CBSE Class 12 — Notes, Chapters & Practice Quizzes

Chapter 5: Nuclei

Summary

The nucleus, made of protons and neutrons (nucleons), has a radius \(R=R_0 A^{1/3}\), giving nearly constant nuclear density independent of mass number. Atomic masses are measured in unified mass units, and the mass of a nucleus is always less than the sum of its constituent nucleons—this mass defect \(\Delta m\) corresponds, through Einstein's \(E=\Delta m\,c^2\), to the binding energy holding the nucleus together. The binding energy per nucleon peaks near mass number 56 (iron), explaining why heavy nuclei release energy on fission and light nuclei on fusion. Radioactivity is the spontaneous decay of unstable nuclei by alpha, beta or gamma emission, governed by the law \(N=N_0 e^{-\lambda t}\), with half-life \(T_{1/2}=\dfrac{0.693}{\lambda}\) and activity proportional to the number of nuclei present. In nuclear fission a heavy nucleus splits, releasing energy and neutrons that can sustain a chain reaction, the basis of reactors and weapons. In fusion, light nuclei combine to form a heavier one with enormous energy release—the process powering the Sun and stars—demonstrating the immense energy locked within the nucleus.

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