Atomic and Nuclear Physics
Atomic and Nuclear Physics tests atomic structure, isotopes, radioactivity, alpha, beta and gamma radiation, half-life, nuclear equations, fission and fusion, and radiation safety. You should be able to interpret decay data and balance nuclear equations.
Core rules and concepts
- An atom has a nucleus of protons and neutrons surrounded by electrons. Atomic number Z = number of protons; mass number A = protons + neutrons.
- Isotopes have the same Z but different numbers of neutrons.
- Alpha decay emits a helium nucleus (⁴₂He), reducing A by 4 and Z by 2. Beta decay emits an electron (⁰₋₁e), leaving A unchanged but increasing Z by 1. Gamma emission changes neither A nor Z.
- Alpha is most ionising but least penetrating; gamma is least ionising but most penetrating.
- Half-life is the time for half the unstable nuclei in a sample to decay. Activity halves every half-life.
- Nuclear equations must conserve both mass number A and atomic number Z. Fission splits a heavy nucleus; fusion joins light nuclei.
Worked example
A radioactive source has a half-life of 6.0 hours and an initial activity of 640 counts/min. Find the activity after 24 hours. Number of half-lives = 24/6.0 = 4. After each half-life the activity halves: 640 → 320 → 160 → 80 → 40 counts/min. So the activity is 640/2⁴ = 40 counts/min. Reasoning: half-life is exponential, not linear. Dividing by 4 because there are four 6-hour periods would be wrong; you divide by 2 four times.
Traps that cost marks
- Swapping mass number and atomic number.
- Forgetting to subtract background count rate before calculating half-life.
- Thinking beta decay changes the mass number; it does not.
- Treating half-life as a linear decrease.
- Ignoring that alpha sources are most dangerous inside the body, while gamma sources are most penetrating externally.