Radioactive decay
Cambridge IGCSE Physics (0625) · Unit 5: Nuclear physics · 10 flashcards
Radioactive decay is topic 5.2.3 in the Cambridge IGCSE Physics (0625) syllabus , positioned in Unit 5 — Nuclear physics , alongside The atom, The nucleus and Detection of radioactivity.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical).
The deck below contains 10 flashcards — covering the precise wording mark schemes reward.
What the Cambridge 0625 syllabus says
2026-2028 syllabus, in our wordsWhat the syllabus expects you to be able to do in this topic, in plain English. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks. For the exact wording, see the official syllabus (PDF).
- Know Know that in radioactive decay an unstable nucleus changes, possibly giving out α-particles or β-particles and/or γ-radiation, and that this happens spontaneously and at random.
- State State that α-decay or β-decay turns the nucleus into the nucleus of a different element.
- Know Know that isotopes of an element can be radioactive because their nucleus has too many neutrons and/or is too heavy. Supplement
- Describe Describe how α-decay, β-decay and γ-emission affect the nucleus, including making it more stable and lowering the number of excess neutrons; in β-emission the nucleus changes as neutron → proton + electron. Supplement
- Use Use decay equations written in nuclide notation to show α-particle, β-particle and γ-radiation emission. Supplement
Cobalt-60 is a radioactive isotope that decays by emitting a beta particle. A sample initially contains 8.0 x 10^12 atoms of Cobalt-60. After a certain time, the number of Cobalt-60 atoms remaining is 2.0 x 10^12. Calculate the number of beta particles emitted during this time.
Answer:
Number of beta particles emitted = Initial number of atoms - Number of atoms remaining
Number of beta particles emitted = (8.0 x 10^12) - (2.0 x 10^12)
Number of beta particles emitted = 6.0 x 10^12
*Explanation: Each Cobalt-60 atom that decays emits one beta particle. Therefore, the number of beta particles emitted is equal to the number of Cobalt-60 atoms that have decayed.*
Describe what is meant by the terms 'spontaneous' and 'random' in the context of radioactive decay.
Answer:
* Spontaneous: The decay of a nucleus occurs on its own, without being triggered or influenced by any external factors such as temperature, pressure, or chemical reactions.
* Random: It is impossible to predict which specific nucleus in a sample will decay next, or when a particular nucleus will decay. The decay events occur randomly and follow statistical probabilities.
The element Polonium-210 (Po-210) undergoes alpha decay to become Lead (Pb). State what happens to the nucleus of the Po-210 atom during this process.
During alpha decay, the Po-210 nucleus emits an alpha particle (Helium nucleus). This results in the Po-210 nucleus changing to a Lead nucleus (Pb), a different element. The number of protons in the nucleus decreases by 2, and the number of neutrons decreases by 2, thus changing the element's identity.
Explain why alpha decay results in the formation of a different element.
Alpha decay involves the emission of an alpha particle, which consists of 2 protons and 2 neutrons, from the nucleus of an atom. The number of protons in the nucleus defines the element. Because alpha decay changes the number of protons (decreases by 2), the atom transforms into a different element with a different atomic number (number of protons).
An isotope of Uranium, Uranium-238, has 92 protons. The stable isotope of Uranium is Uranium-235. Calculate the number of neutrons in Uranium-238, and explain why Uranium-238 is radioactive.
Calculation:
Number of neutrons = Mass number - Number of protons = 238 - 92 = 146 neutrons.
Explanation:
Uranium-238 is radioactive because it has an excess of neutrons in its nucleus compared to the stable Uranium-235. This neutron excess makes the nucleus unstable, causing it to undergo radioactive decay.
State two reasons why an isotope of an element might be radioactive.
1. The isotope may have an excess of neutrons in the nucleus.
2. The nucleus of the isotope may be too heavy (i.e., have a very high nucleon number).
Americium-241 (²⁴¹Am₉₅) undergoes alpha decay. Determine the proton number and nucleon number of the daughter nucleus formed.
Answer:
Alpha decay emits a helium nucleus (⁴He₂).
* Nucleon number (A): 241 - 4 = 237
* Proton number (Z): 95 - 2 = 93
Therefore, the daughter nucleus has a nucleon number of 237 and a proton number of 93.
Describe how beta-minus (β⁻) decay affects the composition of a nucleus. Explain why beta decay often leads to a more stable nucleus.
Answer:
During beta-minus decay, a neutron within the nucleus transforms into a proton and an electron. The electron (β⁻ particle) is emitted from the nucleus. This results in an increase of the proton number by 1, while the nucleon number remains unchanged.
Beta decay often results in a more stable nucleus because it reduces the neutron-to-proton ratio. Nuclei with too many neutrons compared to protons are unstable. The decay converts a neutron into a proton, bringing the nucleus closer to the belt of stability on the nuclide chart.
The Uranium isotope ²³⁸U (atomic number 92) undergoes alpha decay. Write the nuclear decay equation and identify the daughter nucleus.
Equation: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Explanation: Alpha decay releases a helium nucleus (2 protons, 2 neutrons). The mass number decreases by 4 (238 → 234) and the atomic number decreases by 2 (92 → 90), forming Thorium-234.
Describe what happens to the atomic number and mass number of a nucleus when it undergoes beta-minus decay. Use nuclide notation to exemplify a generic beta-minus decay.
During beta-minus decay, a neutron in the nucleus transforms into a proton and an electron (beta-minus particle). The atomic number increases by 1, while the mass number remains the same.
More topics in Unit 5 — Nuclear physics
Radioactive decay sits alongside these Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Related Physics guides
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