Year 10 Science · Unit 2 · Lesson 24

Radioactive Half-Life

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Learning Goals

Identify the odd one out in groups of isotopes and half-life ideas.
Read and interpret data from a real decay scenario.
Calculate the mass or fraction remaining after a whole number of half-lives.

Warm Up

Odd one out

Circle the item that does not belong in each group. Then explain why it doesn't fit in the answer column.

#	Group	Odd one out + reason
1	Carbon-14 Iodine-131 Cobalt-60 Oxygen-16
2	1/2 1/4 1/8 1/6
3	Seconds Days Grams Years
4	Half-life is constant Activity falls over time Mass halves each half-life Decay speeds up when heated
5	After 1 half-life: 1/2 After 2: 1/4 After 3: 1/8 After 4: 1/10

Your Turn

Decay scenario: iodine-131 in medicine

Iodine-131 (¹³¹₅₃I) is used in hospitals to treat thyroid conditions. It has a half-life of about 8 days. A hospital receives a fresh sample with a mass of 80 g. The table below tracks the sample over several half-lives.

Mass after 8 days This is one half-life. How much of the 80 g remains?
Mass after 24 days How many half-lives is 24 days? Work out the mass remaining.
Fraction remaining after 4 half-lives Write the fraction (1/2)ⁿ as a single fraction.
Sketch the decay curve Plot mass against time for 0, 8, 16, 24 and 32 days, then join with a smooth curve.

1. A different isotope has a half-life of 2 days. A sample has a mass of 80 g. Calculate the mass remaining after 6 days. Show your working.

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2. Cobalt-60 (⁶⁰₂₇Co) has a half-life of about 5.3 years. Roughly how many years would it take for a sample to fall to one quarter of its starting amount? Explain how you worked this out.

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Wrap Up

In one sentence, explain why a decay curve gets less steep over time rather than dropping at a steady rate.