Structure of the Atom

Chapter 4 of the Class 9 NCERT Science textbook pieces together the atom from a century of experiments. Cathode rays and radioactivity reveal electrons, protons and neutrons, and the Thomson, Rutherford and Bohr models are judged against evidence, ending with Bohr's orbits and electronic configuration. Valency grows naturally from outer shells, while atomic number, mass number, isotopes and isobars organise diversity. The chapter trains learners to read evidence historically, accepting each model only so far as experiments support it.

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