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 8 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 8 g (t₁/₂ = 8 d) · t = 8 days
2. A radioactive sample has an initial mass of 168 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
N₀ = 168 g (t₁/₂ = 3 d) · t = 9 days
3. A radioactive sample has an initial mass of 288 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
N₀ = 288 g (t₁/₂ = 9 d) · t = 36 days
4. A radioactive sample has an initial mass of 544 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
N₀ = 544 g (t₁/₂ = 2 d) · t = 10 days
5. 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 55 days?
N₀ = 224 g (t₁/₂ = 11 d) · t = 55 days
6. A radioactive sample has an initial mass of 152 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
N₀ = 152 g (t₁/₂ = 3 d) · t = 9 days
7. A radioactive sample has an initial mass of 96 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 15 days?
N₀ = 96 g (t₁/₂ = 5 d) · t = 15 days
8. A radioactive sample has an initial mass of 672 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
N₀ = 672 g (t₁/₂ = 7 d) · t = 35 days
9. 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
10. A radioactive sample has an initial mass of 28 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 28 g (t₁/₂ = 6 d) · t = 12 days
11. A radioactive sample has an initial mass of 60 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 60 g (t₁/₂ = 9 d) · t = 18 days
12. A radioactive sample has an initial mass of 736 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
N₀ = 736 g (t₁/₂ = 2 d) · t = 10 days

12 questions · Seed 44 · 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 8 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
4 g
Number of half-lives = 8 ÷ 8 = 1. Remaining mass = 8 ÷ 2^1 = 4 g.
2. A radioactive sample has an initial mass of 168 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
21 g
Number of half-lives = 9 ÷ 3 = 3. Remaining mass = 168 ÷ 2^3 = 21 g.
3. A radioactive sample has an initial mass of 288 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
18 g
Number of half-lives = 36 ÷ 9 = 4. Remaining mass = 288 ÷ 2^4 = 18 g.
4. A radioactive sample has an initial mass of 544 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
17 g
Number of half-lives = 10 ÷ 2 = 5. Remaining mass = 544 ÷ 2^5 = 17 g.
5. 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 55 days?
7 g
Number of half-lives = 55 ÷ 11 = 5. Remaining mass = 224 ÷ 2^5 = 7 g.
6. A radioactive sample has an initial mass of 152 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
19 g
Number of half-lives = 9 ÷ 3 = 3. Remaining mass = 152 ÷ 2^3 = 19 g.
7. A radioactive sample has an initial mass of 96 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 15 days?
12 g
Number of half-lives = 15 ÷ 5 = 3. Remaining mass = 96 ÷ 2^3 = 12 g.
8. A radioactive sample has an initial mass of 672 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
21 g
Number of half-lives = 35 ÷ 7 = 5. Remaining mass = 672 ÷ 2^5 = 21 g.
9. 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.
10. A radioactive sample has an initial mass of 28 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
7 g
Number of half-lives = 12 ÷ 6 = 2. Remaining mass = 28 ÷ 2^2 = 7 g.
11. A radioactive sample has an initial mass of 60 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
15 g
Number of half-lives = 18 ÷ 9 = 2. Remaining mass = 60 ÷ 2^2 = 15 g.
12. A radioactive sample has an initial mass of 736 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
23 g
Number of half-lives = 10 ÷ 2 = 5. Remaining mass = 736 ÷ 2^5 = 23 g.

12 questions · Seed 44 · 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 8 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
N₀ = 8 g (t₁/₂ = 8 d) · t = 8 days
2. A radioactive sample has an initial mass of 168 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
N₀ = 168 g (t₁/₂ = 3 d) · t = 9 days
3. A radioactive sample has an initial mass of 288 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
N₀ = 288 g (t₁/₂ = 9 d) · t = 36 days
4. A radioactive sample has an initial mass of 544 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
N₀ = 544 g (t₁/₂ = 2 d) · t = 10 days
5. 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 55 days?
N₀ = 224 g (t₁/₂ = 11 d) · t = 55 days
6. A radioactive sample has an initial mass of 152 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
N₀ = 152 g (t₁/₂ = 3 d) · t = 9 days
7. A radioactive sample has an initial mass of 96 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 15 days?
N₀ = 96 g (t₁/₂ = 5 d) · t = 15 days
8. A radioactive sample has an initial mass of 672 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
N₀ = 672 g (t₁/₂ = 7 d) · t = 35 days
9. 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
10. A radioactive sample has an initial mass of 28 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 28 g (t₁/₂ = 6 d) · t = 12 days
11. A radioactive sample has an initial mass of 60 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
N₀ = 60 g (t₁/₂ = 9 d) · t = 18 days
12. A radioactive sample has an initial mass of 736 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
N₀ = 736 g (t₁/₂ = 2 d) · t = 10 days

12 questions · Seed 44 · 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 8 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 8 days?
4 g
Number of half-lives = 8 ÷ 8 = 1. Remaining mass = 8 ÷ 2^1 = 4 g.
2. A radioactive sample has an initial mass of 168 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
21 g
Number of half-lives = 9 ÷ 3 = 3. Remaining mass = 168 ÷ 2^3 = 21 g.
3. A radioactive sample has an initial mass of 288 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 36 days?
18 g
Number of half-lives = 36 ÷ 9 = 4. Remaining mass = 288 ÷ 2^4 = 18 g.
4. A radioactive sample has an initial mass of 544 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
17 g
Number of half-lives = 10 ÷ 2 = 5. Remaining mass = 544 ÷ 2^5 = 17 g.
5. 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 55 days?
7 g
Number of half-lives = 55 ÷ 11 = 5. Remaining mass = 224 ÷ 2^5 = 7 g.
6. A radioactive sample has an initial mass of 152 g and a half-life of 3 days. How many grams of the isotope remain undecayed after 9 days?
19 g
Number of half-lives = 9 ÷ 3 = 3. Remaining mass = 152 ÷ 2^3 = 19 g.
7. A radioactive sample has an initial mass of 96 g and a half-life of 5 days. How many grams of the isotope remain undecayed after 15 days?
12 g
Number of half-lives = 15 ÷ 5 = 3. Remaining mass = 96 ÷ 2^3 = 12 g.
8. A radioactive sample has an initial mass of 672 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
21 g
Number of half-lives = 35 ÷ 7 = 5. Remaining mass = 672 ÷ 2^5 = 21 g.
9. 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.
10. A radioactive sample has an initial mass of 28 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 12 days?
7 g
Number of half-lives = 12 ÷ 6 = 2. Remaining mass = 28 ÷ 2^2 = 7 g.
11. A radioactive sample has an initial mass of 60 g and a half-life of 9 days. How many grams of the isotope remain undecayed after 18 days?
15 g
Number of half-lives = 18 ÷ 9 = 2. Remaining mass = 60 ÷ 2^2 = 15 g.
12. A radioactive sample has an initial mass of 736 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 10 days?
23 g
Number of half-lives = 10 ÷ 2 = 5. Remaining mass = 736 ÷ 2^5 = 23 g.

12 questions · Seed 44 · Age guidance, not curriculum certification