Chemistry CBSE

Failure of Dalton's Theory

The discovery of electrons and protons within the atom disproved key aspects of Dalton's original theory.

Subatomic Particles

Fundamental constituents of an atom, including electrons, protons, and neutrons.

Discovery of Electron

Discovered by J.J. Thomson in 1897.

Cathode Ray Experiment

Thomson's experiment where he observed negatively charged particles moving from the cathode to the anode, which he named electrons.

Discovery of Proton

Discovered by Ernest Goldstein in 1886.

Canal Rays

Goldstein observed positive rays in a gas discharge tube, leading to the discovery of protons.

Discovery of Neutron

Discovered by James Chadwick in 1932; neutrons are located in the atomic nucleus.

Thomson's Model

Proposed by J.J. Thomson in 1897.

Thomson’s Model Summary

The atom is a uniformly distributed sphere of positive charge with electrons embedded inside.

Thomson’s Neutrality

Atoms are electrically neutral because the number of protons equals the number of electrons.

Alternate Names of Thomson Model

Also known as the plum pudding model.

Failure of Thomson’s Model

It could not explain the arrangement of protons and electrons within the atom.

Rutherford's Experiment

Alpha particles were directed at a thin gold foil.

Observation 1

Most alpha particles passed straight through the gold foil undeflected.

Observation 2

Some alpha particles were deflected at large angles.

Observation 3

A small number of alpha particles bounced back towards the source.

Rutherford's Postulate 1

Most of the atom's volume is empty space.

Rutherford's Postulate 2

The atom has a small, dense, positively charged center called the nucleus.

Rutherford's Atomic Model

The nucleus is very small compared to the overall size of the atom.

Rutherford's Postulate 4

Electrons orbit the nucleus in circular paths.

Drawback of Rutherford Model 1

According to classical physics, revolving electrons should lose energy and spiral into the nucleus, causing the atom to collapse.

Drawback of Rutherford Model 2

Atoms are generally stable and do not collapse.

Drawback of Rutherford Model 3

The model could not fully account for the atomic mass, as it only considered protons.

Bohr’s Model

Proposed by Niels Bohr in 1913.

Bohr's Postulate 1

The atom has a dense nucleus that contains most of its mass.

Bohr's Postulate 2

Electrons revolve