New
New
Activity and half-life calculations
I can interpret radioactive half-life graphs and make calculations using values of half-life.
Lesson 6 of 7
New
New
- Year 11
- AQA
Activity and half-life calculations
I can interpret radioactive half-life graphs and make calculations using values of half-life.
Lesson details
Key learning points
- The amount (or activity) of a radioactive isotope repeatedly falls by half in equal amounts of time.
- A radioactive half-life graph shows the amount (or activity) of a radioactive isotope plotted against time.
- In 2 half-lives the amount (or activity) of a radioactive isotope falls to ½ × ½ = ¼ as much.
- In 3 half-lives the amount (or activity) of a radioactive isotope falls to ½ × ½ × ½ = 1/8 as much, and so on.
- Radioactive isotopes with short half-lives decay quickly, emitting most radiation over a short period of time.
Keywords
Activity - the number of decays per second; it is measured in becquerels (Bq)
Radioactive isotopes - contain unstable nuclei that will decay over time and emit ionising radiation
Radioactive half-life - the time taken for the activity of a sample of a radioactive isotope to halve
Common misconception
It is impossible to predict outcomes for random events such as radioactive decay.
Use analogies to show that random nuclear decay can lead to predictable outcomes, such as the randomness in the order popcorn kernels pop, but the predictability of how quickly all the popcorn takes to cook.
Use exam questions to provide lots of practice of half-life calculations in a range of different contexts.
Teacher tip
Content guidance
- Depiction or discussion of sensitive content
Supervision
Adult supervision recommended
Licence
This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).
Lesson video
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Prior knowledge starter quiz
6 Questions
Q1.Which of the following describes what is meant by a radioactive isotope having a half–life of 15 minutes?
Which of the following describes what is meant by a radioactive isotope having a half–life of 15 minutes?
It takes 15 minutes for all of the nuclei to decay.
It takes 15 minutes for one nucleus to decay.
Q2.How do the half–lives of different radioactive isotopes compare?
How do the half–lives of different radioactive isotopes compare?
All radioactive isotopes have the same half–life.
Isotopes of the same element have the same half–life.
Half–life does not depend on the radioactive isotope.
Q3.What effect does increasing the temperature have on the half–life of a radioactive isotope?
What effect does increasing the temperature have on the half–life of a radioactive isotope?
Increasing the temperature increases the half–life.
Increasing the temperature decreases the half–life.
Q4.Starting with the longest, sort the following radioactive isotopes into order of decreasing half–life.
Starting with the longest, sort the following radioactive isotopes into order of decreasing half–life.
Q5.How does the half–life of a carbon–14 atom in a carbon monoxide (CO) molecule compare to its half–life in a carbon dioxide (CO₂) molecule?
How does the half–life of a carbon–14 atom in a carbon monoxide (CO) molecule compare to its half–life in a carbon dioxide (CO₂) molecule?
It is longer when part of the CO molecule.
It is longer when part of the CO₂ molecule.
Q6.A radioactive isotope has a half–life of 10 hours. Match each of the following fractions of the sample remaining to the correct amount of time that has passed.
A radioactive isotope has a half–life of 10 hours. Match each of the following fractions of the sample remaining to the correct amount of time that has passed.
Assessment exit quiz
6 Questions
Q1.The activity of a sample of a radioactive isotope is …
The activity of a sample of a radioactive isotope is …
the total mass of the sample.
the number of nuclei in the sample.
Q2.An activity of 100 becquerel (Bq) means …
An activity of 100 becquerel (Bq) means …
1 nucleus decays every 100 seconds.
there are 100 radioactive nuclei in a sample.
100 nuclei decay after each half–life has passed.
Q3.Which of the following can be used to define what is meant by the half-life of a radioactive isotope?
Which of the following can be used to define what is meant by the half-life of a radioactive isotope?
Half the time it takes for its activity to fall to zero.
Half the time it takes for all of the nuclei to decay.
The time it takes for half of a nucleus to decay.
Q4.A radioactive isotope has an initial activity of 800 Bq and a half–life of 5 minutes. Match each of the following activities of the sample to the correct amount of time that has passed.
A radioactive isotope has an initial activity of 800 Bq and a half–life of 5 minutes. Match each of the following activities of the sample to the correct amount of time that has passed.
Q5.Sodium-24 has a half-life of 15 hours. If a sample of sodium-24 originally has an activity of 500 Bq, what fraction of its original activity will remain after 30 hours?
Sodium-24 has a half-life of 15 hours. If a sample of sodium-24 originally has an activity of 500 Bq, what fraction of its original activity will remain after 30 hours?
½
⅛
0
Q6.After 42 days the activity of a sample of phosphorus–32 has decreased from 400 Bq to 50 Bq. What is the half–life of phosphorus–32?
After 42 days the activity of a sample of phosphorus–32 has decreased from 400 Bq to 50 Bq. What is the half–life of phosphorus–32?
7 days
126 days
168 days
