Ch 13 covers nuclear physics — nuclear size and composition, mass-energy relation, binding energy, radioactivity (α, β, γ), nuclear fission, and fusion.
Nucleus: protons (Z) + neutrons (N). Mass number A = Z + N. Nuclear radius R = R₀A^(1/3). Density is constant (~2.3 × 10¹⁷ kg/m³). Mass defect Δm = (Zm_p + Nm_n) − M_nucleus. Binding energy = Δmc². BE per nucleon vs A curve: peaks at Fe-56 (~8.8 MeV/nucleon). Light nuclei gain energy by fusion, heavy by fission.
Radioactivity: N = N₀e^(−λt). Half-life t₁/₂ = 0.693/λ. α-decay: emits He-4 (Z−2, A−4). β-decay: n → p + e⁻ + ν̄ (Z+1, A same). γ-decay: energy release (no change in Z, A). Nuclear fission: U-235 + neutron → fragments + 2-3 neutrons + energy (~200 MeV). Chain reaction → nuclear reactor. Nuclear fusion: 4H → He + 2e⁺ + energy (Sun's energy source, requires extreme temperature).
Download: https://ncert.nic.in/textbook/pdf/leph205.pdf | Part II: https://ncert.nic.in/textbook/pdf/leph2ps.zip
Iron-56 has the highest binding energy per nucleon (~8.8 MeV). Nuclei lighter than iron can release energy by fusion (moving up the BE/A curve). Nuclei heavier than iron release energy by fission (also moving towards Fe). So iron is at the peak — neither fusion nor fission of iron can release energy. This is why iron is abundant in stellar cores.
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