Class 9 Science Chapter 8 Journey Inside the Atom – NCERT Solutions & Easy Notes Part 1 | PJ Sir
Class 9 Science Chapter 8 Journey Inside the Atom – NCERT Solutions & Easy Notes Part 1 | PJ Sir
Part 1: Journey of Atomic Models – From Dalton to Rutherford
Atoms are the building blocks of matter. Everything around us—living as well as non-living—is ultimately made up of atoms.
But scientists did not always know what was present inside an atom.
For a long time, atoms were considered indivisible. As experiments improved, scientists discovered electrons and the nucleus, and the model of the atom changed step by step.
In Part 1, we will understand this fascinating journey in simple language.
1. Early Idea of the Atom
The idea that matter is made of extremely small particles is more than 2,000 years old.
Acharya Kanada
Ancient Indian thinker Acharya Kanada suggested that if matter or dravya is divided repeatedly, eventually we reach particles that cannot be divided further.
He called these smallest particles parmanus.
His ideas are recorded in the Vaisesika Sutras.
According to this idea:
Parmanus are extremely small.
They cannot be perceived by our senses.
Two parmanus may combine to form a dyad.
Three parmanus may combine to form a triad.
Different combinations eventually form the material world around us.
Leucippus and Democritus
Ancient Greek philosophers Leucippus and Democritus also proposed the existence of indivisible particles.
They called them atomos, meaning indivisible.
These early ideas were philosophical ideas rather than conclusions based on scientific experiments. Chapter 8 Journey Inside the A…
2. Dalton’s Atomic Theory
In 1808, John Dalton proposed an atomic theory based on the scientific experiments available at that time.
Dalton proposed that:
All matter is made up of tiny particles called atoms.
He considered atoms to be indivisible and the fundamental building blocks of matter.
Dalton’s theory was an important step because it provided an early scientific description of how matter is made. Chapter 8 Journey Inside the A…
PJ Sir Trick
Remember the journey:
Kanada → Parmanu
Democritus → Atomos
Dalton → Scientific Atomic Theory
3. Were Atoms Really Indivisible?
For many years, atoms were believed to be the smallest indivisible units of matter.
Later, observations related to radioactivity indicated that atoms could contain smaller particles.
This raised an important question:
What is present inside an atom?
The first major answer came from J. J. Thomson.
4. Discovery of Electron – J. J. Thomson
In 1897, J. J. Thomson studied the conduction of electricity through gases at very low pressure.
He used a glass tube containing two electrodes:
Cathode → Negative electrode
Anode → Positive electrode
When high voltage was applied, rays were observed travelling from the cathode towards the anode.
These rays were called cathode rays.
By studying their behaviour in electric and magnetic fields, Thomson concluded that cathode rays were streams of negatively charged particles.
These particles were later called electrons.
The mass of these particles was much smaller than the mass of an atom.
This was strong evidence that an atom was not indivisible—it contained smaller subatomic components. Chapter 8 Journey Inside the A…
5. Why Are Electrons Present in All Atoms?
An important observation was that the nature of cathode rays did not depend upon:
the material used for the cathode, or
the gas present inside the cathode ray tube.
This indicated that electrons are a fundamental component of all atoms and are present in every element.
The charge of an electron is:
–1.602 × 10⁻¹⁹ C
For convenience, its relative charge is taken as:
Electron = –1 Chapter 8 Journey Inside the A…
30-Second Recall
Electron
Symbol: e⁻
Charge: Negative
Relative charge: –1
Associated discovery: J. J. Thomson
Experiment: Cathode ray experiment
6. Thomson’s Model of the Atom
After discovering electrons, Thomson faced another question.
If electrons are negatively charged, then:
Where is the positive charge in a neutral atom?
To explain this, Thomson proposed a model of the atom.
According to Thomson:
An atom is a sphere of positive charge.
Negatively charged electrons are distributed throughout this positive sphere.
The total positive and negative charges balance each other, making the atom electrically neutral. Chapter 8 Journey Inside the A…
7. Plum Pudding Model
Thomson’s model is commonly known as the Plum Pudding Model.
The chapter also uses a watermelon as a familiar comparison.
Imagine a watermelon:
Red pulp → positively charged matter
Seeds → electrons
Just as seeds are distributed throughout the watermelon, Thomson imagined electrons distributed throughout the positively charged atom.
PJ Sir Trick
Thomson = Watermelon Model
Remember:
Pulp = Positive
Seeds = Negative electrons
8. Problem with Thomson’s Model
Thomson’s model was an important early attempt to explain how positive and negative charges could exist together inside an atom.
But was this model actually correct?
To test it, scientists performed one of the most important experiments in the history of atomic structure:
Rutherford’s Gold Foil Experiment
9. Rutherford’s Gold Foil Experiment
In 1911, Geiger and Marsden, working under Ernest Rutherford, tested Thomson’s model.
They directed a narrow beam of alpha (α) particles towards an extremely thin sheet of gold foil.
Alpha particles are positively charged particles.
According to Thomson’s model, positive charge was spread uniformly throughout the atom.
Therefore, scientists expected most alpha particles to pass straight through the foil, perhaps with only slight deflection.
But the actual observations were surprising. Chapter 8 Journey Inside the A…
10. Observations of the Gold Foil Experiment
Three major observations were made:
1. Most α-particles passed straight through the gold foil without deflection.
2. Some α-particles were deflected through significant angles.
3. A very small number of α-particles bounced back.
The change from the straight path is called scattering.
Therefore, the experiment is also called the:
α-ray scattering experiment
11. What Did Rutherford Conclude?
The observations completely changed our understanding of the atom.
Observation 1
Most α-particles passed straight through.
Conclusion
Most of the atom is empty space.
Observation 2
Some α-particles were deflected.
Conclusion
The positive charge is not spread uniformly throughout the atom.
Observation 3
A few α-particles bounced back.
Conclusion
The positive charge and most of the mass of the atom are concentrated in an extremely small, dense central region.
This region was called the nucleus. Chapter 8 Journey Inside the A…
PJ Sir Trick
Remember Rutherford with:
M-S-F
M → Most passed straight
S → Some were deflected
F → Few bounced back
Therefore:
Most atom = Empty
Centre = Small + Dense + Positive
12. Rutherford’s Model of the Atom
Based on the gold foil experiment, Rutherford proposed that:
Most of the atom is empty space.
At the centre is an extremely small region called the nucleus.
The nucleus contains the positive charge and most of the mass of the atom.
Electrons revolve around the nucleus somewhat like planets revolving around the Sun.
Therefore, Rutherford’s model is also called the:
Planetary Model of the Atom
13. How Small Is the Nucleus?
The chapter gives us a fascinating comparison.
Approximate diameter of atom:
10⁻¹⁰ m
Approximate diameter of nucleus:
10⁻¹⁵ m
The nucleus is therefore extremely small compared with the atom.
Imagine an atom as large as a cricket ground about 100 m across.
Its nucleus would be only about the size of a tiny black pepper grain at the centre. Chapter 8 Journey Inside the A…
14. Limitation of Rutherford’s Model
Rutherford’s model successfully explained the gold foil experiment.
However, it had an important limitation:
It could not explain why atoms are stable.
According to the reasoning discussed in the chapter, an electron moving in a circular path is accelerating.
A negatively charged accelerating electron should lose energy.
As it loses energy, it should move closer and closer to the positively charged nucleus and eventually fall into it.
If this happened:
Atoms should collapse.
But atoms are actually stable.
Therefore, Rutherford’s model could not completely explain atomic stability. Chapter 8 Journey Inside the A…
PJ Sir Trick
Rutherford discovered where the positive charge is concentrated, but could not explain why the electron does not fall into the nucleus.
This problem leads us to the next important atomic model:
Bohr’s Model of the Atom
Important Questions & Answers
Q1. Who proposed the idea of parmanu?
Answer: Acharya Kanada proposed that repeated division of matter would eventually lead to extremely small indivisible particles called parmanus.
Q2. What does the Greek word “atomos” mean?
Answer: Atomos means indivisible.
Q3. Who proposed the scientific atomic theory in 1808?
Answer: John Dalton.
Q4. Who discovered the electron?
Answer: J. J. Thomson.
Q5. What is the charge on an electron?
Answer: An electron has a negative charge. Its relative charge is –1.
Q6. What are cathode rays?
Answer: Cathode rays are streams of negatively charged particles that were identified as electrons.
Q7. Explain Thomson’s model of the atom.
Answer: Thomson proposed that an atom is a positively charged sphere with negatively charged electrons distributed throughout it. The positive and negative charges balance, making the atom neutral.
Q8. Why did Thomson conclude that electrons are present in all atoms?
Answer: The nature of cathode rays was independent of the cathode material and the gas present in the tube. This indicated that electrons are fundamental components of all atoms.
Q9. What were the major observations of Rutherford’s gold foil experiment?
Answer: Most alpha particles passed straight through the foil, some were deflected, and a very small number bounced back.
Q10. What did Rutherford conclude from the gold foil experiment?
Answer: Rutherford concluded that most of the atom is empty space and that its positive charge and most of its mass are concentrated in a very small, dense central nucleus.
Q11. Why is Rutherford’s model called the planetary model?
Answer: Rutherford proposed that electrons revolve around the nucleus somewhat like planets orbiting the Sun.
Q12. What was the main limitation of Rutherford’s atomic model?
Answer: Rutherford’s model could not explain the stability of the atom or why revolving electrons do not lose energy and eventually fall into the nucleus.
30-Second Revision
Acharya Kanada → Parmanu
Democritus → Atomos
Dalton → Atom as indivisible building block
J. J. Thomson → Electron
Thomson Model → Positive sphere + embedded electrons
Gold Foil Experiment → α-particle scattering
Rutherford → Nucleus
Most of atom → Empty space
Nucleus → Small + Dense + Positive
Rutherford Model → Planetary Model
Main limitation → Could not explain atomic stability
What Will We Learn in Part 2?
In Part 2 – Inside the Atom Made Easy, we will study:
Bohr’s Model of Atom
K, L, M, N Energy Levels
Protons, Electrons & Neutrons
Discovery of Neutron
Atomic Number
Mass Number
Important Numericals
Continue with Part 2 → Coming Next
About the Author
Priyank Joshi (PJ Sir)
Chemistry Educator & Mentor
18+ Years of Teaching Experience
JEE | NEET | Class 9–12 Chemistry
The aim of these notes is to help students understand Chemistry through concepts rather than simply memorising information.
Reference: Exploration – Grade 9 Science, Chapter 8: Journey Inside the Atom.
This page contains independently written explanations, examples and learning support prepared by Priyank Joshi (PJ Sir) for students.
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📚 Class 9 Science Chapter 8 – Journey Inside the Atom: Read All Parts
Part 1 – Journey of Atomic Models: Dalton, Thomson & Rutherford
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Part 2 – Bohr Model, Subatomic Particles, Atomic Number & Mass Number
READ PART 2 →
Part 3 – Electronic Configuration, Valence Electrons & Valency
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Part 4 – Isotopes, Isobars & Average Atomic Mass
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