The final section covers atomic structure and nuclear physics: the atom, radioactive decay, half-life, and nuclear energy from fission and fusion.
Atom: nucleus (protons + neutrons) surrounded by electrons. Proton number Z = number of protons. Nucleon number A = protons + neutrons. Isotopes: same proton number, different nucleon number. Rutherford scattering experiment proved the nuclear model: most alpha particles passed through (atom is mostly empty space), some deflected (small dense positive nucleus).
Unstable nuclei decay randomly and spontaneously. Alpha (α): 2 protons + 2 neutrons (helium nucleus), stopped by paper, most ionising, least penetrating. Beta (β): high-speed electron from neutron decay, stopped by aluminium. Gamma (γ): electromagnetic radiation, stopped by thick lead/concrete, least ionising, most penetrating. Nuclear equations must balance Z and A.
Half-life: time for half the radioactive nuclei to decay (or for activity to halve). Does not change with temperature, pressure, or chemical state. After n half-lives: fraction remaining = (½)ⁿ. Determine from decay curve (activity vs time graph). Applications: carbon-14 dating, medical tracers (short half-life), smoke detectors (long half-life).
Fission: heavy nucleus (e.g., U-235) splits into two lighter nuclei + neutrons + energy. Chain reaction: neutrons trigger further fissions. Controlled in nuclear reactors (control rods absorb neutrons). Fusion: light nuclei (e.g., hydrogen) combine to form heavier nucleus + energy. Powers the Sun. Requires extreme temperatures. Both convert mass to energy (E = mc²).
Radioactivity can be dangerous because ionising radiation can damage living cells and DNA, potentially causing cancer or cell death. The danger depends on: type of radiation (alpha is most dangerous inside the body, gamma is most dangerous outside), dose (intensity and duration of exposure), and distance (intensity decreases with distance²). However, controlled use of radioactivity is beneficial: medical imaging (gamma cameras, PET scans), cancer treatment (radiotherapy), and power generation (nuclear reactors).
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