Class 9 Science Chapter 8 Journey Inside the Atom – Revision Notes

Quick revision notes for Class 9 Science Chapter 8 (Journey Inside the Atom). These Class 9 Science Chapter 8 notes are ideal for quick revision just before exams.

Subatomic Particles

• Proton (+, nucleus), Neutron (neutral, nucleus), Electron (−, orbits nucleus).

Atomic Models

• Thomson: plum pudding (uniform + charge, electrons embedded).
• Rutherford: dense central nucleus, electrons orbit in empty space (gold foil experiment).
• Bohr: electrons in fixed energy shells (K, L, M, N…), no energy radiated in orbit.

Shell Filling

• Max electrons per shell: 2n² → K=2, L=8, M=18 (8 as outermost up to Z=18), N=32.

Key Definitions

• Atomic number (Z) = protons = electrons (neutral atom).
• Mass number (A) = protons + neutrons.
• Isotopes: same element, different mass number.
• Isobars: different elements, same mass number.
• Valency: combining capacity, based on valence (outermost shell) electrons.

One-Line Summary

Atomic models evolved from Thomson’s plum pudding to Rutherford’s nuclear model to Bohr’s shell model, and atoms are identified/compared using atomic number, mass number, isotopes, isobars, and valency based on electron shell arrangement.

Written by Satish

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