Half-lives of isotopes commonly used in GCSE questions
Isotope
Half-life
Thorium-232
14,000 million years
Uranium-235
704 million years
Plutonium-239
24,110 years
Carbon-14
5,730 years
Caesium-137
30 years
Cobalt-60
5.27 years
Polonium-210
138 days
Technetium-99m
6 hours
Polonium-218
3 minutes
Isotope
Thorium-232
Half-life
14,000 million years
Isotope
Uranium-235
Half-life
704 million years
Isotope
Plutonium-239
Half-life
24,110 years
Isotope
Carbon-14
Half-life
5,730 years
Isotope
Caesium-137
Half-life
30 years
Isotope
Cobalt-60
Half-life
5.27 years
Isotope
Polonium-210
Half-life
138 days
Isotope
Technetium-99m
Half-life
6 hours
Isotope
Polonium-218
Half-life
3 minutes
Example
Every 5.27 years, the mass of cobalt-60 halves. The number of cobalt-60 atoms halves. The activity of cobalt-60 halves.
Every 5,730 years, the mass of carbon-14 halves. The number of carbon-14 atoms halves. The activity of carbon-14 halves.
If an isotope has a half-life of 6 days, after 6 days the number of parent nuclei will halve, the isotope’s activity will halve and its mass will halve.
Question
The half-life of a radioactive isotope is 27 years. How long will its mass take to fall from 2 g to 0.25 g?
2 g → 1 g → 0.5 g → 0.25 g. Three arrows represent three half-lives. 3 × 27 years = 81 years
Question
The activity of an isotope falls from 600 Bq (becquerel) to 150 Bq in 10 days. What is its half-life?
600 Bq → 300 Bq → 150 Bq. Two arrows represent two half-lives. If two half-lives are ten days, one half-life is five days.
Question
An isotope has a half-life of 30 years. Estimate how long it will take for the number of nuclei to decay to below 200 if the starting number is 8,000?
8,000 → 4,000 → 2,000 → 1,000 → 500 → 250 → 125. It takes five half-lives to fall to 250 nuclei, and six half-lives to 125 nuclei. So somewhere between 5 × 30 = 150 years and 6 × 30 = 180 years.