ATOMS AND RADIOACTIVITY
3 marks · NormalExplain what is meant by the half-life of a radioactive isotope, and why the activity of a radioactive source cannot fall to zero instantly.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
Half-life is the time taken for the activity (or number of radioactive nuclei) to fall to half its initial value. Activity cannot fall to zero instantly because radioactive decay is a random process and nuclei decay at different times.
Must show: Definition: time for activity/nuclei to halve, Random decay process, Activity approaches zero gradually, not instantly
Common mistakes: Not mentioning randomness; Giving imprecise definition
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Defines half-life: time taken for activity or nuclei count to reduce by half.
B2 · 1 mark · Explains decay is random/spread over time.
B3 · 1 mark · States activity approaches zero gradually, never instantly falls to zero.
Worked solution
Half-life is the time taken for the activity or number of undecayed nuclei to fall to half its original value. Decay is random, so nuclei decay at different times, which means the activity reduces gradually and never drops to zero at once.