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 64 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 64 g (t₁/₂ = 2 d) · t = 8 days
2. A radioactive sample has an initial mass of 416 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 25 days?
N₀ = 416 g (t₁/₂ = 5 d) · t = 25 days
3. A radioactive sample has an initial mass of 160 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 60 days?
N₀ = 160 g (t₁/₂ = 12 d) · t = 60 days
4. A radioactive sample has an initial mass of 48 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 36 days?
N₀ = 48 g (t₁/₂ = 12 d) · t = 36 days
5. A radioactive sample has an initial mass of 4 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 6 days?
N₀ = 4 g (t₁/₂ = 6 d) · t = 6 days
6. A radioactive sample has an initial mass of 56 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
N₀ = 56 g (t₁/₂ = 3 d) · t = 9 days
7. A radioactive sample has an initial mass of 200 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
N₀ = 200 g (t₁/₂ = 9 d) · t = 27 days
8. A radioactive sample has an initial mass of 16 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
N₀ = 16 g (t₁/₂ = 11 d) · t = 11 days
9. A radioactive sample has an initial mass of 84 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 84 g (t₁/₂ = 6 d) · t = 12 days
10. A radioactive sample has an initial mass of 48 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 48 g (t₁/₂ = 4 d) · t = 12 days
11. A radioactive sample has an initial mass of 512 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
N₀ = 512 g (t₁/₂ = 2 d) · t = 10 days
12. A radioactive sample has an initial mass of 88 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 33 days?
N₀ = 88 g (t₁/₂ = 11 d) · t = 33 days

12 questions · Seed 46 · 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 64 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 8 days?
4 g
Number of half-lives = 8 ÷ 2 = 4. Remaining mass = 64 ÷ 2^4 = 4 g.
2. A radioactive sample has an initial mass of 416 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 25 days?
13 g
Number of half-lives = 25 ÷ 5 = 5. Remaining mass = 416 ÷ 2^5 = 13 g.
3. A radioactive sample has an initial mass of 160 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 60 days?
5 g
Number of half-lives = 60 ÷ 12 = 5. Remaining mass = 160 ÷ 2^5 = 5 g.
4. A radioactive sample has an initial mass of 48 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 36 days?
6 g
Number of half-lives = 36 ÷ 12 = 3. Remaining mass = 48 ÷ 2^3 = 6 g.
5. A radioactive sample has an initial mass of 4 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 6 days?
2 g
Number of half-lives = 6 ÷ 6 = 1. Remaining mass = 4 ÷ 2^1 = 2 g.
6. A radioactive sample has an initial mass of 56 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
7 g
Number of half-lives = 9 ÷ 3 = 3. Remaining mass = 56 ÷ 2^3 = 7 g.
7. A radioactive sample has an initial mass of 200 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
25 g
Number of half-lives = 27 ÷ 9 = 3. Remaining mass = 200 ÷ 2^3 = 25 g.
8. A radioactive sample has an initial mass of 16 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
8 g
Number of half-lives = 11 ÷ 11 = 1. Remaining mass = 16 ÷ 2^1 = 8 g.
9. A radioactive sample has an initial mass of 84 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
21 g
Number of half-lives = 12 ÷ 6 = 2. Remaining mass = 84 ÷ 2^2 = 21 g.
10. A radioactive sample has an initial mass of 48 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 12 days?
6 g
Number of half-lives = 12 ÷ 4 = 3. Remaining mass = 48 ÷ 2^3 = 6 g.
11. A radioactive sample has an initial mass of 512 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
16 g
Number of half-lives = 10 ÷ 2 = 5. Remaining mass = 512 ÷ 2^5 = 16 g.
12. A radioactive sample has an initial mass of 88 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 33 days?
11 g
Number of half-lives = 33 ÷ 11 = 3. Remaining mass = 88 ÷ 2^3 = 11 g.

12 questions · Seed 46 · 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 64 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 64 g (t₁/₂ = 2 d) · t = 8 days
2. A radioactive sample has an initial mass of 416 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 25 days?
N₀ = 416 g (t₁/₂ = 5 d) · t = 25 days
3. A radioactive sample has an initial mass of 160 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 60 days?
N₀ = 160 g (t₁/₂ = 12 d) · t = 60 days
4. A radioactive sample has an initial mass of 48 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 36 days?
N₀ = 48 g (t₁/₂ = 12 d) · t = 36 days
5. A radioactive sample has an initial mass of 4 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 6 days?
N₀ = 4 g (t₁/₂ = 6 d) · t = 6 days
6. A radioactive sample has an initial mass of 56 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
N₀ = 56 g (t₁/₂ = 3 d) · t = 9 days
7. A radioactive sample has an initial mass of 200 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
N₀ = 200 g (t₁/₂ = 9 d) · t = 27 days
8. A radioactive sample has an initial mass of 16 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
N₀ = 16 g (t₁/₂ = 11 d) · t = 11 days
9. A radioactive sample has an initial mass of 84 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 84 g (t₁/₂ = 6 d) · t = 12 days
10. A radioactive sample has an initial mass of 48 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 48 g (t₁/₂ = 4 d) · t = 12 days
11. A radioactive sample has an initial mass of 512 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
N₀ = 512 g (t₁/₂ = 2 d) · t = 10 days
12. A radioactive sample has an initial mass of 88 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 33 days?
N₀ = 88 g (t₁/₂ = 11 d) · t = 33 days

12 questions · Seed 46 · 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 64 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 8 days?
4 g
Number of half-lives = 8 ÷ 2 = 4. Remaining mass = 64 ÷ 2^4 = 4 g.
2. A radioactive sample has an initial mass of 416 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 25 days?
13 g
Number of half-lives = 25 ÷ 5 = 5. Remaining mass = 416 ÷ 2^5 = 13 g.
3. A radioactive sample has an initial mass of 160 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 60 days?
5 g
Number of half-lives = 60 ÷ 12 = 5. Remaining mass = 160 ÷ 2^5 = 5 g.
4. A radioactive sample has an initial mass of 48 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 36 days?
6 g
Number of half-lives = 36 ÷ 12 = 3. Remaining mass = 48 ÷ 2^3 = 6 g.
5. A radioactive sample has an initial mass of 4 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 6 days?
2 g
Number of half-lives = 6 ÷ 6 = 1. Remaining mass = 4 ÷ 2^1 = 2 g.
6. A radioactive sample has an initial mass of 56 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
7 g
Number of half-lives = 9 ÷ 3 = 3. Remaining mass = 56 ÷ 2^3 = 7 g.
7. A radioactive sample has an initial mass of 200 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 27 days?
25 g
Number of half-lives = 27 ÷ 9 = 3. Remaining mass = 200 ÷ 2^3 = 25 g.
8. A radioactive sample has an initial mass of 16 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 11 days?
8 g
Number of half-lives = 11 ÷ 11 = 1. Remaining mass = 16 ÷ 2^1 = 8 g.
9. A radioactive sample has an initial mass of 84 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
21 g
Number of half-lives = 12 ÷ 6 = 2. Remaining mass = 84 ÷ 2^2 = 21 g.
10. A radioactive sample has an initial mass of 48 g and a half-life of 4 days. How many grams of the isotope remain undecayed after 12 days?
6 g
Number of half-lives = 12 ÷ 4 = 3. Remaining mass = 48 ÷ 2^3 = 6 g.
11. A radioactive sample has an initial mass of 512 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
16 g
Number of half-lives = 10 ÷ 2 = 5. Remaining mass = 512 ÷ 2^5 = 16 g.
12. A radioactive sample has an initial mass of 88 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 33 days?
11 g
Number of half-lives = 33 ÷ 11 = 3. Remaining mass = 88 ÷ 2^3 = 11 g.

12 questions · Seed 46 · Age guidance, not curriculum certification