Atomic, Nuclear and Particle Physics
This section covers Atomic, Nuclear and Particle Physics. The exam tests atomic models, photon energy and spectra, nuclear notation, radioactive decay, half-life, mass defect, binding energy, and the standard model.
Core rules
- A photon carries energy E = hf = hc/λ. Line spectra arise from electron transitions between discrete energy levels.
- Nuclear notation uses A (nucleon number) and Z (proton number). Alpha decay reduces A by 4 and Z by 2; beta-minus decay converts a neutron to a proton and increases Z by 1; gamma emission changes neither A nor Z.
- Radioactive decay is random and spontaneous. Activity decreases by half in each half-life: A = A₀(1/2)^(t/T½).
- Mass defect is the difference between the mass of separate nucleons and the nucleus. Binding energy = mass defect × c².
- The strong nuclear force binds nucleons. Quarks have fractional charge and combine to form hadrons; leptons include electrons and neutrinos.
Worked example
A sample has activity 80 Bq and a half-life of 5 days. After 15 days, three half-lives have passed. The remaining activity is 80 × (1/2)³ = 80 × 1/8 = 10 Bq. The activity has fallen by 70 Bq, not by 80/3 Bq; half-life decay is exponential, not linear.
Traps
Gamma rays have no mass and no charge, so A and Z do not change. Beta-minus emission increases proton number, whereas alpha emission decreases both A and Z. Half-life does not mean all activity disappears after one half-life. Conserved quantities in particle reactions include charge, nucleon number and energy. Do not confuse activity with count rate, which also depends on detection efficiency.