• 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 800 Bq. Its half-life is 6 hours. What is its activity after 12 hours, in Bq? → 200 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 3 minutes. What is its half-life, in minutes? → 3 minPrompt: Count how many times the rate halved.
3. A sample has an activity of 1600 Bq. Its half-life is 2 days. What is its activity after 6 days, in Bq? → 200 BqPrompt: How many half-lives have passed?

• 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.

• 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.

• 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 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.

• 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.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Why can't carbon-14 be used to date dinosaur bones? (Carbon-14 is not radioactive / Bones never contain isotopes / Its half-life is too short; almost none would be left after millions of years / Dinosaurs contained no carbon) → Its half-life is too short; almost none would be left after millions of yearsPrompt: After about 50 000 years too little carbon-14 remains to measure.
2. 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?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-11.