Chapter 8: Journey Inside the AtomClass 9 Science — summary, notes, extra questions & NCERT solutions

Summary

This chapter traces how our picture of the atom developed and what atoms are made of. The idea of indivisible particles is ancient — Acharya Kanada called them parmanus and the Greeks called them atomos — but Dalton (1808) made it scientific. Then J. J. Thomson discovered the negatively charged electron from cathode rays and proposed the plum-pudding model (electrons embedded in a positive sphere). Rutherford’s gold-foil (alpha-scattering) experiment showed most of the atom is empty space with a tiny, dense, positively charged nucleus, giving the planetary model; he also identified the proton. Because an orbiting electron should radiate energy and spiral in, Rutherford’s model could not explain stability, so Bohr proposed fixed energy levels or shells (K, L, M, N… or n = 1, 2, 3…) in which electrons move without losing energy. Chadwick later discovered the neutral neutron, explaining atomic mass. The three subatomic particles are electrons (−1), protons (+1) and neutrons (0). Atomic number Z is the number of protons (it defines the element), and mass number A is protons plus neutrons (nucleons). Electrons fill shells by the Bohr–Bury rule (maximum \(2n^2\) per shell, outermost ≤ 8), giving the electronic configuration; a complete octet (or duplet for the K-shell) makes an atom stable, and valency is the combining capacity. Isotopes have the same Z but different A (e.g. protium, deuterium, tritium), and average atomic mass is the abundance-weighted mean of isotope masses (chlorine ≈ 35.5 u); isobars have the same A but different Z.

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