The shape of the lesson
By the end of the lesson, learners can
- Watch the nucleus change: Trigger alpha and beta decays and see how the nucleon and proton numbers shift.
- Two changes to remember: Alpha drops both numbers; beta keeps the nucleon number but raises the proton number.
- Alpha: nucleon number -4, proton number -2.
- Beta: nucleon number unchanged, proton number +1.
- Gamma: no change to either number.
One picture every idea returns to
In alpha decay the nucleon number falls by 4 and the proton number by 2. In beta decay the nucleon number stays the same and the proton number rises by 1. A decay equation must balance the nucleon numbers and the proton numbers on both sides.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Balancing the nucleus.. When a nucleus decays it does not vanish, it transforms, and the bookkeeping is exact. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Watch the nucleus change | Trigger alpha and beta decays and see how the nucleon and proton numbers shift. Project tab 4 of the lab (Decay equations) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Two changes to remember | Alpha drops both numbers; beta keeps the nucleon number but raises the proton number. Model the worked example Worked example on the board, one line at a time. | Round Table |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 13 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Two changes to remember, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Round Table
One sheet travels round the team. Each student adds one line of working or one idea and passes it on, so the solution is built four hands at a time and every contribution is visible.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| In alpha decay both numbers fall, the nucleon number by 4 and the proton number by 2. | In beta decay the nucleon number does not change; only the proton number rises by 1 as a neutron becomes a proton. Always balance both numbers. |
Radioactive decay equations: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by saying that radioactive decay is random and spontaneous.
Polonium-210 (proton number 84) decays by emitting an alpha particle. State the nucleon number and proton number of the daughter nucleus.
Carbon-14 (proton number 6) decays by emitting a beta particle. State the nucleon number and proton number of the daughter nucleus.
Explain why a nucleus becomes a different element when it emits a beta particle.
A nucleus emits a gamma ray only. State what happens to its nucleon number and proton number, and explain why.
In the decay below, find the missing nucleon number a and proton number b, and state whether it is alpha or beta decay.
uranium (238, 92) → thorium (a, b) + alpha (4, 2)
Answer key and marking notes
- Random: you cannot predict when a particular nucleus will decay. ✓
- Spontaneous: it is not affected by outside conditions such as temperature or pressure. ✓
- nucleon: 210 - 4 = 206 ; proton: 84 - 2 = 82
daughter: nucleon number 206, proton number 82 (lead-206)
- nucleon: stays 14 ; proton: 6 + 1 = 7
daughter: nucleon number 14, proton number 7 (nitrogen-14)
- A neutron changes into a proton, so the proton number increases by 1. ✓
- The element is decided by the proton number, so it becomes the next element. ✓
- Both stay the same. ✓
- Gamma is energy only; no protons or neutrons leave the nucleus. ✓
- a = 234, b = 90. ✓
- It is alpha decay (a helium nucleus is emitted). ✓
3 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
When a nucleus emits an alpha particle, how do its nucleon number and proton number change?
- Anucleon number decreases by 4, proton number decreases by 2
- Bnucleon number decreases by 2, proton number decreases by 4
- Cnucleon number stays the same, proton number increases by 1
- Dboth increase by 2
Radium-226 decays by emitting an alpha particle, forming radon (Rn). What is the nuclide formed?
- Aradon-222 (proton number 86)
- Bradon-230 (proton number 90)
- Cradon-222 (proton number 88)
- Dradon-226 (proton number 86)
When a nucleus emits a beta particle, what happens to its proton number and nucleon number?
- Aproton number increases by 1, nucleon number stays the same
- Bproton number decreases by 1, nucleon number stays the same
- Cboth decrease by 1
- Dproton number stays the same, nucleon number decreases by 1
Quiz key
Losing an alpha particle (2 protons + 2 neutrons) reduces the nucleon number by 4 and the proton number by 2. A change of +1 to the proton number describes beta decay, not alpha decay.
226 − 4 = 222 and 88 − 2 = 86, giving radon-222 with proton number 86. The nucleon and proton numbers must balance on both sides of the equation.
In beta decay a neutron becomes a proton and an electron; the electron is emitted, so the proton number rises by 1 while the total nucleon number is unchanged. The element changes to the next one up.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.