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 18 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
N₀ = 18 g (t₁/₂ = 2 d) · t = 2 days
2. A radioactive sample has an initial mass of 224 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
N₀ = 224 g (t₁/₂ = 11 d) · t = 44 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 36 days?
N₀ = 160 g (t₁/₂ = 12 d) · t = 36 days
4. A radioactive sample has an initial mass of 736 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
N₀ = 736 g (t₁/₂ = 7 d) · t = 35 days
5. A radioactive sample has an initial mass of 24 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 24 g (t₁/₂ = 9 d) · t = 18 days
6. A radioactive sample has an initial mass of 384 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
N₀ = 384 g (t₁/₂ = 9 d) · t = 36 days
7. A radioactive sample has an initial mass of 100 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 14 days?
N₀ = 100 g (t₁/₂ = 7 d) · t = 14 days
8. A radioactive sample has an initial mass of 32 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 32 g (t₁/₂ = 6 d) · t = 18 days
9. A radioactive sample has an initial mass of 36 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 16 days?
N₀ = 36 g (t₁/₂ = 8 d) · t = 16 days
10. A radioactive sample has an initial mass of 84 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 22 days?
N₀ = 84 g (t₁/₂ = 11 d) · t = 22 days
11. A radioactive sample has an initial mass of 80 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 80 g (t₁/₂ = 7 d) · t = 21 days
12. A radioactive sample has an initial mass of 48 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 48 g (t₁/₂ = 8 d) · t = 8 days

12 questions · Seed 104729 · 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 18 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
9 g
Number of half-lives = 2 ÷ 2 = 1. Remaining mass = 18 ÷ 2^1 = 9 g.
2. A radioactive sample has an initial mass of 224 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
14 g
Number of half-lives = 44 ÷ 11 = 4. Remaining mass = 224 ÷ 2^4 = 14 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 36 days?
20 g
Number of half-lives = 36 ÷ 12 = 3. Remaining mass = 160 ÷ 2^3 = 20 g.
4. A radioactive sample has an initial mass of 736 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
23 g
Number of half-lives = 35 ÷ 7 = 5. Remaining mass = 736 ÷ 2^5 = 23 g.
5. A radioactive sample has an initial mass of 24 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
6 g
Number of half-lives = 18 ÷ 9 = 2. Remaining mass = 24 ÷ 2^2 = 6 g.
6. A radioactive sample has an initial mass of 384 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
24 g
Number of half-lives = 36 ÷ 9 = 4. Remaining mass = 384 ÷ 2^4 = 24 g.
7. A radioactive sample has an initial mass of 100 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 14 days?
25 g
Number of half-lives = 14 ÷ 7 = 2. Remaining mass = 100 ÷ 2^2 = 25 g.
8. A radioactive sample has an initial mass of 32 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 18 days?
4 g
Number of half-lives = 18 ÷ 6 = 3. Remaining mass = 32 ÷ 2^3 = 4 g.
9. A radioactive sample has an initial mass of 36 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 16 days?
9 g
Number of half-lives = 16 ÷ 8 = 2. Remaining mass = 36 ÷ 2^2 = 9 g.
10. A radioactive sample has an initial mass of 84 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 22 days?
21 g
Number of half-lives = 22 ÷ 11 = 2. Remaining mass = 84 ÷ 2^2 = 21 g.
11. A radioactive sample has an initial mass of 80 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
10 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 80 ÷ 2^3 = 10 g.
12. A radioactive sample has an initial mass of 48 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
24 g
Number of half-lives = 8 ÷ 8 = 1. Remaining mass = 48 ÷ 2^1 = 24 g.

12 questions · Seed 104729 · 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 18 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
N₀ = 18 g (t₁/₂ = 2 d) · t = 2 days
2. A radioactive sample has an initial mass of 224 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
N₀ = 224 g (t₁/₂ = 11 d) · t = 44 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 36 days?
N₀ = 160 g (t₁/₂ = 12 d) · t = 36 days
4. A radioactive sample has an initial mass of 736 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
N₀ = 736 g (t₁/₂ = 7 d) · t = 35 days
5. A radioactive sample has an initial mass of 24 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 24 g (t₁/₂ = 9 d) · t = 18 days
6. A radioactive sample has an initial mass of 384 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
N₀ = 384 g (t₁/₂ = 9 d) · t = 36 days
7. A radioactive sample has an initial mass of 100 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 14 days?
N₀ = 100 g (t₁/₂ = 7 d) · t = 14 days
8. A radioactive sample has an initial mass of 32 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 32 g (t₁/₂ = 6 d) · t = 18 days
9. A radioactive sample has an initial mass of 36 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 16 days?
N₀ = 36 g (t₁/₂ = 8 d) · t = 16 days
10. A radioactive sample has an initial mass of 84 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 22 days?
N₀ = 84 g (t₁/₂ = 11 d) · t = 22 days
11. A radioactive sample has an initial mass of 80 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 80 g (t₁/₂ = 7 d) · t = 21 days
12. A radioactive sample has an initial mass of 48 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 48 g (t₁/₂ = 8 d) · t = 8 days

12 questions · Seed 104729 · 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 18 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
9 g
Number of half-lives = 2 ÷ 2 = 1. Remaining mass = 18 ÷ 2^1 = 9 g.
2. A radioactive sample has an initial mass of 224 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 44 days?
14 g
Number of half-lives = 44 ÷ 11 = 4. Remaining mass = 224 ÷ 2^4 = 14 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 36 days?
20 g
Number of half-lives = 36 ÷ 12 = 3. Remaining mass = 160 ÷ 2^3 = 20 g.
4. A radioactive sample has an initial mass of 736 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
23 g
Number of half-lives = 35 ÷ 7 = 5. Remaining mass = 736 ÷ 2^5 = 23 g.
5. A radioactive sample has an initial mass of 24 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
6 g
Number of half-lives = 18 ÷ 9 = 2. Remaining mass = 24 ÷ 2^2 = 6 g.
6. A radioactive sample has an initial mass of 384 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
24 g
Number of half-lives = 36 ÷ 9 = 4. Remaining mass = 384 ÷ 2^4 = 24 g.
7. A radioactive sample has an initial mass of 100 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 14 days?
25 g
Number of half-lives = 14 ÷ 7 = 2. Remaining mass = 100 ÷ 2^2 = 25 g.
8. A radioactive sample has an initial mass of 32 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 18 days?
4 g
Number of half-lives = 18 ÷ 6 = 3. Remaining mass = 32 ÷ 2^3 = 4 g.
9. A radioactive sample has an initial mass of 36 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 16 days?
9 g
Number of half-lives = 16 ÷ 8 = 2. Remaining mass = 36 ÷ 2^2 = 9 g.
10. A radioactive sample has an initial mass of 84 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 22 days?
21 g
Number of half-lives = 22 ÷ 11 = 2. Remaining mass = 84 ÷ 2^2 = 21 g.
11. A radioactive sample has an initial mass of 80 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
10 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 80 ÷ 2^3 = 10 g.
12. A radioactive sample has an initial mass of 48 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
24 g
Number of half-lives = 8 ÷ 8 = 1. Remaining mass = 48 ÷ 2^1 = 24 g.

12 questions · Seed 104729 · Age guidance, not curriculum certification