Half Life Quiz: Test Your Understanding of Radioactive Decay

11th Grade

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| Questions: 20 | Updated: Mar 17, 2026

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1. In nuclear notation xAx, A represents:

Charge

Mass number (protons + neutrons)

Atomic radius

Temperature

Concept: meaning of mass number. (A) counts nucleons: protons plus neutrons. It changes during alpha decay but stays the same during gamma emission.

Explanation

Concept: meaning of mass number. (A) counts nucleons: protons plus neutrons. It changes during alpha decay but stays the same during gamma emission.

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About This Quiz

Half Life Quiz: Test Your Understanding Of Radioactive Decay - Quiz

This quiz focuses on the principles of radioactive decay, specifically the concept of half-life. It evaluates your understanding of how radioactive substances decay over time and the calculations involved. Engaging with this material is essential for learners interested in nuclear physics, chemistry, and environmental science, as it provides foundational knowledge... see morecritical for further studies in these fields. see less

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2. In xAx, Z is the atomic number (number of protons).

True

False

Concept: atomic number definition. (Z) determines the element. Changing (Z) changes which element the nucleus is.

Explanation

Concept: atomic number definition. (Z) determines the element. Changing (Z) changes which element the nucleus is.

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3. In alpha decay, the nucleus emits an alpha particle written as 24He (also written):

One electron

24He

A photon

A neutron only

Concept: alpha particle in equations. An alpha particle is a helium-4 nucleus (2 protons + 2 neutrons), written 24He. Emitting it reduces the parent nucleus’s mass number by 4 and atomic number by 2.

Explanation

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4. After alpha decay, the new mass number is (A - ____).

Concept: alpha decay changes (A). Alpha particles carry 4 nucleons away. So the parent’s mass number drops by 4.

Explanation

Concept: alpha decay changes (A). Alpha particles carry 4 nucleons away. So the parent’s mass number drops by 4.

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5. After alpha decay, the atomic number decreases by 2.

True

False

Concept: alpha decay changes (Z). Losing an alpha particle means losing two protons. That moves the element two places back on the periodic table.

Explanation

Concept: alpha decay changes (Z). Losing an alpha particle means losing two protons. That moves the element two places back on the periodic table.

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6. In beta-minus (β-) decay, the atomic number (Z):

Decreases by 2

Stays the same

Increases by 1

Increases by 2

Concept: beta-minus effect on (Z). A neutron turns into a proton (plus an electron and antineutrino). The mass number stays the same, but (Z) increases by 1.

Explanation

Concept: beta-minus effect on (Z). A neutron turns into a proton (plus an electron and antineutrino). The mass number stays the same, but (Z) increases by 1.

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7. In beta-minus decay, the mass number (A) stays the same.

True

False

Concept: beta decay doesn’t change nucleon count. A neutron becomes a proton, so the total number of nucleons remains constant. Only the balance changes.

Explanation

Concept: beta decay doesn’t change nucleon count. A neutron becomes a proton, so the total number of nucleons remains constant. Only the balance changes.

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8. Gamma emission changes the nucleus by:

Changing it into a new element

Changing (A) and (Z)

Releasing energy without changing (A) or (Z)

Emitting a helium nucleus

Concept: gamma emission. Gamma rays are photons emitted when an excited nucleus drops to a lower energy state. The element and nucleon numbers stay the same.

Explanation

Concept: gamma emission. Gamma rays are photons emitted when an excited nucleus drops to a lower energy state. The element and nucleon numbers stay the same.

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9. The half-life is the time for the number of undecayed nuclei (or activity) to fall to ______ of its original value.

Concept: half-life definition. Half-life describes exponential decay in a simple way. After each half-life, the remaining amount halves again.

Explanation

Concept: half-life definition. Half-life describes exponential decay in a simple way. After each half-life, the remaining amount halves again.

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10. Half-life is constant for a given isotope and does not depend on how much of the isotope you start with.

True

False

Concept: half-life as a property. Half-life depends on nuclear structure, not sample size. Larger samples simply start with more nuclei, but the fraction decaying per time is the same.

Explanation

Concept: half-life as a property. Half-life depends on nuclear structure, not sample size. Larger samples simply start with more nuclei, but the fraction decaying per time is the same.

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11. If a sample has a half-life of 5 years, after 10 years the remaining fraction is:

1/3

1/2

1/4

1/8

Concept: repeated halving. 10 years is two half-lives. Two halvings gives (1/2 → 1/4).

Explanation

Concept: repeated halving. 10 years is two half-lives. Two halvings gives (1/2 → 1/4).

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12. Activity decreases over time because fewer undecayed nuclei remain.

True

False

Concept: activity proportional to number of nuclei. With fewer radioactive nuclei, fewer decays occur per second. That reduces activity as time passes.

Explanation

Concept: activity proportional to number of nuclei. With fewer radioactive nuclei, fewer decays occur per second. That reduces activity as time passes.

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13. On a graph of activity vs time, radioactive decay usually looks like:

A straight line decreasing at constant rate

A curve that drops quickly at first then more slowly

A curve that rises then falls

A constant line

Concept: exponential decay shape. Exponential decay is fast at the start when there are many nuclei. It slows as fewer nuclei remain.

Explanation

Concept: exponential decay shape. Exponential decay is fast at the start when there are many nuclei. It slows as fewer nuclei remain.

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14. After each half-life, the same fraction (half) of what remains decays, not the same absolute amount.

True

False

Concept: exponential vs linear. The decay rate depends on how many nuclei are left. So halving repeats, but the absolute change gets smaller each time.

Explanation

Concept: exponential vs linear. The decay rate depends on how many nuclei are left. So halving repeats, but the absolute change gets smaller each time.

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15. If 80 g of a radioactive isotope becomes 10 g after a certain time, how many half-lives passed?

Concept: counting halvings. 80 → 40 (1), 40 → 20 (2), 20 → 10 (3). That’s three half-lives.

Explanation

Concept: counting halvings. 80 → 40 (1), 40 → 20 (2), 20 → 10 (3). That’s three half-lives.

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16. The unit of activity is the becquerel (Bq), meaning ______ decay per second.

Concept: becquerel definition. 1 Bq equals 1 decay each second. It measures decay rate directly.

Explanation

Concept: becquerel definition. 1 Bq equals 1 decay each second. It measures decay rate directly.

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17. A long half-life means a substance decays slowly and stays radioactive for a long time.

True

False

Concept: long half-life implications. Slow decay means lower activity for a given number of atoms, but it persists. This matters for waste storage and dating.

Explanation

Concept: long half-life implications. Slow decay means lower activity for a given number of atoms, but it persists. This matters for waste storage and dating.

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18. In a balanced nuclear equation, you must conserve:

Only energy

Mass number and atomic number

Only electron count

Only volume

Concept: conservation in nuclear equations. The total nucleon number and charge must balance. This helps determine missing particles in decay equations.

Explanation

Concept: conservation in nuclear equations. The total nucleon number and charge must balance. This helps determine missing particles in decay equations.

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19. Beta decay involves the weak interaction (qualitatively), while alpha decay is related to nuclear forces and stability.

True

False

Concept: different decay mechanisms. Beta decay changes a neutron–proton balance, which involves the weak force. Alpha decay involves tunneling and nuclear binding/stability.

Explanation

Concept: different decay mechanisms. Beta decay changes a neutron–proton balance, which involves the weak force. Alpha decay involves tunneling and nuclear binding/stability.

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20. The best overall summary is:

Half-life depends on temperature

Decay equations conserve (A) and (Z), and half-life describes exponential decrease of nuclei/activity over time

Gamma decay changes the element

Beta-minus decreases atomic number

Concept: decay + half-life recap. Alpha and beta change nuclear composition in predictable ways, while gamma releases energy. Half-life captures the statistical, exponential nature of decay.

Explanation

Concept: decay + half-life recap. Alpha and beta change nuclear composition in predictable ways, while gamma releases energy. Half-life captures the statistical, exponential nature of decay.

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