Questions about Half-Life
Half-Life – Multiple-Choice Questions
1. What is the definition of half-life in radioactive decay?
A) The time it takes for all atoms to decay
B) The time it takes for half of the radioactive atoms to decay
C) The time it takes for atoms to gain energy
D) The time it takes for an atom to fuse
E) The time it takes for radiation to stop
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2. Which of the following quantities decreases during radioactive decay?
A) Temperature
B) Atomic number
C) Mass number
D) Number of radioactive nuclei
E) Volume of gas released
3. If a substance has a half-life of 10 hours, how much of a 200 g sample remains after 20 hours?
A) 25 g
B) 50 g
C) 100 g
D) 75 g
E) 200 g
4. What happens to the rate of decay as a radioactive sample becomes smaller?
A) It increases
B) It stops
C) It decreases
D) It stays constant
E) It doubles
5. How many half-lives have passed if only 12.5% of the original sample remains?
A) 1
B) 2
C) 3
D) 4
E) 5
6. If a 160 g sample decays to 20 g in 3 half-lives, what is the amount left after one half-life?
A) 20 g
B) 40 g
C) 80 g
D) 120 g
E) 100 g
7. A scientist finds that 1/16 of a radioactive isotope remains. How many half-lives passed?
A) 2
B) 3
C) 4
D) 5
E) 6
8. Which of the following best describes the graph of radioactive decay over time?
A) Linear
B) Exponential increase
C) Exponential decay
D) Parabolic
E) Constant
9. After 5 half-lives, what fraction of the original radioactive sample remains?
A) 1/2
B) 1/4
C) 1/10
D) 1/16
E) 1/32
10. A 100 g radioactive sample has a half-life of 1 day. How much remains after 3 days?
A) 12.5 g
B) 25 g
C) 50 g
D) 75 g
E) 10 g
11. Which factor does NOT affect the half-life of a substance?
A) Temperature
B) Pressure
C) Volume
D) Chemical state
E) None of the above affect it
12. The half-life of carbon-14 is approximately 5,730 years. What percentage of carbon-14 remains after 11,460 years?
A) 75%
B) 50%
C) 25%
D) 12.5%
E) 6.25%
13. What is true about the half-life of a given isotope?
A) It varies depending on temperature
B) It is constant and characteristic of that isotope
C) It depends on sample size
D) It changes with pressure
E) It decreases over time
14. A sample loses half its mass every 8 hours. How much remains after 24 hours?
A) 1/2
B) 1/4
C) 1/8
D) 1/16
E) None
15. Which statement about half-life is correct?
A) It measures the total life span of an atom
B) It is when a radioactive isotope becomes stable
C) It is the time taken for the activity to reach zero
D) It is the time for half the radioactive atoms to decay
E) It changes over time as atoms decay
16. If a 48 g sample is reduced to 6 g after 3 half-lives, what is the amount after 2 half-lives?
A) 6 g
B) 12 g
C) 18 g
D) 24 g
E) 36 g
17. Which nuclear property determines the half-life of an isotope?
A) Atomic radius
B) Electron configuration
C) Nuclear stability
D) Melting point
E) Molecular weight
18. A student observes that a 40 g sample of a substance becomes 5 g after a certain time. How many half-lives passed?
A) 2
B) 3
C) 4
D) 5
E) 6
19. Which equation relates the remaining quantity of a substance to its half-life?
A) N = N₀ × 1/t
B) N = N₀ × e^(-λt)
C) N = N₀/2^n
D) N = λ × t
E) N = 2 × N₀ × t
20. If a radioactive material has a short half-life, it means:
A) It is not dangerous
B) It decays slowly
C) It decays quickly
D) It lasts a long time
E) It gains mass quickly
Answer Key with Explanations
1. B – Half-life is the time for half the radioactive atoms to decay.
2. D – The number of radioactive nuclei decreases over time.
3. B – After 2 half-lives (20 hours), 200 → 100 → 50 g.
4. D – The decay rate stays constant but the number of decays per second decreases.
5. C – 100 → 50 → 25 → 12.5%; 3 half-lives.
6. C – After one half-life: 160 → 80 g.
7. C – 1 → ½ → ¼ → ⅛ → 1/16 = 4 half-lives.
8. C – Radioactive decay is an exponential decay process.
9. E – After 5 half-lives: 1/32 remains.
10. A – 100 → 50 → 25 → 12.5 g after 3 days.
11. E – Half-life is unaffected by external conditions.
12. C – Two half-lives = 5,730 × 2 = 11,460 → 25% remains.
13. B – Half-life is constant and depends only on the isotope.
14. C – 24 hours = 3 half-lives → 1/8 remains.
15. D – Half-life = time for half of the radioactive atoms to decay.
16. B – 48 → 24 → 12 → 6; after 2 half-lives: 12 g.
17. C – Nuclear stability influences half-life.
18. C – 40 → 20 → 10 → 5 g = 3 half-lives.
19. C – N = N₀ / 2^n, where n = number of half-lives.
20. C – Short half-life means rapid decay.
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