Radioactivity covers atomic structure, the three types of nuclear radiation, half-life, nuclear equations, uses and dangers of radiation, and the processes of fission and fusion.
Atom: nucleus (protons + neutrons) surrounded by electrons. Atomic number Z = protons; mass number A = protons + neutrons. Isotopes: same Z, different A. Unstable nuclei decay: alpha (α) — 2 protons + 2 neutrons (helium nucleus), stopped by paper, most ionising. Beta (β) — high-speed electron from neutron → proton, stopped by aluminium. Gamma (γ) — EM radiation, stopped by thick lead/concrete, least ionising. Background radiation: from rocks, cosmic rays, medical, food.
Half-life: time for half the radioactive nuclei to decay (or activity to halve). Used in dating (carbon-14), medical tracers (technetium-99m), and thickness monitoring. Nuclear equations balance: mass numbers and atomic numbers must balance. Alpha decay: ᴬ_Z X → ᴬ⁻⁴_(Z−2) Y + ⁴₂He. Beta decay: ᴬ_Z X → ᴬ_(Z+1) Y + ⁰₋₁e. Nuclear fission: large nucleus splits into two smaller nuclei + neutrons + energy. Chain reaction: neutrons cause further fissions. Nuclear fusion: small nuclei join to form larger nucleus + energy. Fusion powers stars. Conditions: extremely high temperature and pressure.
Carbon-14 is a radioactive isotope absorbed by living organisms from CO₂ in the atmosphere. While alive, the ratio of C-14 to C-12 is constant (replenished). When the organism dies, it stops absorbing C-14, and the existing C-14 decays with a half-life of about 5730 years. By measuring the remaining C-14 activity and comparing it to the expected activity of a living sample, scientists can calculate how many half-lives have passed. For example, if the activity is ¼ of the living value, 2 half-lives have passed → age ≈ 11,460 years. Carbon dating is useful up to about 50,000 years (after that too little C-14 remains to measure reliably).
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