The shape of the lesson
By the end of the lesson, learners can
- Count down by halves: Watch the activity fall to a half, then a quarter, then an eighth, one half-life at a time.
- Fractions after each half-life: One half-life leaves a half, two leave a quarter, three leave an eighth.
- Half-life: time for the activity (or undecayed nuclei) to fall to half.
- After n half-lives: fraction left = one over two to the power n.
- Time: number of half-lives multiplied by the half-life.
One picture every idea returns to
The half-life is the time for half the radioactive nuclei in a sample to decay, which is the same as the time for the activity to fall to half. Decay is random, but each isotope has a fixed half-life.
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, Halving, and halving again.. You cannot say when one nucleus will decay, but you can say exactly how long it takes for half of a whole sample to go. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Count down by halves | Watch the activity fall to a half, then a quarter, then an eighth, one half-life at a time. Project tab 5 of the lab (Half-life) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Fractions after each half-life | One half-life leaves a half, two leave a quarter, three leave an eighth. Model the worked example Worked example on the board, one line at a time. | Rally Coach |
| 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, 12 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 Fractions after each half-life, 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.
Rally Coach
Pairs share one worksheet. One partner solves the first question aloud while the other coaches, checking each line and offering a prompt rather than the answer; they swap roles for the next question.
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 |
|---|---|
| Each half-life halves what is left, it does not subtract a fixed amount. | After two half-lives a quarter remains, after three an eighth, never reaching zero in a fixed number of steps. Decay is random but the half-life is constant. |
Half-life: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Define the half-life of a radioactive source.
A source has an activity of 480 counts per second. Its half-life is 2 hours. What is the activity after 6 hours?
A sample contains 8000 undecayed nuclei. The half-life is 5 days. How many remain undecayed after 15 days?
The activity of a source falls from 1600 counts per second to 200 counts per second in 24 minutes. Calculate the half-life.
A student says that after two half-lives a radioactive source has completely decayed. Explain why this is wrong, and state how much is actually left.
A radioactive tracer is to be injected into a patient. Explain why a source with a short half-life of a few hours is more suitable than one with a half-life of many years.
Answer key and marking notes
The time taken for half the undecayed nuclei (or for the activity) to fall to half. ✓
- half-lives = 6 / 2 = 3 ; halve three times: 480 to 240 to 120 to 60
activity after 6 hours = 60 counts per second
- half-lives = 15 / 5 = 3 ; 8000 to 4000 to 2000 to 1000
1000 nuclei remain undecayed
- 1600 to 800 to 400 to 200 is 3 halvings
half-life = 24 / 3 = 8
half-life = 8 minutes
- Each half-life removes half of what is left, not half of the original each time. ✓
- After two half-lives a quarter is still left. ✓
- A short half-life means the source decays away quickly. ✓
- So the patient is not exposed to radiation for a long time. ✓
2 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.
What is meant by the half-life of a radioactive isotope?
- Athe time taken for half of the radioactive nuclei in a sample to decay
- Bthe time taken for all the nuclei to decay
- Cthe time taken for the mass of the sample to double
- Dhalf the time the source has existed
A radioactive isotope has a half-life of 6 hours. A sample contains 80 g of the isotope. How much remains after 18 hours?
- A10 g
- B20 g
- C40 g
- D0 g
Quiz key
The half-life is the time for half of the undecayed nuclei to decay (equivalently, the time for the activity to fall to half). It does not depend on how much you start with.
18 hours is 3 half-lives, so the mass halves three times: 80 → 40 → 20 → 10 g. Option B halves only twice; the sample never reaches exactly zero.
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.