⚛ Class 9 Science – Journey Inside the Atom Original Visual Revision Notes
Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.
💡 Think about this first! Imagine an atom as a giant cricket stadium. The nucleus is like a tiny marble placed right at the center of the pitch — so small it is almost invisible. The electrons are like birds flying in wide circles around the stands. And here is the amazing part — most of the stadium is completely empty! Scientists once thought atoms were solid like a raisin cake. Then brilliant experiments proved otherwise. In this lesson, we follow that exciting detective story step by step.
🏛️ Topic 1: Early Ideas About Atoms
Acharya Kanada called tiny particles parmanu.
He said parmanu cannot be divided further.
Democritus was an ancient Greek thinker.
Democritus named tiny particles atomos.
Atomos means something that cannot be cut.
🔬 Topic 2: Discovery of the Electron
J.J. Thomson studied rays inside glass tubes.
This tube is called a cathode ray tube.
He found tiny negative particles inside atoms.
These particles were later named electrons.
Thomson made this discovery in the year 1897.
🍉 Topic 3: Thomson's Atomic Model
Thomson pictured the atom as a sphere.
The sphere carried positive charge all over.
Electrons were embedded inside this positive sphere.
People called it the plum pudding model.
It is also called the watermelon model.
🥇 Topic 4: The Gold Foil Experiment
Geiger and Marsden did this experiment in 1911.
They worked under scientist Ernest Rutherford.
They aimed alpha particles at thin gold foil.
Most alpha particles passed straight through the foil.
A few particles bounced almost straight back.
🭪 Topic 5: Rutherford's Atomic Model
Rutherford said every atom has a tiny center.
This tiny center is called the nucleus.
The nucleus holds all the positive charge.
Electrons move around the nucleus like planets.
Most of the atom is empty space.
🎡 Topic 6: Bohr's Model of the Atom
Niels Bohr gave his atomic model in 1913.
Electrons move in fixed paths called shells.
Shells are also known as energy levels.
Electrons do not lose energy inside shells.
Electrons jump shells by gaining or losing energy.
🧩 Topic 7: Subatomic Particles
Atoms are made of three tiny particles.
Electrons carry a negative electric charge.
Protons carry a positive electric charge.
Neutrons carry no electric charge at all.
Protons and neutrons live inside the nucleus.
🔢 Topic 8: Atomic Number and Mass Number
Atomic number equals the number of protons.
Atomic number is written using the letter Z.
Mass number equals protons plus neutrons together.
Mass number is written using the letter A.
These two numbers identify every element uniquely.
🐚 Topic 9: Electron Arrangement in Shells
Electrons fill shells named K, L, M, N.
Each shell can hold a fixed maximum number.
The rule used is two times n squared.
The outermost shell holds at most eight electrons.
Inner shells must fill up before outer ones.
👯 Topic 10: Isotopes and Isobars
Isotopes have the same number of protons.
But isotopes have different numbers of neutrons.
Isotopes of an element behave the same chemically.
Isobars have the same mass number as each other.
But isobars have different atomic numbers.
🎨 Section 2 — Easy Illustrations
Fig 1: Thomson's model compared with Rutherford's model
Fig 2: The gold foil experiment revealed a tiny dense nucleus
Fig 3: Bohr's model with K, L and M shells
Fig 4: Protium, deuterium and tritium are isotopes of hydrogen
Fig 5: Electron shells of sodium, chlorine and argon
🔬 Section 3 — Worked Examples
💡 Example 1: Finding Subatomic Particles
An element has atomic number 12 and mass number 24. Find its protons, neutrons and electrons.
1
Protons = atomic number = 12.
2
Neutrons = mass number − protons = 24 − 12 = 12.
3
Electrons = protons in a neutral atom = 12.
Neutrons = Mass Number − Atomic Number
Answer: 12 protons, 12 neutrons, 12 electrons.
💡 Example 2: Electronic Configuration
Write the electron arrangement for an atom with atomic number 16.
1
Fill the K shell first with 2 electrons.
2
Fill the L shell next with 8 electrons.
3
Remaining electrons = 16 − 2 − 8 = 6 in the M shell.
Fill shells as K, then L, then M
Answer: The configuration is 2, 8, 6.
💡 Example 3: Finding Valency
An element has 3 electrons in its outermost shell. Find its valency.
1
Check whether losing or gaining is easier.
2
Losing 3 electrons is easier than gaining 5.
3
So the element loses these 3 electrons.
Valency = electrons lost, gained or shared
Answer: The valency of this element is 3.
📋 Section 4 — Cheat Sheet
Term
Meaning
Parmanu
Kanada's word for a tiny indivisible particle.
Atomos
Greek word meaning cannot be cut.
Electron
Negatively charged particle inside an atom.
Proton
Positively charged particle inside the nucleus.
Neutron
Neutral particle inside the nucleus.
Nucleus
Tiny dense center of an atom.
Atomic Number (Z)
Number of protons in an atom.
Mass Number (A)
Sum of protons and neutrons.
Shell
Fixed path where electrons move.
Valence Shell
The outermost shell of an atom.
Valence Electrons
Electrons present in the outermost shell.
Valency
Combining power based on outer electrons.
Octet Rule
Eight outer electrons make a shell stable.
Isotopes
Same protons but different number of neutrons.
Isobars
Same mass number but different atomic numbers.
🏠 Section 5 — Take-Home Handout
Part A: Fill in the Blanks
1. The nucleus lies at the _______ of an atom.
2. Electrons carry a _______ charge.
3. Protons carry a _______ charge.
4. Neutrons carry _______ charge.
5. The outermost shell is called the _______ shell.
Part B: True or False
1. Thomson discovered the nucleus of the atom.
2. The gold foil experiment used alpha particles.
3. James Chadwick discovered the neutron.
4. Isotopes have different atomic numbers.
5. The K shell can hold at most two electrons.
Part C: Solve These
1. An atom has 17 protons and 18 neutrons. Find its mass number.
2. An atom has atomic number 11. Write its electron arrangement.
3. An element has 6 electrons in its outer shell. Find its valency.
Part D: Match the Columns
Column A
Column B
Your Answer
1. Electron
A. No charge, found in nucleus
2. Proton
B. Same mass number, different protons
3. Neutron
C. Carries a negative charge
4. Isotope
D. Same protons, different neutrons
5. Isobar
E. Positive charge, found in nucleus
Part E: Draw and Label
Draw a Bohr model atom with three shells. Label K, L, M and nucleus.
Q3. An element has 6 electrons in its outer shell. Find its valency.
1
Outer shell has 6 electrons. A full outer shell needs 8 electrons.
2
Gaining 2 electrons is easier than losing 6 electrons.
3
So the element gains 2 electrons to become stable.
Answer: Valency = 2.
Part D: Match the Columns — Answers
Column A
Match
Column B
1. Electron
→ C
Carries a negative charge
2. Proton
→ E
Positive charge, found in nucleus
3. Neutron
→ A
No charge, found in nucleus
4. Isotope
→ D
Same protons, different neutrons
5. Isobar
→ B
Same mass number, different protons
🔄 Section 6 — Revision Flashcards
Light side = Question. Dark side = Answer.
Who used the word parmanu for atoms?
Acharya Kanada.
Who discovered the electron?
J.J. Thomson, in 1897.
What is Thomson's atomic model called?
Plum pudding model.
What did the gold foil experiment show?
Atoms have a tiny dense nucleus.
Who discovered the proton?
Ernest Rutherford.
Who gave the shell model of the atom?
Niels Bohr, in 1913.
Who discovered the neutron?
James Chadwick, in 1932.
What does the 2n² rule tell us?
The maximum electrons a shell can hold.
🧠 Section 7 — Mind Map
Fig 6: Mind map of journey inside the atom
⚠ This is independently created study material aligned with the Class 9 Science syllabus. Not affiliated with or endorsed by NCERT or any government body.