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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 pass through the packaging and kill microbes / They heat the equipment to 1000 °C / They make the equipment radioactive / They are stopped by plastic) → They pass through the packaging and kill microbesPrompt: Gamma is very penetrating.
  3. Nuclear radiation: which statement is NOT true? (Background radiation comes from natural and man-made sources. / Radioactive decay is random. / Gamma rays are part of the electromagnetic spectrum. / We can predict exactly when a particular nucleus will decay.) → We can predict exactly when a particular nucleus will decay.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 1200 Bq. Its half-life is 12 minutes. What is its activity after 24 minutes, in Bq? → 300 BqPrompt: How many half-lives have passed?
  2. The count rate of a radioactive source falls from 1200 counts/min to 600 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 1600 undecayed nuclei. How many remain undecayed after 2 half-lives? → 400Prompt: After each half-life, half of the remaining nuclei are left.
  2. The count rate of a radioactive source falls from 1600 counts/min to 200 counts/min in 9 minutes. What is its half-life, in minutes? → 3 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 2 half-lives?

    Answer

  2. Question 2

    A sample has an activity of 1200 Bq. Its half-life is 8 hours. What is its activity after 32 hours, in Bq?

    AnswerBq

  3. Question 3

    The count rate of a radioactive source falls from 1600 counts/min to 400 counts/min in 20 minutes. What is its half-life, in minutes?

    Answermin

  4. Question 4

    A sample contains 800 undecayed nuclei. How many remain undecayed after 2 half-lives?

    Answer

  5. Question 5

    A sample has an activity of 2000 Bq. Its half-life is 2 hours. What is its activity after 8 hours, in Bq?

    AnswerBq

  6. Question 6

    The count rate of a radioactive source falls from 640 counts/min to 160 counts/min in 16 minutes. What is its half-life, in minutes?

    Answermin

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Answer key: Half-life calculations

  1. 1. 400

  2. 2. 75 Bq

  3. 3. 10 min

  4. 4. 200

  5. 5. 125 Bq

  6. 6. 8 min