Journey Inside the Atom

 

Class 9

Chapter 8 - Journey Inside the Atom

(Till Bohr's Atomic Model)

1. What is an Atom?

The word 'atom' means 'indivisible'. Democritus and Maharishi Kanad proposed that matter is made of tiny particles called atoms. Modern science proved that atoms contain subatomic particles: electrons, protons and neutrons.

2. Discovery of the Electron

Cathode Ray Experiment (J.J. Thomson, 1897)

J.J. Thomson passed electricity through a gas kept at very low pressure in a discharge tube. He observed rays travelling from the cathode (negative electrode) to the anode (positive electrode). These were called cathode rays.

Observations

• Cathode rays travelled in straight lines.
• They produced fluorescence on the glass wall.
• They were deflected towards the positive plate, showing that they carry a negative charge.
• Their properties remained the same irrespective of the gas used.

Conclusion

Cathode rays are made up of tiny negatively charged particles called electrons, which are present in the atoms of all elements.

Charge of electron: –1.6 × 10⁻¹⁹ C

Mass of electron: 9.1 × 10⁻³¹ kg

Remember: Cathode rays = Electrons = Negative charge

3. Discovery of the Proton

Canal Ray Experiment (Eugen Goldstein)

Goldstein used a discharge tube with a perforated cathode. Along with cathode rays, another set of rays travelled in the opposite direction through the holes in the cathode. These rays were called canal rays.

Observations

• Canal rays travelled in straight lines.
• They were deflected towards the negative plate.
• Hence, they carried a positive charge.

Conclusion

Canal rays contain positively charged particles called protons.

Charge of proton: +1.6 × 10⁻¹⁹ C

Mass of proton: 1.67 × 10⁻²⁷ kg

Remember: Canal rays = Protons = Positive charge

 

4. Thomson's Atomic Model

After discovering the electron, J.J. Thomson proposed the first model of the atom.

Main Features

• The atom is a uniform sphere of positive charge.
• Negatively charged electrons are embedded throughout the sphere.
• The positive and negative charges balance each other, making the atom electrically neutral.

Example

The atom can be compared to:

• A plum pudding, where the plums represent electrons.
• A watermelon, where the seeds represent electrons embedded inside the fruit.

Limitation

This model could not explain:

• The presence of the nucleus.
• Rutherford's scattering experiment.

 

5. Rutherford's Experiment

Aim

To study how positive charge and mass are distributed inside an atom.

Experiment

A beam of fast-moving alpha particles was directed at a very thin gold foil. A fluorescent screen surrounding the foil detected the path of the particles.

Observations and Inferences

Observation	Inference
Most alpha particles passed straight through the foil.	Most of the atom is empty space.
A few particles were deflected.	Positive charge occupies only a small part of the atom.
Very few particles bounced back.	The atom has a tiny, dense, positively charged nucleus containing almost all its mass.

 

6. Rutherford's Nuclear Model

Based on his experiment, Rutherford proposed that:

• Every atom has a small, dense and positively charged nucleus at its centre.
• Nearly all the mass of the atom is concentrated in the nucleus.
• Electrons revolve around the nucleus.
• Most of the atom consists of empty space.

Drawbacks

Rutherford's model could not explain why:

• Electrons do not lose energy while revolving.
• Electrons do not fall into the nucleus.
• Atoms remain stable.

 

7. Bohr's Atomic Model

Niels Bohr improved Rutherford's model by introducing the concept of fixed energy levels.

Postulates of Bohr's Model

1. Electrons revolve around the nucleus only in certain fixed paths called shells or energy levels.
2. While moving in these shells, electrons do not lose energy.
3. Each shell has a fixed amount of energy.
4. An electron absorbs energy to move to a higher shell and releases energy when it returns to a lower shell.

Energy Levels

Shell	Symbol	Maximum Electrons
First	K	2
Second	L	8
Third	M	18
Fourth	N	32

Formula

The maximum number of electrons that can be accommodated in a shell is:

Maximum electrons = 2n²

where n is the shell number.

Example:

• K shell (n = 1): 2 × 1² = 2 electrons
• L shell (n = 2): 2 × 2² = 8 electrons
• M shell (n = 3): 2 × 3² = 18 electrons
• N shell (n = 4): 2 × 4² = 32 electrons

Comparison of Atomic Models

Thomson's Model	Rutherford's Model	Bohr's Model
An atom is a positively charged sphere with embedded electrons.	An atom has a dense nucleus with electrons revolving around it.	Electrons revolve only in fixed energy levels around the nucleus.
No nucleus.	Nucleus discovered.	Explained atomic stability.
Could not explain the scattering experiment.	Could not explain the stability of atoms.	Successfully explained why atoms are stable.