Atomic and nuclear physics: structure of matter, radioactivity, ionising radiation
What this domain tests
Atomic and nuclear physics covers the structure of the atom, isotopes, the three main types of radioactivity, decay equations, half-life, the uses and hazards of ionising radiation, and the difference between fission and fusion. The exam wants balanced nuclear equations and clear safety reasoning.
Core rules
- Atom: a small, dense nucleus of protons (Z) and neutrons (N) surrounded by electrons; mass number A = Z + N. A neutral atom has as many electrons as protons; an ion has gained or lost electrons.
- Isotopes have the same Z but different N, for example carbon-12 and carbon-14.
- Alpha radiation is a helium nucleus (⁴₂He) with charge +2: strongly ionising, stopped by paper or a few centimetres of air.
- Beta radiation is a fast electron emitted when a neutron turns into a proton: stopped by a few millimetres of aluminium.
- Gamma radiation is a high-energy electromagnetic wave with no charge and no mass: weakened by thick lead or concrete.
- Decay equations conserve A and Z. Alpha decay lowers A by 4 and Z by 2; beta decay keeps A and raises Z by 1.
- Half-life is the time for half of the undecayed nuclei to decay: after n half-lives the amount left is N₀/2ⁿ.
- Ionising radiation damages DNA; protection means distance, shielding and limiting exposure time. Background radiation comes from radon, rocks, cosmic rays and medicine.
- Fission splits a heavy nucleus such as uranium-235 in a chain reaction (power stations); fusion joins light nuclei in the Sun and releases more energy per kilogram.
Worked example
A sample contains 64 mg of a radioisotope with a half-life of 6 hours. How much remains after 24 hours?
n = 24/6 = 4 half-lives. Remaining mass = 64 / 2⁴ = 64/16 = 4 mg. A quick equation check: ²²²₈₆Rn → ²¹⁸₈₄Po + ⁴₂He balances, because 222 = 218 + 4 and 86 = 84 + 2.
Traps that cost marks
- Calling an alpha particle an electron - it is a helium nucleus.
- Forgetting that gamma radiation is uncharged and therefore not deflected by electric or magnetic fields.
- Writing unbalanced equations: check both the top (mass) and the bottom (charge) numbers.
- Thinking half-life halves the whole sample's mass at once rather than the number of undecayed nuclei.
- Swapping fission and fusion.
- Using becquerels (activity) and sieverts (dose) as if they were the same unit.