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Science lesson plan · 11th grade (Year 12 in England) · 60 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 · 6 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 1200 Bq. Its half-life is 30 hours. What is its activity after 90 hours, in Bq? → 150 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 45 minutes. What is its half-life, in minutes? → 15 minPrompt: Count how many times the rate halved.
  3. A sample has an activity of 1600 Bq. Its half-life is 6 days. What is its activity after 18 days, in Bq? → 200 BqPrompt: How many half-lives have passed?

Hook · 6 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 · 12 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 · 12 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 12.5 % of the carbon-14 found in living wood. The half-life of carbon-14 is 5730 years. How old is the wood, in years? → 17190Prompt: 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 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? → 5Prompt: Find what fraction of the original potassium is left.

You do: independent practice · 17 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 · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. 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? → 5Prompt: Find what fraction of the original potassium is left.
  2. Why must an igneous rock be dated rather than a sedimentary rock for an accurate age? (Sedimentary rocks are always younger than 1000 years / Igneous rock starts its radioactive "clock" when it crystallises; sediment grains come from older rocks / Sedimentary rocks have no atoms / Igneous rocks have no radioactivity) → Igneous rock starts its radioactive "clock" when it crystallises; sediment grains come from older rocksPrompt: When does the clock start?

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

eurekami

Date rocks and fossils with radioactive decay

Name

Date

Score/ 8

  1. Question 1

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

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

  2. Question 2

    A piece of ancient wood has 12.5 % 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

  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 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

  4. Question 4

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

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

  5. Question 5

    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

  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 can't carbon-14 be used to date dinosaur bones?

    • A. Carbon-14 is not radioactive
    • B. Its half-life is too short; almost none would be left after millions of years
    • C. Bones never contain isotopes
    • D. Dinosaurs contained no carbon

  8. Question 8

    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?

    Answer

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Answer key: Date rocks and fossils with radioactive decay

  1. 1. D. meteorites and the oldest mineral grains, dated with uranium–lead

  2. 2. 17190

  3. 3. 5

  4. 4. A. uranium-238 (half-life about 4.5 billion years)

  5. 5. 11460

  6. 6. 3.75

  7. 7. B. Its half-life is too short; almost none would be left after millions of years

  8. 8. 22920