Class 9 | journey inside the atom

Rediscovering Atomic Theory

The idea that matter is made up of tiny particles is very old. Scientists gradually developed the modern atomic theory through observations and experiments.

Kanada’s Idea of Atoms

Kanada was an ancient Indian philosopher and scientist associated with the Vaisheshika school of philosophy.

• He proposed that matter is made up of extremely small particles called Parmanu.
• According to his ideas, a Parmanu was the smallest indivisible unit of matter.
• He suggested that these tiny particles could combine to form larger forms of matter.
• His ideas were developed through philosophical reasoning, rather than modern laboratory experiments.

John Dalton's Atomic Theory

John Dalton developed a scientific atomic theory in the early 19th century based on experimental observations and the laws of chemical combination.

Main Postulates of Dalton's Atomic Theory

1. Matter is made up of very small particles called atoms.
2.  Atom of the same element are identical in their mass and chemical properties.
3. Modern science has shown that atoms of the same element can have different masses because of isotopes.
4. Atoms of different elements have different properties.
5. Atoms combine in simple whole-number ratios to form compounds.
6. Atoms are neither created nor destroyed during a chemical reaction. They are rearranged.

Example

Hydrogen and oxygen combine to form water:

Hydrogen + Oxygen → Water

The atoms combine in a fixed ratio to form molecules of water.

Thomson's Model of Atom

After the discovery of the electron, scientists knew that atoms were not completely indivisible.

J. J. Thomson proposed a model of the atom in 1904.

His model is popularly known as the Plum Pudding Model.

Plum Pudding Model

According to Thomson:

An atom is a sphere of uniformly distributed positive charge.

Electrons are embedded inside this positively charged sphere.

The total positive charge is equal to the total negative charge of electrons.

Therefore, an atom as a whole is electrically neutral.

Why is it called the Plum Pudding Model?

Thomson imagined the atom somewhat like a plum pudding:

Pudding → positive charge

Plums/raisins → electrons

Limitation of Thomson's Model

Thomson's model could not explain the results of Rutherford's alpha-particle scattering experiment.

Rutherford's experiment later showed that:

• Most of the atom is empty space.
• Almost all the positive charge and most of the mass are concentrated in a tiny nucleus.

Therefore, Thomson's Plum Pudding Model was eventually rejected and replaced by Rutherford's nuclear model.