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Science lesson plan · 11th grade (Year 12 in England) · 45 minutes

Date rocks and fossils with radioactive decay

Earth and rocks

CCSS NGSS.HS-ESS1-5CCSS NGSS.HS-ESS1-6CCSS NGSS.SEP 5England: GCSE Physics: Atomic structure (half-life uses)England: A level Geology/Physics links

Objective and success criteria

Students will: Date rocks and fossils with radioactive decay.

  • 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: Half-life calculations; Earth's 4.6-billion-year history on a timeline

Leads to: the next skill in this area.

Materials: Rock samples, Hand lenses, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 5 min

  • Quick retrieval from earlier learning (Half-life calculations; Earth's 4.6-billion-year history on a timeline). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. A sample has an activity of 2000 Bq. Its half-life is 5 hours. What is its activity after 15 hours, in Bq? → 250 BqPrompt: How many half-lives have passed?
  2. The count rate of a radioactive source falls from 1200 counts/min to 150 counts/min in 15 minutes. What is its half-life, in minutes? → 5 minPrompt: Count how many times the rate halved.
  3. A sample has an activity of 1200 Bq. Its half-life is 2 minutes. What is its activity after 6 minutes, in Bq? → 150 BqPrompt: How many half-lives have passed?

Hook · 5 min

  • Pass round different rocks and ask how each one might have formed.
  • Share the objective: Date rocks and fossils with radioactive decay. Read the success criteria together.

I do: model it · 10 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: A piece of ancient wood has 50 % of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. How old is the wood, in years?
  • Step 1: 100 → 50 %: 1 half-lives
  • Step 2: 1 × 5730 = 5730
  • Step 3: So the answer is: 5730
  • Answer: 5730
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 10 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 piece of ancient wood has 50 % of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. How old is the wood, in years? → 5730Prompt: How many times has 100 % been halved to reach this value?
  2. In a volcanic rock, potassium-40 decays to argon-40 with a half-life of 1.25 billion years. The rock now contains 7 atoms of argon-40 for every 1 atom of potassium-40 (assume all the argon came from decay). How old is the rock, in billions of years? → 3.75Prompt: Find what fraction of the original potassium is left.

You do: independent practice · 10 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 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A piece of ancient wood has 6.25 % of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. How old is the wood, in years? → 22920Prompt: How many times has 100 % been halved to reach this value?
  2. In a volcanic rock, potassium-40 decays to argon-40 with a half-life of 1.25 billion years. The rock now contains 7 atoms of argon-40 for every 1 atom of potassium-40 (assume all the argon came from decay). How old is the rock, in billions of years? → 3.75Prompt: Find what fraction of the original potassium is left.

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: Half-life calculations.
  • Use concrete materials: rock samples.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • 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-11.

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

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Date rocks and fossils with radioactive decay

Name

Date

Score/ 8

  1. Question 1

    A piece of ancient wood has 50 % of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. How old is the wood, in years?

    Answer

  2. Question 2

    Which isotope is best for dating the oldest rocks on Earth (billions of years)?

    • A. carbon-14 (half-life 5730 years)
    • B. uranium-238 (half-life about 4.5 billion years)
    • C. iodine-131 (half-life 8 days)
    • D. radon-222 (half-life about 4 days)

  3. Question 3

    In a volcanic rock, potassium-40 decays to argon-40 with a half-life of 1.25 billion years. The rock now contains 1 atoms of argon-40 for every 1 atom of potassium-40 (assume all the argon came from decay). How old is the rock, in billions of years?

    Answer

  4. Question 4

    A piece of ancient wood has 25 % of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. How old is the wood, in years?

    Answer

  5. Question 5

    Radiometric dating gives the age of Earth as about 4.5 billion years. Which samples gave this result?

    • A. dinosaur bones dated with carbon-14
    • B. meteorites and the oldest mineral grains, dated with uranium–lead
    • C. ice cores
    • D. tree rings

  6. Question 6

    In a volcanic rock, potassium-40 decays to argon-40 with a half-life of 1.25 billion years. The rock now contains 7 atoms of argon-40 for every 1 atom of potassium-40 (assume all the argon came from decay). How old is the rock, in billions of years?

    Answer

  7. Question 7

    Why must an igneous rock be dated rather than a sedimentary rock for an accurate age?

    • A. Sedimentary rocks have no atoms
    • B. Igneous rock starts its radioactive "clock" when it crystallises; sediment grains come from older rocks
    • C. Sedimentary rocks are always younger than 1000 years
    • D. Igneous rocks have no radioactivity

  8. Question 8

    In a volcanic rock, potassium-40 decays to argon-40 with a half-life of 1.25 billion years. The rock now contains 15 atoms of argon-40 for every 1 atom of potassium-40 (assume all the argon came from decay). How old is the rock, in billions of years?

    Answer

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Answer key: Date rocks and fossils with radioactive decay

  1. 1. 5730

  2. 2. B. uranium-238 (half-life about 4.5 billion years)

  3. 3. 1.25

  4. 4. 11460

  5. 5. B. meteorites and the oldest mineral grains, dated with uranium–lead

  6. 6. 3.75

  7. 7. B. Igneous rock starts its radioactive "clock" when it crystallises; sediment grains come from older rocks

  8. 8. 5