Chapter 8 — Journey Inside the Atom

Chapter 8 of the Class 9 Science NCERT textbook, "Journey Inside the Atom", traces atomic theory from ancient thinkers through Dalton, Thomson, Rutherford, and Bohr, explaining subatomic particles, atomic number, mass number, electronic configuration, valency, isotopes, and isobars.

• Evolution of the Atom — Ideas about the atom evolved from Acharya Kanada and Greek thinkers to Dalton's 1808 theory, Thomson's plum pudding model, Rutherford's nuclear model, and Bohr's shells, each refining the picture with new evidence.
• Discovering Subatomic Particles — Thomson discovered the electron in 1897 and Chadwick the neutron in 1932, explaining why atomic masses exceed the mass of protons alone; neutrons occur in every nucleus except hydrogen.
• The Nuclear Model — Rutherford's gold foil experiment showed an atom is mostly empty space with a tiny dense positive nucleus, while Bohr proposed electrons revolving in fixed energy shells (K, L, M, N) holding up to 2n².
• Describing Atoms — Atomic number is the proton count and mass number is protons plus neutrons; valency governs bonding, while isotopes share atomic number but differ in mass, and isobars share mass number but differ in element.

1. 01J. J. Thomson discovered electrons in 1897 using cathode ray tube experiments and proposed the plum pudding (watermelon) model of the atom.
2. 02Rutherford's gold foil experiment (1911) showed that most of an atom is empty space with a tiny, dense, positively charged nucleus at the centre; the atom's diameter is ~10⁻¹⁰ m and the nucleus ~10⁻¹⁵ m.
3. 03Bohr's model (1913) proposed that electrons revolve in fixed energy shells (K, L, M, N) without losing energy; the maximum electrons per shell follow the formula 2n².
4. 04James Chadwick discovered the neutron in 1932, explaining why atomic masses are greater than the sum of protons alone; neutrons are found in all nuclei except hydrogen.
5. 05Atomic number (Z) equals the number of protons; mass number (A) equals protons plus neutrons; valency is the number of electrons gained, lost, or shared to complete the outermost shell octet.
6. 06Isotopes are atoms of the same element with the same atomic number but different mass numbers (e.g., three isotopes of hydrogen: protium, deuterium, tritium); isobars are atoms of different elements with the same mass number but different atomic numbers.