Chapter at a glance
The history of atomic models from Thomson to Rutherford and Bohr, subatomic particles, atomic and mass numbers, electron shells, valency, isotopes and isobars.
What this chapter covers
- 8.1 Rediscovering the Roots of Atomic Theory
- 8.2 A Short Historical Journey Through Atomic Models
- 8.3 Components of the Atom
- 8.4 Symbols of Elements
- 8.5 Atomic Number
- 8.6 Mass Number
- 8.7 Distribution of Electrons in Shells
- 8.8 Valency
- 8.9 A Deeper Look into Atomic Structure
Key points
- Thomson: electrons embedded in a positive sphere. Rutherford: tiny dense positive nucleus with mostly empty space. Bohr: electrons in fixed shells K, L, M, N. Chadwick discovered the neutron.
- Atomic number = number of protons; mass number = protons + neutrons.
- An octet in the outer shell (a duplet for helium) makes an atom stable; valency is its combining capacity.
- Isotopes have the same atomic number but different mass numbers; isobars have the same mass number but different atomic numbers.
Read the NCERT chapter
Read this chapter in the official NCERT textbook Exploration (2026-27 edition) for free on ncert.nic.in.
Source: NCERT, Exploration, Grade 9 (2026-27). NCERT chapter PDF. Spotted a mistake? Email edura.class9.yt@gmail.com. Last updated 11 October 2026.