This are detailed, easy-to-write Class 11 Chemistry Chapter 2 (Structure of
Atom) notes in English, suitable for school notebooks and exam
preparation:
1. Introduction: Structure of Atom
Atoms are the basic building blocks of all matter.
Ancient ideas suggested matter is made up of very small indivisible
particles called “atoms.” Modern science confirms atoms are made
of subatomic particles: electrons, protons, and neutrons.
2. Discovery of Sub-Atomic Particles
Electron:*Discovered by J.J. Thomson using cathode ray experiments;
electrons have a negative charge.
Proton:*Discovered by Goldstein using canal rays; protons carry a
positive charge.
Neutron:*Discovered by James Chadwick; neutrons are neutral (no
charge).
3. Atomic Models
a. Thomson’s Model (Plum Pudding Model)
Atom is a sphere of positive charge with negatively charged
electrons embedded in it.
Could not explain certain experimental results.
b. Rutherford’s Nuclear Model
Most of the mass and all the positive charge of atom is concentrated
in a small nucleus.
Electrons revolve around the nucleus in orbits.
Most of atom is empty space.
Could not explain atomic stability and spectra.
c. Bohr’s Model
Electrons revolve in fixed circular orbits called “energy levels”
(shells) without emitting energy.
Energy is absorbed/emitted when electrons jump between shells.
4. Atomic Number and Mass Number
