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# From uranium to lead

Years 9 and 10 Chemistry · Inside the atom

<!-- Speaker note: Students trace a decay sequence, track atomic and mass numbers at each step, and explain how the sequence ends at a stable nucleus (AC9S9U06, node 5, an integration node). About 22 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 1 min, Wider about 1 min). -->

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## Today

**We are learning to** an unstable nucleus often decays into another unstable one, and a decay chain ends only when a stable nucleus is reached. Each alpha decay lowers the atomic number by 2 and the mass number by 4; each beta decay raises the atomic number by 1 and leaves the mass number alone. Uranium-238 reaches stable lead-206 after eight alpha and six beta decays.

- I can follow a decay chain from an unstable isotope to a stable one.
- I can work out the atomic and mass numbers after each alpha or beta decay.
- I can say why a chain stops.

<!-- Speaker note: Before class: A short integration tutorial that follows the half-life lesson. Set the width with `?width=minimum`, `typical` (default) or `wider`. The decay board stops at a stable isotope and says whether each step is how the isotope really decays, so students can't wander into impossible chains. The uranium-238 chain used here is the main series: eight alpha and six beta decays, with a few side branches in the full chart that are left out. Board notes: the board counts alpha (`x`) and beta (`y`) decays. Half-lives vary from a fraction of a second to billions of years and aren't shown here. Teacher: In your own words: the first four steps of the chain. -->

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## Why it matters

> A grain of uranium doesn't decay once. It sets off a chain of fourteen steps, passing through radon, a gas that seeps into some basements, and ends as lead. How can you follow it?

<!-- Speaker note: You know alpha and beta decay and half-life. Now put them in a row. -->

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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 uranium-238 nucleus decays by emitting an alpha particle. What is the new nucleus like?**

- The same uranium, only lighter
- Thorium-234, which is also unstable and decays again
- Stable lead straight away
- A pile of loose protons

<!-- Speaker note: What happens: Thorium-234 (atomic number 90, mass number 234), which is unstable too. The chain carries on through a series of alpha and beta decays until a stable nucleus, lead-206, is reached. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. Ask the 'stable lead' group what makes a nucleus stable. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fdecay-chains%2Fstep-2&code=widget%3A+atom%0Amode%3A+decay%0Aisotope%3A+radon-222%0A&fresh=1}}

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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what is the same about every alpha step, and every beta step. -->

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## What happened?

* Thorium-234 (atomic number 90, mass number 234), which is unstable too. The chain carries on through a series of alpha and beta decays until a stable nucleus, lead-206, is reached.

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

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## Atomic number

**Uranium-238 has atomic number 92. After one alpha decay, what is the new atomic number?**

* Answer: 90

<!-- Speaker note: An alpha particle carries away 2 protons, so 92 − 2 = 90 (thorium). -->

<!-- Speaker note: Teacher: the number questions and 'why it stops' are the success criteria. -->

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## Mass number

**And the new mass number, after the alpha decay from mass number 238?**

* Answer: 234

<!-- Speaker note: An alpha particle has 4 particles (2 protons and 2 neutrons), so 238 − 4 = 234. -->

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## Beta step

**Thorium-234 (atomic number 90) then decays by beta emission. What is the new atomic number?**

* Answer: 91

<!-- Speaker note: A neutron becomes a proton, so the atomic number goes up by 1: 91 (protactinium). The mass number stays at 234. -->

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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>

## Total fall

**Uranium-238 ends as lead-206. By how much does the mass number fall in total?**

* Answer: 32

<!-- Speaker note: 238 − 206 = 32. -->

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## Alpha count

**Only alpha decays change the mass number, by 4 each time. How many alpha decays are in the chain?**

* Answer: 8

<!-- Speaker note: 32 ÷ 4 = 8 alpha decays. They lower the atomic number by 16, from 92 to 76; beta decays raise it back to 82. -->

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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 stops

**Why does the chain stop at lead-206?**

- It has become too small
- It has run out of energy
- Its nucleus has a stable balance of protons and neutrons, so it doesn't decay

* Answer: Its nucleus has a stable balance of protons and neutrons, so it doesn't decay

<!-- Speaker note: Uranium-238's nucleus is too big, with too many neutrons, to last. Alpha decays make it smaller; beta decays turn spare neutrons into protons. When it reaches a mix of protons and neutrons that lasts, nothing more happens. -->

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## Elaborate: radon in the home

**Radon-222 is a gas made in the uranium chain, and it seeps up from rock into some buildings. Why is it of more concern than the solid isotopes before it?**

- It is hotter
- It has a longer half-life than uranium
- As a gas it can be breathed in, and it decays to more radioactive solids inside the lungs

* Answer: As a gas it can be breathed in, and it decays to more radioactive solids inside the lungs

<!-- Speaker note: The earlier solids stay in the rock. A gas gets out and can be breathed in, so its later decay products are released next to lung tissue. Good ventilation helps. What this step is for: Radon is the best-known step in the chain, because it is a gas and shows why the order of the steps matters in the real world. -->

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## Elaborate: missing mass

**The total mass of the products of a decay is slightly less than the mass of the original nucleus. What does physics say happened to the 'missing' mass?**

- It became the energy carried away by the radiation (E = mc²)
- It was destroyed
- It left as a gas

* Answer: It became the energy carried away by the radiation (E = mc²)

<!-- Speaker note: A tiny amount of mass becomes a large amount of energy, which is why a small amount of radioactive material releases so much energy. What this step is for: The energy released in decay links to mass, which is the idea behind nuclear power. -->

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## Evaluate: look back

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

**Revision summary**

- A **decay chain** is a series of decays that ends only at a **stable** nucleus.
- **Alpha** decay: atomic number −2, mass number −4. **Beta** decay: atomic number +1, mass number unchanged.
- Uranium-238 ends as lead-206 after **8 alpha** and **6 beta** decays.
- Alpha decays make the nucleus smaller and beta decays fix its balance of neutrons to protons, until it is stable.

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## Exit ticket

**Radium-226 (atomic number 88) decays by alpha emission. What are the new atomic and mass numbers?**

* Atomic number 86, mass number 222: radon-222.

<!-- 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: word equations and balancing equations. In class: draw the decay series on a large wall chart with a bar for each isotope's half-life; ask students to colour the alpha and beta steps. -->