Topic 7 explores the structure of matter at the atomic and subatomic level: atomic models, nuclear stability, radioactive decay, nuclear energy, and the fundamental particles of the standard model.
Rutherford scattering proved the nuclear model: tiny dense positive nucleus surrounded by electrons. Bohr model: electrons in discrete energy levels. Transitions between levels: ΔE = hf (photon emission/absorption). Energy level diagrams. Line spectra as evidence for quantised energy levels. Ground state vs excited states.
Unstable nuclei decay randomly and spontaneously. Alpha (α): ⁴₂He emitted, Z decreases by 2, A by 4. Beta-minus (β⁻): neutron → proton + electron + antineutrino, Z increases by 1. Gamma (γ): photon emitted, no change in Z or A. Half-life (t₁/₂): time for half the nuclei to decay. N = N₀(½)^(t/t₁/₂). Activity A = λN = A₀e⁻λᵗ.
Mass defect: Δm = Z·mₚ + N·mₙ − m_nucleus. Binding energy: E = Δmc². Binding energy per nucleon curve: peak at Fe-56 (most stable). Fission: heavy nucleus splits into lighter fragments (above Fe). Fusion: light nuclei combine (below Fe). Both release energy because products have higher binding energy per nucleon.
Quarks (up, down, charm, strange, top, bottom) and leptons (electron, muon, tau, and their neutrinos) are fundamental. Force carriers (gauge bosons): photon (EM), W±/Z⁰ (weak), gluon (strong). Higgs boson gives particles mass. Hadrons: baryons (3 quarks, e.g., proton = uud) and mesons (quark-antiquark pair). Conservation laws: charge, baryon number, lepton number, strangeness.
Fission: a heavy nucleus (e.g., U-235) splits into two lighter fragments when hit by a neutron. Used in nuclear power plants. Fusion: two light nuclei (e.g., hydrogen isotopes) combine to form a heavier nucleus. Powers the Sun. Both release energy because the products have higher binding energy per nucleon than the reactants. Fusion releases more energy per kilogram and produces less radioactive waste, but requires extreme temperatures (~10⁸ K) to overcome electrostatic repulsion.
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