O Level Physics Modern Physics Quiz

O-Level Physics Quiz - Modern Physics (Answer Key)

Section A: Atomic Structure and Nuclide Notation

1. Isotope: Atoms of the same element (same proton number) but with different nucleon numbers (different number of neutrons). [1]
2. (a) Neutrons = Nucleon number - Proton number = $37 - 17 = 20$. [1] (b) $^{37}_{17}\text{Cl}$ (Element is Chlorine, but the notation $^{37}_{17}\text{X}$ is acceptable if element symbol is unknown). [1]
3. Proton number: The number of protons in the nucleus; defines the element. [1] Nucleon number: The total number of protons and neutrons in the nucleus. [1]
4. Nucleon number: $238 - 4 = 234$. [1] Proton number: $92 - 2 = 90$. [1]
5. Electrons are arranged in shells (or energy levels) around the nucleus. [1] They occupy these shells in a specific order, filling inner shells before outer ones. [1]

Section B: Radioactive Decay and Radiation

6. Alpha particle: A helium nucleus consisting of 2 protons and 2 neutrons. [1]
7. Beta radiation has moderate penetrating power (stopped by a few mm of aluminium). [1] Gamma radiation has very high penetrating power (stopped by thick lead or concrete). [1]
8. Gamma radiation consists of high-energy electromagnetic waves. [1] These are more likely to strip electrons from atoms/molecules compared to the larger, slower alpha particles (Wait: Correction - Alpha is more ionising due to charge; Gamma is less ionising but more penetrating. Correction for mark scheme: Gamma is less ionising than alpha. If student explains gamma's lack of charge, award marks). [2]
9. $^{14}_{7}\text{N}$ [2] (Proton number increases by 1, nucleon number stays same).
10. Random: It is impossible to predict exactly which nucleus will decay at any given moment. [2]
11. (a) Background radiation. [1] (b) Cosmic rays / Radon gas / Natural rocks. [1]
12. Beta particles are stopped by a few mm of aluminium; lead is much denser. [1] The count rate would drop significantly/to background levels. [1]

Section C: Half-Life and Applications

13. Half-life: The time taken for half of the radioactive nuclei in a sample to decay. [1] OR The time taken for the activity of a sample to decrease to half its initial value. [1]
14. $1200 \rightarrow 600 \rightarrow 300 \rightarrow 150$. [1] This is 3 half-lives. [1] Half-life = $15 \text{ days} / 3 = 5 \text{ days}$. [1]
15. $24 \text{ hours} / 6 \text{ hours} = 4$ half-lives. [1] Fraction = $(1/2)^4 = 1/16$. [1]
16. (a) Gamma radiation. [1] (b) Long half-life: Source lasts longer, reducing the need for frequent, expensive replacements. [1] Short half-life (tracers): Ensures the radiation does not remain in the patient's body for too long, reducing biological damage. [2]
17. Nuclear Fission: A heavy nucleus (e.g., Uranium) absorbs a neutron. [1] It becomes unstable and splits into two smaller daughter nuclei. [1] Energy and more neutrons are released. [1]
18. Nuclear Fusion: Two light nuclei (e.g., Hydrogen) collide at very high temperatures/pressures. [1] They fuse to form a heavier nucleus (e.g., Helium). [1] A large amount of energy is released. [1]
19. Nuclear fusion releases significantly more energy per unit mass than nuclear fission. [2]
20. Property: High penetrating power (Gamma). [1] Justification: The radiation must be able to pass through the soil/ground to be detected at the surface. [1] Property: Moderate half-life. [1] Justification: Long enough to find the leak, but short enough to decay away after the job is done. [1]