Chapter 13 — Nuclei

Chapter 13 of the Class 12 Physics NCERT textbook, "Nuclei", covers the structure, composition, and properties of atomic nuclei, explaining nuclear forces, radioactivity, binding energy, and energy release through fission and fusion.

• Inside the nucleus — The chapter examines the nucleus of protons and neutrons, introducing isotopes, isobars, and isotones, and the surprising result that nuclear density stays nearly constant regardless of size.
• Mass, energy, and binding — Einstein's mass-energy relation explains the mass defect: nuclei weigh less than their parts, and this missing mass becomes the binding energy holding nucleons together.
• The nuclear force and radioactivity — A short-range force far stronger than electrical repulsion binds nucleons, while unstable nuclei shed energy through alpha, beta, and gamma decay.
• Fission and fusion — Splitting heavy nuclei (fission) or joining light ones (fusion) releases immense energy — powering reactors and the sun, respectively — millions of times more than chemical reactions.

1. 01The atomic nucleus contains protons and neutrons (nucleons); its radius follows R = R0A^(1/3) where R0 = 1.2 fm, and nuclear density is approximately 2.3 × 10^17 kg/m^3, independent of mass number A.
2. 02Mass defect ΔM = [Zmp + (A−Z)mn] − M leads to nuclear binding energy Eb = ΔMc^2; binding energy per nucleon peaks around 8.75 MeV for A = 56 (iron).
3. 03The nuclear force is short-ranged, much stronger than the Coulomb force, and acts equally between neutron-neutron, proton-neutron, and proton-proton pairs.
4. 04Radioactivity involves three decay types: alpha decay (helium nucleus emitted), beta decay (electrons or positrons emitted), and gamma decay (high-energy photons emitted).
5. 05Nuclear fission of U-235 releases approximately 200 MeV per fissioning nucleus — about a million times more energy than a typical chemical reaction.
6. 06Nuclear fusion powers the sun via the proton-proton cycle, in which four hydrogen nuclei fuse into one helium-4 nucleus releasing 26.7 MeV of energy.