The time for half the radioactive nuclei to decay; each further half-life halves the rest again.

Does gamma change the atomic number?

No, gamma only carries away energy and leaves mass and atomic numbers unchanged.

S5 Physics Radioactivity, Paper 1: half-life calculations and three radiations

2026-11-01

Tutor Prep Guide · Physics

Each half-life halves the amount; alpha removes four from the mass number, while beta raises the atomic number by one.

Focus　Count how many half-lives have passed, then divide by two that many times.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 3 · Intermediate 4 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Radioactivity
Level	S4 to S5

02　Questions (10 items)

FoundationA radioactive sample of half-life 5 days has 80 g at the start. How much remains after 15 days?
FoundationWhich radiation is stopped by a sheet of paper?
FoundationA source of half-life 8 hours gives 400 counts per minute. What is the count rate after 24 hours?
IntermediateCarbon-14 has a half-life of 5 730 years. What percentage of the original amount remains after 17 190 years?
IntermediateRadium-226 (mass number 226, atomic number 88) emits an alpha particle. Give the mass number and atomic number of the new nucleus.
IntermediateA sample of half-life 2 hours starts at 64 g. How long until only 4 g remains?
IntermediateArrange alpha, beta and gamma in order of increasing penetrating power.
ChallengeWhat happens inside the nucleus during beta decay?
ChallengeA source of half-life 10 minutes has an activity of 1 000 Bq. Find the activity after 40 minutes.
Error redoCorrect the statement: “After two half-lives every nucleus has decayed.”

03　Answers and one-line notes

#	Answer	One-line note
1	10 g	15 days is three half-lives, so 80/2^3 = 10 g.
2	Alpha particles	Alpha has the weakest penetration and a sheet of paper stops it.
3	50 counts per minute	24 hours is three half-lives, so 400/8 = 50 counts per minute.
4	12.5%	17 190 years is three half-lives, leaving (1/2)^3 = 12.5%.
5	222 and 86	An alpha particle removes 4 from the mass number and 2 from the atomic number.
6	8 hours	64 to 4 takes four halvings, that is 8 hours.
7	alpha < beta < gamma	Alpha is easiest to stop, gamma penetrates most.
8	A neutron turns into a proton, so the mass number is unchanged and the atomic number rises by 1	A neutron turns into a proton and an electron leaves, so the atomic number rises by 1.
9	62.5 Bq	40 minutes is four half-lives, so 1000/2^4 = 62.5 Bq.
10	It should read: one quarter remains, so three quarters have decayed.	A half-life halves the number of nuclei, so a quarter remains and decay never finishes at once.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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