Practise Half-lives and random decay. 15 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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A sample of a radioactive isotope has an activity of 800 Bq. The half-life of the isotope is 2 days. What is the activity of the sample after 2 days?
400 Bq
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is meant by the half-life of a radioactive isotope? Tick (✓) one box.[1]
The time it takes for the number of unstable nuclei in a sample to halve
Half of the time it takes for all the nuclei in a sample to decay
The time it takes for the mass number of a nucleus to halve
The time it takes for a nucleus to lose half of its protons
(b) A sample of a radioactive isotope has an activity of 800 Bq. The half-life of the isotope is 2 days. What is the activity of the sample after 2 days?[1]
(c) What is the activity of the sample after 6 days?[1]
(d) Radioactive decay is random. What does this mean?[1]
Show the answer and mark scheme
(a)Answer: The time it takes for the number of unstable nuclei in a sample to halve
(b)Answer: 400 Bq
400 (Bq)
(c)Answer: 100 Bq
100 (Bq)
(d)Answer: You cannot predict when a particular nucleus will decay.
it is not possible to predict which nucleus will decay next / when a particular nucleus will decay
Question 2Medium3 marks
After 3 half-lives, the activity of a radioactive sample is 15 Bq. Calculate its initial activity.[3]
Show the answer and mark scheme
Answer: 120 Bq
after 3 half-lives, activity = initial activity ÷ 8
initial activity = 15 × 8
120 (Bq)
Question 3Hard7 marks
Technetium-99m is a radioactive isotope with a half-life of 6.0 hours.
(a) Calculate the fraction of the original technetium-99m nuclei in a sample that remain after 24 hours.[2]
(b) A sample of a different radioactive isotope has an activity of 6400 Bq. After 40 hours, its activity is 200 Bq. Calculate the half-life of this isotope.[3]
(c) What fraction of the initial activity of a radioactive sample has been lost after 3 half-lives?[2]
Show the answer and mark scheme
(a)Answer: 1/16
24 ÷ 6.0 = 4 half-lives
(½)4 = 1/16
(b)Answer: 8.0 hours
6400 ÷ 200 = 32
32 = 25, so 40 hours is 5 half-lives
half-life = 40 ÷ 5 = 8.0 (hours)
(c)Answer: 7/8
activity remaining = (½)3 = 1/8 of the initial activity