ATOMS AND RADIOACTIVITY
3 marks · NormalA radioactive sample has an initial activity of . After 16 hours, its activity is .
Calculate the half-life of this isotope.
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4 hours (±0.5)
Must show: Shows number of half-lives or uses correct equation and values, Final answer 4 hours
Also accept: Use of decay curve logic
Common mistakes: Calculates only one half-life; Uses wrong equation
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Correctly uses half-life or decay equation or shows steps (splits activity to original/halved progression or uses ).
M2 · 1 mark · Finds the number of half-lives = 4 ().
A1 · 1 mark · Calculates 16 ÷ 4 = 4 hours (accept follow-through if reasonable). (follow-through)
Worked solution
3200 → 1600 (1st), 1600 → 800 (2nd), 800 → 400 (3rd), 400 → 200 (4th): 4 half-lives.