Half-life of radioactive atoms
Watch a crowd of radioactive atoms decay. Choose an isotope used in hospitals or for dating old bones, let half-lives go by, and see what share of the atoms is left and how long that took.
Core concept
Radioactive atoms decay at random, but in each half-life half of those left decay: after n half-lives 100% × (½)ⁿ remain, whether the half-life is hours or thousands of years
Topic: Science around us
Some atoms are unstable: one day, at random, their nucleus breaks up and gives out radiation. We can never say when one atom will decay, but with billions of atoms the pattern is exact: in a fixed time called the half-life, half of them decay. After one half-life 50% are left, after two 25%, after three 12.5%: 100% × (½)ⁿ. Each isotope has its own half-life. Technetium-99m (6 hours) helps doctors scan bones and is soon gone from the body; iodine-131 (8 days) treats the thyroid; cobalt-60 (5.27 years) is used in cancer machines; and carbon-14 (5730 years) lets scientists date old wood and bones.
What you will learn
- Explain what a half-life is.
- Find the share of atoms left after a number of half-lives.
- Work out the time for n half-lives of a given isotope.
Before you start
- Know powers and fractions such as (½)³ = 1/8.
Materials
- None
Equipment
- None
Procedure
- Record iodine-131 after 2 half-lives.
- Record it after 1, 3 and 10 half-lives.
- Choose carbon-14 and record 2 half-lives.
How this simulation is simplified
- Half-lives: technetium-99m 6 hours, iodine-131 8 days, cobalt-60 5.27 years, carbon-14 5730 years.
- The time is in the isotope's own unit: hours, days or years.