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Science lesson plan · 10th grade (Year 11 in England) · 30 minutes

Half-life calculations

Atoms, elements and the periodic table

CCSS NGSS.HS-PS1-8CCSS NGSS.SEP 5England: GCSE Combined Science: Atomic structure (radioactivity) (half-lives)

Objective and success criteria

Students will: Half-life calculations.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Alpha, beta and gamma radiation

Leads to: Date rocks and fossils with radioactive decay

Materials: Periodic tables, Atom model kits, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 4 min

  • Quick retrieval from earlier learning (Alpha, beta and gamma radiation). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Which type of radiation: the most strongly ionising? (alpha (α) / beta (β) / gamma (γ)) → alpha (α)Prompt: The more ionising the radiation, the less penetrating it is.
  2. Why are gamma rays used to sterilise medical equipment in sealed packs? (They are stopped by plastic / They make the equipment radioactive / They heat the equipment to 1000 °C / They pass through the packaging and kill microbes) → They pass through the packaging and kill microbesPrompt: Gamma is very penetrating.
  3. Nuclear radiation: which statement is NOT true? (Alpha radiation is the most dangerous if the source is inside the body. / Radioactive decay is random. / Beta particles are helium nuclei. / Gamma rays are part of the electromagnetic spectrum.) → Beta particles are helium nuclei.Prompt: Which radiation needs lead to stop it?

Hook · 3 min

  • Show the periodic table and ask what patterns students can see before anything is explained.
  • Share the objective: Half-life calculations. Read the success criteria together.

I do: model it · 7 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: A sample has an activity of 1600 Bq. Its half-life is 6 days. What is its activity after 24 days, in Bq?
  • Step 1: 24 ÷ 6 = 4
  • Step 2: 1600 ÷ 2^4 = 100
  • Step 3: So the answer is: 100 Bq
  • Answer: 100 Bq
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 6 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. A sample has an activity of 800 Bq. Its half-life is 2 minutes. What is its activity after 8 minutes, in Bq? → 50 BqPrompt: How many half-lives have passed?
  2. The count rate of a radioactive source falls from 1600 counts/min to 800 counts/min in 10 minutes. What is its half-life, in minutes? → 10 minPrompt: Count how many times the rate halved.

You do: independent practice · 7 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A sample contains 6400 undecayed nuclei. How many remain undecayed after 3 half-lives? → 800Prompt: After each half-life, half of the remaining nuclei are left.
  2. The count rate of a radioactive source falls from 960 counts/min to 240 counts/min in 8 minutes. What is its half-life, in minutes? → 4 minPrompt: Count how many times the rate halved.

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Alpha, beta and gamma radiation.
  • Use concrete materials: periodic tables.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Move on to: Date rocks and fossils with radioactive decay.
  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-10.

Free lesson plan from eurekami.com/lesson-plan-builder

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Half-life calculations

Name

Date

Score/ 6

  1. Question 1

    A sample contains 1600 undecayed nuclei. How many remain undecayed after 4 half-lives?

    Answer

  2. Question 2

    The count rate of a radioactive source falls from 640 counts/min to 160 counts/min in 10 minutes. What is its half-life, in minutes?

    Answermin

  3. Question 3

    A sample has an activity of 3200 Bq. Its half-life is 5 hours. What is its activity after 5 hours, in Bq?

    AnswerBq

  4. Question 4

    A sample contains 12800 undecayed nuclei. How many remain undecayed after 4 half-lives?

    Answer

  5. Question 5

    The count rate of a radioactive source falls from 960 counts/min to 480 counts/min in 15 minutes. What is its half-life, in minutes?

    Answermin

  6. Question 6

    A sample has an activity of 2000 Bq. Its half-life is 30 days. What is its activity after 30 days, in Bq?

    AnswerBq

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Answer key: Half-life calculations

  1. 1. 100

  2. 2. 5 min

  3. 3. 1600 Bq

  4. 4. 800

  5. 5. 15 min

  6. 6. 1000 Bq