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# What's inside an atom?

Years 9 and 10 Chemistry · Inside the atom

<!-- Speaker note: Students describe the nuclear atom (protons and neutrons in the nucleus, electrons in shells), use atomic and mass numbers to count each particle, and connect the model to the evidence that replaced earlier ones (AC9S9U06, node 1, a hinge node). About 26 minutes (Engage and Explore 7 min · Explain 12 min · Evaluate 3 min · Typical adds 4 min, Wider about 4 min). -->

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## Today

**We are learning to** an atom has a tiny, dense nucleus of protons and neutrons, with electrons in shells around it. The number of protons (the atomic number) decides the element; protons plus neutrons give the mass number.

- I can describe the nuclear atom.
- I can use the atomic number and mass number to find the protons, neutrons and electrons.
- I can describe the relative mass and charge of protons, neutrons and electrons.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace a timeline of atomic models or a gold-foil analogy activity. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge: students arrive with Year 7 particles that have no inside, so the model has to be extended explicitly. The prediction asks for neutrons from the two numbers before students meet the rule; the usual slips are taking the mass number as the neutrons, or adding the numbers. Board notes: the Bohr model is drawn not to scale (the nucleus would be about 100 000 times smaller than the atom). Shells fill 2, 8, 8, 2; this lesson doesn't need shells beyond "electrons around the nucleus" (Year 10 Shells does). Protons up to 20, neutrons up to 24. Teacher: In your own words: the labelled carbon-12 diagram with working is the evidence. -->

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## Why it matters

> If an atom were the size of a football stadium, its nucleus would be a marble on the centre spot. Yet that marble holds nearly all the mass.

<!-- Speaker note: In Year 7 the particles in the particle model had no parts. This lesson opens one up. -->

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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

**A carbon atom has atomic number 6 and mass number 12. How many neutrons does it have?**

- 18
- 6
- None: the mass is all protons
- 12

<!-- Speaker note: What happens: 6. The mass number counts protons plus neutrons, and the atomic number counts protons, so 12 − 6 = 6 neutrons. A neutral carbon atom also has 6 electrons. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally 6 / 12 / 18. The 12s have mixed up mass number and neutrons. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Finside-atom%2Fstep-2&code=widget%3A+atom%0Atarget%3A+carbon-12%0A&fresh=1}}

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

<!-- Speaker note: Use **+** and **−** for protons, neutrons and electrons. The board names the element from the protons, and adds up the mass number and the charge. Red: protons. Grey: neutrons. Blue: electrons. -->

<!-- Speaker note: Teacher: ask what changed on the board when they added a neutron (mass number, not the name). -->

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## What happened?

* 6. The mass number counts protons plus neutrons, and the atomic number counts protons, so 12 − 6 = 6 neutrons. A neutral carbon atom also has 6 electrons.

<!-- 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>

## Where the mass is

**Where is nearly all of an atom's mass?**

- Spread evenly through the atom
- In the electrons, because they fill most of the space
- In the nucleus: protons and neutrons have mass 1 each, an electron about 1/1840

* Answer: In the nucleus: protons and neutrons have mass 1 each, an electron about 1/1840

<!-- Speaker note: Protons and neutrons each have a relative mass of 1; an electron about 1/1840. So the nucleus, a tiny fraction of the atom's size, holds nearly all its mass. -->

<!-- Speaker note: Teacher: the two number checks are the node's success criteria; the gold foil question is 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>

## Three particles

**Proton, neutron or electron?**

Groups: Proton · Neutron · Electron

Charge +1 · No charge · Charge −1 · Mass about 1/1840 of a proton · In the nucleus; its number decides the element · In the nucleus; changing its number changes the mass, not the element · In shells around the nucleus

* Answer: Charge +1 → Proton; No charge → Neutron; Charge −1 → Electron; Mass about 1/1840 of a proton → Electron; In the nucleus; its number decides the element → Proton; In the nucleus; changing its number changes the mass, not the element → Neutron; In shells around the nucleus → Electron

<!-- 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>

## Neutrons

**Sodium has atomic number 11 and mass number 23. How many neutrons does a sodium atom have?**

* Answer: 12

<!-- Speaker note: Mass number minus atomic number: 23 − 11 = 12 neutrons. -->

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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>

## Electrons

**How many electrons does a neutral sodium atom have?**

* Answer: 11

<!-- Speaker note: Neutral means no overall charge: as many electrons (−) as protons (+), so 11. -->

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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>

## The evidence

**Rutherford fired alpha particles at thin gold foil. Most went straight through, but a few bounced back. What did that show?**

- An atom is mostly empty space, with a tiny, dense, positive nucleus
- The positive charge is spread evenly through the atom
- Atoms are solid balls

* Answer: An atom is mostly empty space, with a tiny, dense, positive nucleus

<!-- Speaker note: Only a tiny, dense, positive centre could turn a few alpha particles right round while letting most pass. The model of the atom changed because of this evidence. -->

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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: chlorine

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Finside-atom%2Fstep-4&code=widget%3A+atom%0Atarget%3A+chlorine-35%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: A bigger atom with an odd mass number checks the counting rule without the symmetry of carbon (6, 6, 6). -->

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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>

## One more proton

**You add one proton to a chlorine atom. What is it now?**

- A chlorine ion
- Argon: a different element
- Still chlorine, just heavier

* Answer: Argon: a different element

<!-- Speaker note: The atomic number (protons) decides the element: 18 protons is argon. -->

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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: an ion

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Finside-atom%2Fstep-5&code=widget%3A+atom%0Astart%3A+11%2C+12%2C+11%0Atarget%3A+Na%2B+23%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: Changing only electrons shows charge comes from the electron count while the element stays the same, ready for ions in bonding. -->

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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>

## Where the charge came from

**How did you give sodium a charge of +1?**

- Added a proton
- Added a neutron
- Took away one electron, so there is one more proton than electrons

* Answer: Took away one electron, so there is one more proton than electrons

<!-- Speaker note: Losing an electron leaves 11 protons (+11) and 10 electrons (−10): a charge of +1. It is still sodium. -->

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## Evaluate: look back

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

**Revision summary**

- An atom has a tiny, dense **nucleus** of **protons** (charge +1, mass 1) and **neutrons** (no charge, mass 1), with **electrons** (charge −1, mass about 1/1840) in **shells** around it.
- **Atomic number** = protons; it decides the element. **Mass number** = protons + neutrons.
- A neutral atom has as many electrons as protons. An **ion** has gained or lost electrons.
- The nuclear model replaced earlier models because of evidence, such as Rutherford's gold foil experiment.

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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

**Oxygen has atomic number 8 and mass number 16. How many protons, neutrons and electrons does a neutral oxygen atom have?**

* 8 protons, 8 neutrons (16 − 8), 8 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: isotopes (node 2), then alpha, beta and gamma decay (node 3), half-life (node 4); decay chains are a class task (node 5). In Year 10, Shells uses the same board to arrange electrons. In class: a timeline of atomic models (Dalton, Thomson, Rutherford, Bohr) with the evidence for each change; a "mystery box" gold-foil analogy (rolling marbles at a hidden shape). -->