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.

New variation Try interactive challenge Browse templates

Choose “Save as PDF” to keep a copy. Use your browser’s paper setting for A4 or Letter.

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 608 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
N₀ = 608 g (t₁/₂ = 8 d) · t = 40 days
2. A radioactive sample has an initial mass of 288 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 24 days?
N₀ = 288 g (t₁/₂ = 6 d) · t = 24 days
3. A radioactive sample has an initial mass of 64 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
N₀ = 64 g (t₁/₂ = 8 d) · t = 40 days
4. A radioactive sample has an initial mass of 4 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 4 g (t₁/₂ = 12 d) · t = 12 days
5. A radioactive sample has an initial mass of 512 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 55 days?
N₀ = 512 g (t₁/₂ = 11 d) · t = 55 days
6. A radioactive sample has an initial mass of 192 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
N₀ = 192 g (t₁/₂ = 12 d) · t = 48 days
7. A radioactive sample has an initial mass of 128 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 128 g (t₁/₂ = 7 d) · t = 21 days
8. A radioactive sample has an initial mass of 368 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
N₀ = 368 g (t₁/₂ = 12 d) · t = 48 days
9. A radioactive sample has an initial mass of 544 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
N₀ = 544 g (t₁/₂ = 7 d) · t = 35 days
10. A radioactive sample has an initial mass of 16 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
N₀ = 16 g (t₁/₂ = 2 d) · t = 2 days
11. A radioactive sample has an initial mass of 240 g and a half-life of 10 days. How many grams of the isotope remain undecayed after 40 days?
N₀ = 240 g (t₁/₂ = 10 d) · t = 40 days
12. A radioactive sample has an initial mass of 88 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 24 days?
N₀ = 88 g (t₁/₂ = 12 d) · t = 24 days

12 questions · Seed 45 · 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 608 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
19 g
Number of half-lives = 40 ÷ 8 = 5. Remaining mass = 608 ÷ 2^5 = 19 g.
2. A radioactive sample has an initial mass of 288 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 24 days?
18 g
Number of half-lives = 24 ÷ 6 = 4. Remaining mass = 288 ÷ 2^4 = 18 g.
3. A radioactive sample has an initial mass of 64 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
2 g
Number of half-lives = 40 ÷ 8 = 5. Remaining mass = 64 ÷ 2^5 = 2 g.
4. A radioactive sample has an initial mass of 4 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 12 days?
2 g
Number of half-lives = 12 ÷ 12 = 1. Remaining mass = 4 ÷ 2^1 = 2 g.
5. A radioactive sample has an initial mass of 512 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 55 days?
16 g
Number of half-lives = 55 ÷ 11 = 5. Remaining mass = 512 ÷ 2^5 = 16 g.
6. A radioactive sample has an initial mass of 192 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
12 g
Number of half-lives = 48 ÷ 12 = 4. Remaining mass = 192 ÷ 2^4 = 12 g.
7. A radioactive sample has an initial mass of 128 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
16 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 128 ÷ 2^3 = 16 g.
8. A radioactive sample has an initial mass of 368 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
23 g
Number of half-lives = 48 ÷ 12 = 4. Remaining mass = 368 ÷ 2^4 = 23 g.
9. A radioactive sample has an initial mass of 544 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
17 g
Number of half-lives = 35 ÷ 7 = 5. Remaining mass = 544 ÷ 2^5 = 17 g.
10. A radioactive sample has an initial mass of 16 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
8 g
Number of half-lives = 2 ÷ 2 = 1. Remaining mass = 16 ÷ 2^1 = 8 g.
11. A radioactive sample has an initial mass of 240 g and a half-life of 10 days. How many grams of the isotope remain undecayed after 40 days?
15 g
Number of half-lives = 40 ÷ 10 = 4. Remaining mass = 240 ÷ 2^4 = 15 g.
12. A radioactive sample has an initial mass of 88 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 24 days?
22 g
Number of half-lives = 24 ÷ 12 = 2. Remaining mass = 88 ÷ 2^2 = 22 g.

12 questions · Seed 45 · 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 608 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
N₀ = 608 g (t₁/₂ = 8 d) · t = 40 days
2. A radioactive sample has an initial mass of 288 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 24 days?
N₀ = 288 g (t₁/₂ = 6 d) · t = 24 days
3. A radioactive sample has an initial mass of 64 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
N₀ = 64 g (t₁/₂ = 8 d) · t = 40 days
4. A radioactive sample has an initial mass of 4 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 12 days?
N₀ = 4 g (t₁/₂ = 12 d) · t = 12 days
5. A radioactive sample has an initial mass of 512 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 55 days?
N₀ = 512 g (t₁/₂ = 11 d) · t = 55 days
6. A radioactive sample has an initial mass of 192 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
N₀ = 192 g (t₁/₂ = 12 d) · t = 48 days
7. A radioactive sample has an initial mass of 128 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
N₀ = 128 g (t₁/₂ = 7 d) · t = 21 days
8. A radioactive sample has an initial mass of 368 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
N₀ = 368 g (t₁/₂ = 12 d) · t = 48 days
9. A radioactive sample has an initial mass of 544 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
N₀ = 544 g (t₁/₂ = 7 d) · t = 35 days
10. A radioactive sample has an initial mass of 16 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
N₀ = 16 g (t₁/₂ = 2 d) · t = 2 days
11. A radioactive sample has an initial mass of 240 g and a half-life of 10 days. How many grams of the isotope remain undecayed after 40 days?
N₀ = 240 g (t₁/₂ = 10 d) · t = 40 days
12. A radioactive sample has an initial mass of 88 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 24 days?
N₀ = 88 g (t₁/₂ = 12 d) · t = 24 days

12 questions · Seed 45 · 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 608 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
19 g
Number of half-lives = 40 ÷ 8 = 5. Remaining mass = 608 ÷ 2^5 = 19 g.
2. A radioactive sample has an initial mass of 288 g and a half-life of 6 days. How many grams of the isotope remain undecayed after 24 days?
18 g
Number of half-lives = 24 ÷ 6 = 4. Remaining mass = 288 ÷ 2^4 = 18 g.
3. A radioactive sample has an initial mass of 64 g and a half-life of 8 days. How many grams of the isotope remain undecayed after 40 days?
2 g
Number of half-lives = 40 ÷ 8 = 5. Remaining mass = 64 ÷ 2^5 = 2 g.
4. A radioactive sample has an initial mass of 4 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 12 days?
2 g
Number of half-lives = 12 ÷ 12 = 1. Remaining mass = 4 ÷ 2^1 = 2 g.
5. A radioactive sample has an initial mass of 512 g and a half-life of 11 days. How many grams of the isotope remain undecayed after 55 days?
16 g
Number of half-lives = 55 ÷ 11 = 5. Remaining mass = 512 ÷ 2^5 = 16 g.
6. A radioactive sample has an initial mass of 192 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
12 g
Number of half-lives = 48 ÷ 12 = 4. Remaining mass = 192 ÷ 2^4 = 12 g.
7. A radioactive sample has an initial mass of 128 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 21 days?
16 g
Number of half-lives = 21 ÷ 7 = 3. Remaining mass = 128 ÷ 2^3 = 16 g.
8. A radioactive sample has an initial mass of 368 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 48 days?
23 g
Number of half-lives = 48 ÷ 12 = 4. Remaining mass = 368 ÷ 2^4 = 23 g.
9. A radioactive sample has an initial mass of 544 g and a half-life of 7 days. How many grams of the isotope remain undecayed after 35 days?
17 g
Number of half-lives = 35 ÷ 7 = 5. Remaining mass = 544 ÷ 2^5 = 17 g.
10. A radioactive sample has an initial mass of 16 g and a half-life of 2 days. How many grams of the isotope remain undecayed after 2 days?
8 g
Number of half-lives = 2 ÷ 2 = 1. Remaining mass = 16 ÷ 2^1 = 8 g.
11. A radioactive sample has an initial mass of 240 g and a half-life of 10 days. How many grams of the isotope remain undecayed after 40 days?
15 g
Number of half-lives = 40 ÷ 10 = 4. Remaining mass = 240 ÷ 2^4 = 15 g.
12. A radioactive sample has an initial mass of 88 g and a half-life of 12 days. How many grams of the isotope remain undecayed after 24 days?
22 g
Number of half-lives = 24 ÷ 12 = 2. Remaining mass = 88 ÷ 2^2 = 22 g.

12 questions · Seed 45 · Age guidance, not curriculum certification