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# What comes out of a decaying nucleus?

Years 9 and 10 Chemistry · Inside the atom

<!-- Speaker note: Students describe alpha, beta and gamma decay by what leaves the nucleus, explain how alpha and beta decay change the atomic and mass numbers, and compare how far each gets (AC9S9U06, node 3, a hinge node). About 27 minutes (Engage and Explore 8 min · Explain 12 min · Evaluate 3 min · Typical adds 5 min, Wider about 5 min). -->

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## Today

**We are learning to** an unstable nucleus decays by giving out alpha particles (2 protons and 2 neutrons), beta particles (fast electrons made in the nucleus) or gamma rays (energy). Alpha and beta decay change the element; gamma does not. Alpha is stopped by paper, beta by aluminium, and gamma is only reduced by lead.

- I can describe what is given out in alpha, beta and gamma decay.
- I can explain how alpha and beta decay change the atomic number and mass number.
- I can compare how far alpha, beta and gamma get through materials.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace a teacher demonstration with sealed sources and a Geiger counter (or a video of one). Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge: students often run the three kinds together (gamma "turns it into a new element"; beta electrons "come from the shells"), or cannot track the numbers. The prediction asks for the product of one alpha decay before students meet the rule; the usual slips are changing only the mass number, or moving the wrong way in the table. Board notes: the decay board takes any isotope up to uranium (`isotope: radon-222`). It knows how about 25 common isotopes really decay (the uranium-238 and radon-222 chains, carbon-14, iodine-131, cobalt-60, caesium-137, americium-241 and others) and says so; for the rest it says it has no data. Students can still press any button: a "wrong" decay is a good discussion ("In nature C-14 decays by beta, not alpha"). The nucleus shows at most 60 particles, in the right proportion of protons to neutrons; it is not to scale. Beta here is beta-minus only (no positrons). The shield board is schematic: real ranges depend on energy. Teacher: In your own words: the table plus the radon equation is the evidence. -->

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## Why it matters

> Radon is a gas that seeps out of some rocks and into houses. It is invisible and has no smell, yet the air in a closed cellar can carry atoms that are turning, one by one, into lead.

<!-- Speaker note: Isotopes of an element have different numbers of neutrons, and some are unstable. This lesson asks what an unstable nucleus gives out, and what it becomes. -->

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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

**Radon-222 has 86 protons. Its nucleus gives out an alpha particle: 2 protons and 2 neutrons. What is left?**

- Radon-220 (86 protons)
- Polonium-218 (84 protons)
- Radon-222: nothing changes
- Radium-226 (88 protons)

<!-- Speaker note: What happens: Polonium-218. Losing 2 protons takes the atomic number from 86 to 84, which is polonium; losing 4 particles in all takes the mass number from 222 to 218. A different number of protons means a different element. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. Radon-220 means 'only the mass changed'; radium-226 means the numbers went the wrong way. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fdecay%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/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask which button changed the element's name, and which changed only the energy. -->

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## What happened?

* Polonium-218. Losing 2 protons takes the atomic number from 86 to 84, which is polonium; losing 4 particles in all takes the mass number from 222 to 218. A different number of protons means a different element.

<!-- 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>

## Three kinds

**Alpha, beta or gamma?**

Groups: Alpha · Beta · Gamma

2 protons and 2 neutrons (a helium nucleus) · A fast electron made when a neutron turns into a proton · Energy only: no mass and no charge · Atomic number down 2, mass number down 4 · Atomic number up 1, mass number the same · Atomic number and mass number both the same · Stopped by a sheet of paper or a few centimetres of air · Stopped by a few millimetres of aluminium · Reduced by thick lead or concrete, never stopped completely

* Answer: 2 protons and 2 neutrons (a helium nucleus) → Alpha; A fast electron made when a neutron turns into a proton → Beta; Energy only: no mass and no charge → Gamma; Atomic number down 2, mass number down 4 → Alpha; Atomic number up 1, mass number the same → Beta; Atomic number and mass number both the same → Gamma; Stopped by a sheet of paper or a few centimetres of air → Alpha; Stopped by a few millimetres of aluminium → Beta; Reduced by thick lead or concrete, never stopped completely → Gamma

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the sort and the two number checks are the node's success criteria; the 'why' question links stopping power to damage. -->

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## Alpha

**Uranium-238 (92 protons) decays by alpha decay. What is the mass number of the new nucleus?**

* Answer: 234

<!-- Speaker note: An alpha particle takes 4 particles (2 protons, 2 neutrons): 238 − 4 = 234. The atomic number drops to 90, which is thorium: thorium-234. -->

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## Beta

**Carbon-14 (6 protons) decays by beta decay. What is the atomic number of the new nucleus?**

* Answer: 7

<!-- Speaker note: A neutron turns into a proton, so 6 + 1 = 7 protons: nitrogen. The mass number stays 14, so carbon-14 becomes nitrogen-14, which is stable. -->

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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>

## Gamma

**A nucleus gives out a gamma ray. What changes?**

- It becomes the next element
- It loses 4 from its mass number
- Only its energy: it is the same isotope, calmer

* Answer: Only its energy: it is the same isotope, calmer

<!-- Speaker note: Gamma is energy, with no particles, so the protons and neutrons stay the same. Gamma rays usually come straight after an alpha or beta decay, carrying away the new nucleus's extra energy. -->

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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>

## Stopping power

**A detector picks up radiation from a source through paper, but nothing once 5 mm of aluminium is in the way. What is the source giving out?**

- Alpha
- Gamma
- Beta

* Answer: Beta

<!-- Speaker note: Through paper but stopped by a few millimetres of aluminium: beta. -->

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## Why

**Why is alpha stopped so much sooner than gamma?**

- Paper absorbs helium
- Alpha particles are big, slow and charged, so they hit and knock electrons off atoms almost at once; gamma has no mass or charge
- Alpha particles have less energy than gamma rays

* Answer: Alpha particles are big, slow and charged, so they hit and knock electrons off atoms almost at once; gamma has no mass or charge

<!-- Speaker note: The bigger and more charged the radiation, the more it collides with atoms in its path and the sooner it gives up its energy. That also makes alpha the most damaging if its source gets inside the body. -->

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## Elaborate: radiation in medicine

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fdecay%2Fstep-4&code=widget%3A+atom%0Amode%3A+decay%0Aisotope%3A+iodine-131%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: Matching the kind of radiation to a job (treat from inside, image from outside) uses penetrating power as a reason, not a fact to recall. -->

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## Why beta

**Iodine-131 collects in the thyroid gland and gives out beta particles. Why does that suit treating thyroid cancer?**

- Beta turns cancer cells into xenon
- Beta goes straight through the body, so it does no harm
- Beta travels only a few millimetres in the body, so it damages cells near the iodine and few others

* Answer: Beta travels only a few millimetres in the body, so it damages cells near the iodine and few others

<!-- Speaker note: Inside the body, beta travels only millimetres, so its energy is delivered to the thyroid itself. -->

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## Seeing with gamma

**To make a picture of an organ, doctors inject a substance that gives out gamma rays and use a camera outside the body. Why gamma?**

- It gets out of the body to the camera, and it is less damaging on the way than alpha or beta
- It is stopped inside the organ
- It changes the organ into another element

* Answer: It gets out of the body to the camera, and it is less damaging on the way than alpha or beta

<!-- Speaker note: Gamma passes out of the body, so a camera can detect where it came from. Isotopes for imaging also have short half-lives, so they don't stay active for long (half-life is next). -->

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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: in the home

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fdecay%2Fstep-5&code=widget%3A+atom%0Amode%3A+decay%0Aisotope%3A+americium-241%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: The same alpha that a sheet of paper stops is safe in a smoke alarm and dangerous when breathed in: penetration and harm depend on where the source is. -->

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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>

## On the ceiling

**Why is the americium in a smoke alarm safe to have on the ceiling?**

- Americium does not decay
- Its alpha particles are stopped by a few centimetres of air and the alarm's case
- Alpha particles can't harm living cells

* Answer: Its alpha particles are stopped by a few centimetres of air and the alarm's case

<!-- Speaker note: Alpha cannot get out of the alarm. Inside, it knocks electrons off air molecules so a small current flows; smoke reduces the current, and the alarm sounds. -->

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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>

## In the lungs

**Radon gas gives out alpha particles too. Why is breathing it in a health risk, when the alarm is not?**

- Breathing makes radon decay faster
- Inside the lungs there is nothing between the alpha particles and living cells, and alpha does the most damage close up
- Radon's alpha particles are a different kind that go through walls

* Answer: Inside the lungs there is nothing between the alpha particles and living cells, and alpha does the most damage close up

<!-- Speaker note: Outside the body, skin and air stop alpha. Inside, alpha delivers all its energy to the cells next to it. That is why homes over some rocks are tested for radon and ventilated. -->

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## Evaluate: look back

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

**Revision summary**

- An unstable nucleus decays by giving out radiation. It is random for one atom, but predictable for many.
- **Alpha**: 2 protons and 2 neutrons (a helium nucleus). Atomic number −2, mass number −4: a new element. Stopped by paper or a few centimetres of air.
- **Beta**: a neutron turns into a proton and a fast electron, which leaves. Atomic number +1, mass number the same: a new element. Stopped by a few millimetres of aluminium.
- **Gamma**: energy only, usually just after alpha or beta. No change to the atomic or mass number. Reduced by thick lead or concrete.
- The more massive and charged the radiation, the sooner it is stopped, and the more damage it does close up.

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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

**Cobalt-60 (27 protons) decays by beta decay and gives out gamma rays. What is it now, and which of the radiation would get through 1 cm of aluminium?**

* Nickel-60 (28 protons, mass number still 60). Only the gamma rays get through the aluminium.

<!-- 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: How long does radioactivity last? (half-life, node 4). Node 5, decay chains, is a class task: the decay board with `isotope: uranium-238` lets students follow the real chain to lead-206 (the board says when each step is the real one and when it reaches a stable isotope). In class: a teacher demonstration or video of sealed sources, absorbers and a Geiger counter; a card sort of uses (smoke alarm, thyroid treatment, medical imaging, thickness gauges for paper and foil, checking welds in aircraft). -->