Course: Combined Science + Separate Physics | Tier: Foundation + Higher (HT parts marked) | Time: about 60 minutes (you can split it into two 30-minute sessions)
✏️ You need: your exercise book, a pen, a coloured pen for corrections and a calculator. Work through the steps in order. Write every answer before you open it.
🗺️ Your route through this page
Step 1 Rate yourself (3 min) → Step 2 Watch, learn and check – 5 chunks (35 min) → Step 3 Exam questions (12 min) → Step 4 6-mark answer (8 min) → Step 5 Review (2 min)
☢️ what is radioactive decay → 🧱 alpha, beta and gamma → 🧮 nuclear equations → ⏳ half-life → ⚠️ irradiation and contamination
Step 1 – Rate yourself 🚦
Copy this table into your book. Give each statement a colour now: 🔴 I can’t do this yet · 🟠 I’m not sure · 🟢 I can do this. You’ll rate yourself again at the end.
| I can… | Start | End |
|---|---|---|
| explain what radioactive decay and activity are | ||
| describe alpha, beta and gamma radiation and how far they penetrate | ||
| complete nuclear equations for alpha and beta decay | ||
| explain what half-life means and use it in calculations | ||
| explain the difference between irradiation and contamination |
Step 2 – Watch, learn and check 🎬📖
Do each chunk in order:
1. 🎬 Watch the video and write 3 key facts in your book.
2. 📖 Read the notes.
3. ✅ Answer the quick check without looking back. Correct any mistakes in coloured pen.
Videos by Freesciencelessons (YouTube). If a video won’t play on your network, just read the notes – they cover everything you need.
Chunk A: What is radioactive decay? ☢️
🎬 Watch: Radioactivity
📖 Read:
- Some atoms have an unstable nucleus. To become more stable, the nucleus gives out radiation. This is called radioactive decay.
- Decay is random – we can’t predict which nucleus will decay next, or when.
- Activity = the number of nuclei that decay each second. It is measured in becquerels (Bq). 1 Bq = 1 decay per second.
- Count rate = the number of decays recorded each second by a detector, such as a Geiger–Müller tube.
- The nucleus can give out alpha particles, beta particles, gamma rays or neutrons.
✅ Quick check A
1. Why does a nucleus give out radiation?
2. What is the unit of activity?
3. What does “random” mean for radioactive decay?
Show answers
1. It is unstable – giving out radiation makes it more stable
2. The becquerel (Bq)
3. You can’t predict which nucleus will decay or when
Chunk B: Alpha, beta and gamma 🧱
🎬 Watch: Properties of Alpha, Beta and Gamma Radiation. While you watch, copy the table below into your book and fill it in.
📖 Read:
| Alpha (α) | Beta (β) | Gamma (γ) | |
|---|---|---|---|
| What is it? | 2 protons + 2 neutrons (a helium nucleus) | a fast-moving electron from the nucleus | an electromagnetic wave |
| Stopped by | 📄 paper or skin | 🥫 a few mm of aluminium | 🧱 thick lead or concrete (reduced, not fully stopped) |
| Range in air | a few cm | about 1 m | a very long way |
| Ionising power | 🔴 strongly ionising | 🟠 moderately ionising | 🟢 weakly ionising |
💡 Pattern: the more ionising the radiation, the less penetrating it is. Alpha is the most ionising but the least penetrating.
✅ Quick check B
1. What is an alpha particle made of?
2. Which radiation is stopped by a few mm of aluminium?
3. Which radiation is the most ionising?
Show answers
1. 2 protons and 2 neutrons
2. Beta
3. Alpha
Chunk C: Nuclear equations 🧮
🎬 Watch: Nuclear Equations. Pause after each example and try it yourself first.
📖 Read: in a nuclear equation, the top numbers (mass numbers) and the bottom numbers (atomic numbers) must add up the same on both sides.
| Decay | Mass number | Atomic number | Symbol |
|---|---|---|---|
| Alpha | goes down by 4 | goes down by 2 | 42He |
| Beta | stays the same | goes up by 1 (a neutron turns into a proton) | 0−1e |
| Gamma | no change | no change | γ |
✅ Worked examples
Alpha: 22688Ra → 22286Rn + 42He (226 = 222 + 4 ✔ 88 = 86 + 2 ✔)
Beta: 146C → 147N + 0−1e (14 = 14 + 0 ✔ 6 = 7 + (−1) ✔)
✅ Quick check C
1. Uranium-238 (atomic number 92) emits an alpha particle. What are the new mass number and atomic number?
2. Iodine-131 (atomic number 53) emits a beta particle. What are the new mass number and atomic number?
3. What happens to the numbers when a gamma ray is emitted?
Show answers
1. Mass number 238 − 4 = 234; atomic number 92 − 2 = 90 (thorium)
2. Mass number stays 131; atomic number 53 + 1 = 54 (xenon)
3. Nothing changes – gamma is just energy
Chunk D: Half-life ⏳
🎬 Watch: Half Life
📖 Read: the half-life of a radioactive isotope is the time it takes for:
• the number of unstable nuclei in a sample to halve, or
• the activity (or count rate) to fall to half its starting value.
✅ Worked example: a sample has an activity of 800 Bq. Its half-life is 2 days. What is its activity after 6 days?
Step 1 – number of half-lives: 6 ÷ 2 = 3
Step 2 – halve 3 times: 800 → 400 → 200 → 100 Bq
📈 From a graph: find the starting activity, halve it, read across to the curve and down to the time axis. That time is the half-life.
(HT) You may be asked for the net decline as a ratio. After 2 half-lives, ¼ of the activity is left, so the activity has fallen by ¾. The ratio of final to starting activity is 1 : 4.
✅ Quick check D
1. Define half-life.
2. A sample has an activity of 600 Bq and a half-life of 5 minutes. What is its activity after 10 minutes?
3. How many half-lives does it take for the activity to fall to one-eighth?
Show answers
1. The time for the number of unstable nuclei (or the activity) to halve
2. 10 ÷ 5 = 2 half-lives: 600 → 300 → 150 Bq
3. 3 (½ → ¼ → ⅛)
Chunk E: Irradiation and contamination ⚠️
🎬 Watch: Irradiation and Contamination
📖 Read:
| Irradiation | Contamination | |
|---|---|---|
| What it means | being exposed to radiation from a source outside the body | radioactive atoms get on or inside the body (or an object) |
| Does the object become radioactive? | No | Yes – the atoms keep decaying |
| When does it stop? | as soon as you move away or the source is shielded | it carries on until the atoms are removed or decay |
| Protection | lead shielding; keep your distance; limit the time | gloves; tongs; masks; sealed sources |
💡 Alpha is a trick question! Outside the body, alpha is the least dangerous – skin stops it. Inside the body (contamination), it is the most dangerous – it is strongly ionising and damages cells nearby.
✅ Quick check E
1. Which one makes an object radioactive: irradiation or contamination?
2. Give two ways to reduce irradiation.
3. Why is alpha most dangerous inside the body?
Show answers
1. Contamination
2. Any two: shielding (e.g. lead) · keep a distance · limit the time of exposure
3. It is strongly ionising, so it causes lots of damage to nearby cells
Step 3 – Exam questions ✏️
Answer in full sentences. The marks tell you how many points to make. The questions follow the same order as the chunks. Mark your work in coloured pen.
Q1 (Chunk A) What is meant by radioactive decay? [1 mark]
Mark scheme
an unstable nucleus gives out radiation to become more stable (1)
Q2 (Chunk B) Which type of radiation is (a) the most ionising, (b) the most penetrating? [2 marks]
Mark scheme
(a) alpha (1)
(b) gamma (1)
Q3 (Chunk C) Polonium-210 has atomic number 84. It decays by emitting an alpha particle to form lead. Give the mass number and atomic number of the lead nucleus. [2 marks]
Mark scheme
mass number = 210 − 4 = 206 (1)
atomic number = 84 − 2 = 82 (1)
Q4 (Chunk D) A sample has an activity of 1200 Bq. Its half-life is 3 hours. Calculate its activity after 9 hours. [2 marks]
Mark scheme
9 ÷ 3 = 3 half-lives (1)
1200 → 600 → 300 → 150 Bq (1)
Q5 (Chunk E) Explain the difference between irradiation and contamination. [2 marks]
Mark scheme
irradiation is being exposed to radiation from a source outside the object; the object does not become radioactive (1)
contamination is when radioactive atoms get on or into an object, so it becomes radioactive (1)
Q6 (Chunks B and E) Explain why an alpha source is fairly safe outside the body but very dangerous if swallowed. [2 marks]
Mark scheme
outside: alpha is stopped by the skin / has a short range, so it can’t reach living cells (1)
inside: alpha is strongly ionising, so it damages nearby cells / can cause cancer (1)
Step 4 – Write a 6-mark answer 📝
Q7 A paper factory uses a radioactive source and a detector to check the paper is the right thickness. The paper passes between the source and the detector. Explain which type of radiation should be used, why the other two would not work, and why the source should have a long half-life. [6 marks]
🧩 Planning frame – use your table from Chunk B and these sentence starters:
1. “Beta radiation should be used because…”
2. “If the paper gets thicker, the count rate…”
3. “Alpha would not work because…”
4. “Gamma would not work because…”
5. “The source needs a long half-life so that…”
Model answer and mark scheme
Model answer: Beta radiation should be used because some of it passes through the paper and some is absorbed. If the paper gets thicker, more beta is absorbed, so the count rate at the detector goes down. If the paper gets thinner, the count rate goes up, so the machine can adjust the rollers. Alpha would not work because it is stopped completely by paper, so the count rate would be zero whatever the thickness. Gamma would not work because it passes straight through paper, so the count rate would hardly change when the thickness changes. The source needs a long half-life so that its activity stays almost the same for a long time. This means any change in the count rate is caused by the paper thickness, not by the source decaying, and the source does not need replacing often.
Level 3 (5–6): chooses beta with a clear explanation, explains why alpha and gamma don’t work, and explains the long half-life. Level 2 (3–4): chooses beta with some explanation, and deals with one other type or the half-life. Level 1 (1–2): simple points, e.g. “alpha is stopped by paper”.
Step 5 – Review 🔁
1. Go back to your Step 1 table and fill in the End column.
2. For anything still 🔴 or 🟠, re-watch that chunk’s video, then write one question to ask your teacher.
3. Write down your score for Step 3 (out of 11) and Step 4 (out of 6).
☢️ What next?
Still 🔴? → History of the atom made simple · Fill the gaps: Atomic Structure · Draw the Atomic Structure diagrams
Mostly 🟢? → Atomic Structure quiz · Test (Foundation) · Test (Higher)
Back to the Atomic Structure topic page
