Chapter 13 — Nuclei

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

Chapter 13 of NCERT Class 12 Physics (Part II) explores the atomic nucleus in depth. It covers atomic masses, isotopes, isobars, and isotones, and describes how neutrons were discovered by James Chadwick in 1932. Nuclear size follows R = R0A^(1/3) with R0 = 1.2 fm, giving a nearly constant nuclear density of about 2.3 × 10^17 kg/m^3. Einstein's mass-energy relation E = mc^2 explains nuclear binding energy and mass defect. The chapter also covers three types of radioactive decay (alpha, beta, gamma), nuclear fission of uranium-235 releasing ~200 MeV per event, and nuclear fusion powering the sun through the proton-proton cycle releasing 26.7 MeV per helium nucleus formed.

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.