ check target these. Board notes: the ball models are flat (2D), atoms not to scale, and molecules are rigid. Water is drawn bent and carbon dioxide straight, as in reality. Teacher: In your own words: carbon dioxide four ways is the evidence. -->

---

## Why it matters

> H₂O, a drawing of three balls, and the word 'water' all mean the same thing. Why do chemists need so many ways to say it?

<!-- Speaker note: You can tell elements, compounds and mixtures apart by their particles. This lesson asks how to show them. -->

---

<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

**What does the formula H₂O tell you about a water particle?**

- It is a mixture of hydrogen and oxygen
- It has 2 hydrogen atoms and 2 oxygen atoms
- It is a bent shape with the oxygen in the middle
- It has 2 hydrogen atoms and 1 oxygen atom

<!-- Speaker note: What happens: 2 hydrogen atoms and 1 oxygen atom. The small number belongs to the symbol just before it; no number means 1. A formula tells you which atoms and how many, but not the shape: for that you need a model. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. '2 hydrogen and 2 oxygen' applies the 2 to both symbols. -->

---

<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-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fmodels%2Fstep-2&code=widget%3A+atoms%0Ashow%3A+H2O+x5%2C+CO2+x4%2C+CH4+x4%0Acaption%3A+Three+compounds.+Inspect+each+one.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/models/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: on the no-symbols board, ask how they could tell which particle is water. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 25px; }</style>

---

<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 hydrogen atoms and 1 oxygen atom. The small number belongs to the symbol just before it; no number means 1. A formula tells you which atoms and how many, but not the shape: for that you need a model.

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

---

<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>

## Which shows what

**Match each representation to what it shows best.**

| | |
|---|---|
| `Symbol (O)` | A. Which atoms are joined to which |
| `Formula (CO₂)` | B. Which atoms, and how many of each |
| `Structural formula (O=C=O)` | C. Whether it is an element, compound or mixture, and how much of each |
| `3D model` | D. The shape of the particle |
| `Particle diagram of a box` | E. Which element |

* Answer: Symbol (O) → E. Which element; Formula (CO₂) → B. Which atoms, and how many of each; Structural formula (O=C=O) → A. Which atoms are joined to which; 3D model → D. The shape of the particle; Particle diagram of a box → C. Whether it is an element, compound or mixture, and how much of each

<!-- Speaker note: Each shows something; none shows everything. -->

<!-- Speaker note: Teacher: the matching and the two number checks are the success criteria. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 17px; }</style>

---

<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>

## Atoms in methane

**How many atoms are in one particle of methane, CH₄?**

* Answer: 5

<!-- Speaker note: 1 carbon and 4 hydrogen: 5 atoms. -->

---

<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>

## Big and small numbers

**How many oxygen atoms are in 3CO₂ (three carbon dioxide particles)?**

* Answer: 6

<!-- Speaker note: Each CO₂ has 2 oxygen atoms; 3 × 2 = 6. The big number in front counts particles; the small one counts atoms in each. -->

---

<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>

## Choosing one

**You want to show a friend that air is a mixture of nitrogen and oxygen. Which is best?**

- The formulas N₂ and O₂
- A particle diagram of a box with separate N₂ and O₂ particles
- A 3D model of one oxygen particle

* Answer: A particle diagram of a box with separate N₂ and O₂ particles

<!-- Speaker note: Pick the representation that shows what matters for the job: here, separate particles side by side. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 28px; }</style>

---

<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 each leaves out

**Every representation leaves something out. Match each to one of its limits.**

| | |
|---|---|
| `Symbol (O)` | A. Slow to build or draw, and no use in a written equation |
| `Formula (H₂O)` | B. It doesn't show how many atoms are joined in each particle (oxygen gas is O₂) |
| `Structural formula (H–O–H)` | C. It doesn't show which atoms are joined to which, or the shape |
| `3D model` | D. Drawn flat, it can hide the real 3D shape (water is bent) |

* Answer: Symbol (O) → B. It doesn't show how many atoms are joined in each particle (oxygen gas is O₂); Formula (H₂O) → C. It doesn't show which atoms are joined to which, or the shape; Structural formula (H–O–H) → D. Drawn flat, it can hide the real 3D shape (water is bent); 3D model → A. Slow to build or draw, and no use in a written equation

<!-- Speaker note: Symbols and formulas are quick to write; a structural formula shows the joins; a 3D model shows the shape. Each has a limit, which is why chemists use several. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 16px; }</style>

---

<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: three more

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fmodels%2Fstep-4&code=widget%3A+atoms%0Ashow%3A+H2O2+x4%2C+CO+x4%2C+NH3+x4%0Acaption%3A+Three+more+substances.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/models/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Reading formulas for less familiar substances, with the model to check, builds fluency. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 25px; }</style>

---

<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>

## One more oxygen

**Hydrogen peroxide is H₂O₂ and water is H₂O. What is the difference?**

- H₂O₂ is a mixture of water and oxygen
- H₂O₂ has one more oxygen atom in each particle, so it is a different compound
- None: both are water

* Answer: H₂O₂ has one more oxygen atom in each particle, so it is a different compound

<!-- Speaker note: Change one small number and you have a new substance with new properties. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 28px; }</style>

---

<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: where models mislead

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fmodels%2Fstep-5&code=widget%3A+atoms%0Ashow%3A+H2O+x5%2C+CO2+x4%2C+CH4+x4%0Acaption%3A+Three+compounds.+Inspect+each+one.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/models/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Knowing what a model leaves out (size, motion, the real shape in 3D) is part of using it well. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 25px; }</style>

---

<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 it leaves out

**Which of these does this flat ball model NOT show truthfully?**

- How many atoms are in each particle
- Which atoms are in each particle
- The real 3D shape, the real sizes and the huge empty space between particles

* Answer: The real 3D shape, the real sizes and the huge empty space between particles

<!-- Speaker note: It is flat, the atoms aren't to scale, and real gas particles are much further apart. Every model simplifies; a good scientist knows how. -->

---

## Evaluate: look back

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

**Revision summary**

- A **symbol** (O, Na) names an element.
- A **formula** (H₂O, CO₂) gives the types of atom and how many of each. The small number counts the atom before it; a big number in front counts particles (3CO₂).
- A **structural formula** shows which atoms are joined; a **3D model** shows the shape; a **particle diagram** shows elements, compounds and mixtures.
- No single representation shows everything: choose the one that suits the job.

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 24px; }</style>

---

<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

**How many atoms of each element are in 2NH₃?**

* 2 nitrogen and 6 hydrogen (each NH₃ has 1 N and 3 H, and there are 2 particles).

<!-- 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: Same particles, new look? (physical change, AC9S8U07 node 1). In Year 9 formulas become balanced equations. In class: model kits for H₂O, CO₂, CH₄, NH₃; draw the same substance four ways and swap with a partner to decode. -->