ScienceAges 16-18

Radioactive half-life decay

Calculate the remaining activity (Bq) of a radioactive sample after a whole number of half-lives. Review suitability for your learners before teaching.

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Worksheet StudioPractice pack · Ages 16-18

Radioactive half-life decay

Take your time. Show your thinking.

1. A radioactive sample has an initial mass of 160 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 160 g (t₁/₂ = 7 d) · t = 21 days
2. A radioactive sample has an initial mass of 512 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 15 days?
N₀ = 512 g (t₁/₂ = 3 d) · t = 15 days
3. A radioactive sample has an initial mass of 92 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 92 g (t₁/₂ = 9 d) · t = 18 days
4. A radioactive sample has an initial mass of 24 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 24 g (t₁/₂ = 8 d) · t = 8 days
5. A radioactive sample has an initial mass of 144 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 144 g (t₁/₂ = 7 d) · t = 21 days
6. A radioactive sample has an initial mass of 22 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 4 days?
N₀ = 22 g (t₁/₂ = 4 d) · t = 4 days
7. A radioactive sample has an initial mass of 112 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
N₀ = 112 g (t₁/₂ = 11 d) · t = 44 days
8. A radioactive sample has an initial mass of 352 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
N₀ = 352 g (t₁/₂ = 12 d) · t = 48 days
9. A radioactive sample has an initial mass of 40 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 6 days?
N₀ = 40 g (t₁/₂ = 3 d) · t = 6 days
10. A radioactive sample has an initial mass of 56 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
N₀ = 56 g (t₁/₂ = 9 d) · t = 27 days
11. A radioactive sample has an initial mass of 576 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 45 days?
N₀ = 576 g (t₁/₂ = 9 d) · t = 45 days
12. A radioactive sample has an initial mass of 14 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
N₀ = 14 g (t₁/₂ = 11 d) · t = 11 days

12 questions · Seed 42 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 16-18

Radioactive half-life decay

Answers and reasoning. Review before teaching.

1. A radioactive sample has an initial mass of 160 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
20 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 160 ÷ 2^3 = 20 g.
2. A radioactive sample has an initial mass of 512 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 15 days?
16 g
Number of half-lives = 15 ÷ 3 = 5. Remaining mass = 512 ÷ 2^5 = 16 g.
3. A radioactive sample has an initial mass of 92 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
23 g
Number of half-lives = 18 ÷ 9 = 2. Remaining mass = 92 ÷ 2^2 = 23 g.
4. A radioactive sample has an initial mass of 24 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
12 g
Number of half-lives = 8 ÷ 8 = 1. Remaining mass = 24 ÷ 2^1 = 12 g.
5. A radioactive sample has an initial mass of 144 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
18 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 144 ÷ 2^3 = 18 g.
6. A radioactive sample has an initial mass of 22 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 4 days?
11 g
Number of half-lives = 4 ÷ 4 = 1. Remaining mass = 22 ÷ 2^1 = 11 g.
7. A radioactive sample has an initial mass of 112 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
7 g
Number of half-lives = 44 ÷ 11 = 4. Remaining mass = 112 ÷ 2^4 = 7 g.
8. A radioactive sample has an initial mass of 352 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
22 g
Number of half-lives = 48 ÷ 12 = 4. Remaining mass = 352 ÷ 2^4 = 22 g.
9. A radioactive sample has an initial mass of 40 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 6 days?
10 g
Number of half-lives = 6 ÷ 3 = 2. Remaining mass = 40 ÷ 2^2 = 10 g.
10. A radioactive sample has an initial mass of 56 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
7 g
Number of half-lives = 27 ÷ 9 = 3. Remaining mass = 56 ÷ 2^3 = 7 g.
11. A radioactive sample has an initial mass of 576 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 45 days?
18 g
Number of half-lives = 45 ÷ 9 = 5. Remaining mass = 576 ÷ 2^5 = 18 g.
12. A radioactive sample has an initial mass of 14 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
7 g
Number of half-lives = 11 ÷ 11 = 1. Remaining mass = 14 ÷ 2^1 = 7 g.

12 questions · Seed 42 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 16-18

Radioactive half-life decay

Take your time. Show your thinking.

1. A radioactive sample has an initial mass of 160 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 160 g (t₁/₂ = 7 d) · t = 21 days
2. A radioactive sample has an initial mass of 512 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 15 days?
N₀ = 512 g (t₁/₂ = 3 d) · t = 15 days
3. A radioactive sample has an initial mass of 92 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 92 g (t₁/₂ = 9 d) · t = 18 days
4. A radioactive sample has an initial mass of 24 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 24 g (t₁/₂ = 8 d) · t = 8 days
5. A radioactive sample has an initial mass of 144 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 144 g (t₁/₂ = 7 d) · t = 21 days
6. A radioactive sample has an initial mass of 22 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 4 days?
N₀ = 22 g (t₁/₂ = 4 d) · t = 4 days
7. A radioactive sample has an initial mass of 112 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
N₀ = 112 g (t₁/₂ = 11 d) · t = 44 days
8. A radioactive sample has an initial mass of 352 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
N₀ = 352 g (t₁/₂ = 12 d) · t = 48 days
9. A radioactive sample has an initial mass of 40 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 6 days?
N₀ = 40 g (t₁/₂ = 3 d) · t = 6 days
10. A radioactive sample has an initial mass of 56 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
N₀ = 56 g (t₁/₂ = 9 d) · t = 27 days
11. A radioactive sample has an initial mass of 576 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 45 days?
N₀ = 576 g (t₁/₂ = 9 d) · t = 45 days
12. A radioactive sample has an initial mass of 14 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
N₀ = 14 g (t₁/₂ = 11 d) · t = 11 days

12 questions · Seed 42 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 16-18

Radioactive half-life decay

Answers and reasoning. Review before teaching.

1. A radioactive sample has an initial mass of 160 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
20 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 160 ÷ 2^3 = 20 g.
2. A radioactive sample has an initial mass of 512 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 15 days?
16 g
Number of half-lives = 15 ÷ 3 = 5. Remaining mass = 512 ÷ 2^5 = 16 g.
3. A radioactive sample has an initial mass of 92 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
23 g
Number of half-lives = 18 ÷ 9 = 2. Remaining mass = 92 ÷ 2^2 = 23 g.
4. A radioactive sample has an initial mass of 24 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
12 g
Number of half-lives = 8 ÷ 8 = 1. Remaining mass = 24 ÷ 2^1 = 12 g.
5. A radioactive sample has an initial mass of 144 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
18 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 144 ÷ 2^3 = 18 g.
6. A radioactive sample has an initial mass of 22 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 4 days?
11 g
Number of half-lives = 4 ÷ 4 = 1. Remaining mass = 22 ÷ 2^1 = 11 g.
7. A radioactive sample has an initial mass of 112 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
7 g
Number of half-lives = 44 ÷ 11 = 4. Remaining mass = 112 ÷ 2^4 = 7 g.
8. A radioactive sample has an initial mass of 352 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
22 g
Number of half-lives = 48 ÷ 12 = 4. Remaining mass = 352 ÷ 2^4 = 22 g.
9. A radioactive sample has an initial mass of 40 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 6 days?
10 g
Number of half-lives = 6 ÷ 3 = 2. Remaining mass = 40 ÷ 2^2 = 10 g.
10. A radioactive sample has an initial mass of 56 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
7 g
Number of half-lives = 27 ÷ 9 = 3. Remaining mass = 56 ÷ 2^3 = 7 g.
11. A radioactive sample has an initial mass of 576 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 45 days?
18 g
Number of half-lives = 45 ÷ 9 = 5. Remaining mass = 576 ÷ 2^5 = 18 g.
12. A radioactive sample has an initial mass of 14 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
7 g
Number of half-lives = 11 ÷ 11 = 1. Remaining mass = 14 ÷ 2^1 = 7 g.

12 questions · Seed 42 · Age guidance, not curriculum certification