Atomic and Nuclear Physics
Core concepts
Atomic models progressed from Thomson's plum-pudding model to Rutherford's nuclear model and Bohr's quantised energy levels. Atoms emit and absorb photons when electrons transition between levels, with photon energy equal to the energy difference: E = hf = ΔE. Emission spectra are discrete lines; absorption spectra show dark lines at the same wavelengths.
The nucleus contains protons and neutrons, held together by the strong nuclear force, which acts over very short ranges and overcomes proton-proton repulsion. Isotopes have the same proton number but different neutron numbers. Mass defect is converted to binding energy via E = mc²; binding energy per nucleon peaks near iron, explaining both fusion in light nuclei and fission in heavy nuclei.
Radioactive decay is random and spontaneous. Activity A = λN, with half-life t½ = ln2/λ. Alpha decay reduces mass number by 4 and atomic number by 2; beta-minus decay converts a neutron to a proton with emission of an electron and antineutrino; gamma emission changes energy but not particle numbers. Conservation of mass number and charge must be checked in every nuclear equation.
Traps
Forgetting that photon energy equals the energy difference between levels, not the level energy itself; misapplying half-life (it is not a fixed lifetime); ignoring neutrinos in beta decay; confusing mass number with atomic mass; assuming binding energy per nucleon always increases.
Worked example
A sample has a half-life of 8 days. After 24 days, three half-lives have passed, so the fraction remaining is (½)³ = 1/8, or 12.5%. Rule of thumb: count half-lives first, then apply the power of one-half.