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

Journey Inside the Atom moves through the three classic atomic models, Thomson, Rutherford, and Bohr, and the rules for filling electron shells, and this page lines up the definitions of atomic number and mass number alongside them.

Last Updated: September 23, 2026

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.

Frequently Asked Questions

Why was the discovery of the electron and proton important for understanding atomic structure?
The discovery of subatomic particles like the electron and proton showed that the atom, once thought to be indivisible, actually has an internal structure made of charged particles, which led to the development of various atomic models.

What is the main limitation of the early atomic models that the chapter discusses?
Early models like the plum pudding model could not explain experimental observations such as the scattering of alpha particles, which eventually led to the nuclear model of the atom proposed after the gold foil experiment.

Written by Satish

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