How can I teach half-life using a classroom model?
Use dice, coins or sweets: each round, remove the ones that 'decay' and plot how many remain. Pupils see the halving pattern and the randomness of decay.
Radioactive decay is abstract, but a simple model makes it tangible and links to how meteorites are dated.
The coin model. Give each group 100 coins, or counters with one marked side. Each coin represents an atom of a radioactive parent isotope. Pupils shake the coins in a tray and tip them out. Every coin showing heads has 'decayed' and is removed, perhaps placed in a separate pile as the daughter isotope. Count the remaining coins and record. Repeat until none remain.
The dice model. Using dice, decide that a six represents decay. The probability per round is one in six, giving a slower decline and a half-life of about four rounds. This shows that different isotopes decay at different rates.
Analysis. Pupils plot the number of undecayed atoms against the number of rounds. The curve halves at roughly regular intervals: this is the half-life. Combining class data gives a smoother curve and shows that decay is random for individual atoms but predictable for large numbers.
Link to meteorites. Explain that scientists measure the ratio of parent to daughter atoms in a meteorite's minerals. Knowing the half-life, they work out how long the clock has been running. Uranium-238 has a half-life of about 4.5 billion years, conveniently close to the age of meteorites.
Extension. Pupils can be given a parent-to-daughter ratio and asked to estimate how many half-lives have passed. Discuss assumptions, such as no atoms escaping and no daughter present at the start.
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