Class 9 Science Chapter 8 Journey Inside the Atom – NCERT Solutions & Easy Notes Part 2 | PJ Sir

Part 2: Inside the Atom – Bohr Model, Subatomic Particles, Atomic Number & Mass Number

Part 1 me humne atomic models ki journey samjhi — Dalton se Thomson aur Rutherford tak.

Rutherford ne bataya ki atom ke centre me ek very small, dense and positively charged nucleus hota hai aur atom ka most part empty space hota hai.

Lekin Rutherford ka model ek important question ka answer nahi de paya:

Electrons nucleus me girte kyon nahi hain?

Isi problem ko samjhane ke liye Niels Bohr ne atom ka ek improved model propose kiya.

1. Bohr’s Model of the Atom

In 1913, Niels Bohr proposed a new model to explain the stability of atoms.

According to Bohr:

Electrons nucleus ke around randomly move nahi karte.

They move only in certain fixed circular paths.

These fixed paths are called:

Stationary States

Orbits

Shells

Each shell has a definite amount of energy, so these shells are also called:

Energy Levels

The shells are represented as:

K, L, M, N...

n = 1, 2, 3, 4...

So:

K shell → n = 1

L shell → n = 2

M shell → n = 3

N shell → n = 4

The K shell is nearest to the nucleus and has the lowest energy.

As we move farther from the nucleus, the energy of the shells increases.

2. Why Do Electrons Not Fall into the Nucleus?

This was the major problem with Rutherford’s model.

Bohr proposed that an electron moving in an allowed stationary state does not lose energy.

Therefore, it can continue moving around the nucleus without continuously losing energy and collapsing into it.

Electrons cannot exist between the allowed shells.

An electron can move from one shell to another by absorbing or releasing a fixed amount of energy equal to the energy difference between the two levels.

PJ Sir Trick

Remember:

K → L → M → N

means

1 → 2 → 3 → 4

And:

Distance from nucleus ↑ → Energy ↑

So:

K < L < M < N in energy

3. Rutherford Model vs Bohr Model

Rutherford Model

Electrons revolve around the nucleus.

But the model could not satisfactorily explain why atoms remain stable.

Bohr Model

Electrons move only in specific allowed energy levels.

While an electron remains in an allowed stationary state, its energy remains constant.

Therefore, Bohr’s model provided an explanation for atomic stability.

4. Three Important Subatomic Particles

By this stage, scientists understood that atoms contain smaller particles.

The three important subatomic particles are:

Electron → e⁻ → Relative charge = –1

Proton → p⁺ → Relative charge = +1

Neutron → n⁰ → Relative charge = 0

Electrons are present outside the nucleus.

Protons and neutrons are present inside the nucleus.

PJ Sir Trick

Nucleus = P + N

Outside nucleus = E

P = Proton

N = Neutron

E = Electron

5. Discovery of Neutron

Rutherford’s model showed that most of the mass of an atom is concentrated in its nucleus.

But scientists noticed an interesting problem.

Hydrogen has one proton.

Helium has two protons.

Still, helium has approximately four times the mass of hydrogen rather than simply twice its mass.

This suggested that something else must also be present inside the nucleus.

In 1932, James Chadwick discovered another subatomic particle.

It had:

Nearly the same mass as a proton

But no electrical charge

This particle was called the:

Neutron

Symbol:

n⁰

Neutrons are found in the nuclei of atoms except ordinary hydrogen, according to the chapter.

The mass of an atom therefore comes mainly from the protons and neutrons packed inside its nucleus.

6. Proton, Neutron and Electron – Quick Comparison

Electron

Symbol: e⁻
Relative charge: –1
Location: Outside nucleus

Proton

Symbol: p⁺
Relative charge: +1
Location: Nucleus

Neutron

Symbol: n⁰
Relative charge: 0
Location: Nucleus

PJ Sir Trick

Remember charges using:

P = Positive

N = Neutral

E = Negative

7. Why Is an Atom Electrically Neutral?

A normal atom is electrically neutral.

Why?

Because the number of positively charged protons is equal to the number of negatively charged electrons.

Therefore:

Number of Protons = Number of Electrons

Positive charge and negative charge balance each other.

Example:

Hydrogen:

1 proton + 1 electron

Helium:

2 protons + 2 electrons

Therefore, both atoms are electrically neutral.

8. Atomic Number

The atoms of different elements differ in the number of protons present in their nuclei.

The number of protons in the nucleus of an atom is called its Atomic Number.

Atomic number is represented by:

Therefore:

Atomic Number (Z) = Number of Protons

For a neutral atom:

Number of Protons = Number of Electrons

Therefore:

Z = Number of Protons = Number of Electrons

The atomic number determines the identity of an element and is related to its chemical behaviour.

9. Examples of Atomic Number

Hydrogen

Number of protons = 1

Therefore:

Z = 1

Number of electrons in neutral H atom = 1

Helium

Number of protons = 2

Therefore:

Z = 2

Number of electrons = 2

Carbon

Number of protons = 6

Therefore:

Z = 6

Number of electrons = 6

PJ Sir Trick

For a neutral atom:

Z = p = e

Atomic Number = Protons = Electrons

10. Mass Number

Protons and neutrons are present inside the nucleus.

Together, protons and neutrons are called:

Nucleons

The total number of protons and neutrons present in the nucleus is called the:

Mass Number

Mass number is represented by:

Therefore:

Mass Number = Number of Protons + Number of Neutrons

A = p + n

The mass of an electron is almost negligible compared with proton and neutron mass, so it is ignored in this calculation.

11. How to Calculate Number of Neutrons?

We know:

A = p + n

And:

p = Z

Therefore:

A = Z + n

So:

Number of Neutrons = Mass Number – Atomic Number

n = A – Z

PJ Sir Super Trick

Remember only these:

Z = p

For neutral atom:

p = e

Therefore:

Z = p = e

And:

A = p + n

So:

n = A – Z

This one trick solves most basic atomic-number and mass-number numericals.

12. Example – Hydrogen

Hydrogen has:

Protons = 1

Neutrons = 0

Therefore:

Mass Number = 1 + 0 = 1

So:

A = 1

Z = 1

13. Example – Helium

Helium has:

Protons = 2

Neutrons = 2

Therefore:

A = 2 + 2 = 4

So:

Mass Number = 4

Atomic Number = 2

14. Example – Lithium

Lithium has:

Protons = 3

Neutrons = 4

Therefore:

A = 3 + 4 = 7

So:

Mass Number = 7

Atomic Number = 3

15. Standard Representation of an Atom

An atom can be represented using:

A = Mass Number

Z = Atomic Number

along with the symbol of the element.

For carbon:

Symbol = C

Atomic Number = 6

Mass Number = 12

So carbon is represented as:

¹²₆C

Here:

12 = Mass Number

6 = Atomic Number

Therefore:

Protons = 6

Electrons = 6

Neutrons = 12 – 6 = 6

16. Solved Numerical 1

An atom has:

Atomic Number = 26

Mass Number = 56

Find the number of protons, electrons and neutrons.

Solution

Atomic Number:

Z = 26

Therefore:

Protons = 26

For a neutral atom:

Electrons = 26

Neutrons:

n = A – Z

n = 56 – 26

n = 30

Answer

Protons = 26

Electrons = 26

Neutrons = 30

17. Solved Numerical 2

An atom contains:

Protons = 20

Mass Number = 41

Find the number of neutrons.

Solution

n = A – Z

Here:

A = 41

Z = Number of protons = 20

Therefore:

n = 41 – 20 = 21

Answer

Number of neutrons = 21

18. Solved Numerical 3

An atom has:

Neutrons = 18

Atomic Number = 17

Find its mass number.

Solution

Atomic Number = Number of protons

Therefore:

Protons = 17

We know:

A = p + n

Therefore:

A = 17 + 18

A = 35

Answer

Mass Number = 35

Important Questions & Answers

Q1. Who proposed the model of fixed energy levels?

Answer: Niels Bohr proposed the atomic model with fixed energy levels or shells.

Q2. In which year did Bohr propose his atomic model?

Answer: Bohr proposed his atomic model in 1913.

Q3. What are stationary states?

Answer: The fixed allowed paths or shells in which electrons can move around the nucleus with definite energy are called stationary states.

Q4. Name the first four shells.

Answer: K, L, M and N.

Q5. Which shell is nearest to the nucleus?

Answer: K shell.

Q6. Who discovered the neutron?

Answer: James Chadwick discovered the neutron in 1932.

Q7. What are the three major subatomic particles?

Answer: Electron, proton and neutron.

Q8. What is atomic number?

Answer: The number of protons present in the nucleus of an atom is called its atomic number.

Q9. What symbol is used for atomic number?

Answer: Z

Q10. What is mass number?

Answer: The total number of protons and neutrons in the nucleus of an atom is called its mass number.

Q11. What symbol is used for mass number?

Answer: A

Q12. What are nucleons?

Answer: Protons and neutrons present in the nucleus are collectively called nucleons.

Q13. Write the formula for finding neutrons.

Answer:

Number of Neutrons = Mass Number – Atomic Number

n = A – Z

Q14. Why is an atom electrically neutral?

Answer: A neutral atom contains equal numbers of positively charged protons and negatively charged electrons, so the charges balance each other.

30-Second Revision

Bohr → Fixed Energy Levels

Shells → K, L, M, N

K → Closest to nucleus + lowest energy

Electron → e⁻ → –1

Proton → p⁺ → +1

Neutron → n⁰ → 0

James Chadwick → Neutron

Atomic Number → Z = Number of Protons

For neutral atom:

Z = p = e

Mass Number → A = p + n

Therefore:

n = A – Z

What Will We Learn in Part 3?

In Part 3 – Electronic Configuration & Valency Made Easy, we will study:

Distribution of Electrons

2n² Rule

K, L, M, N Shell Filling

Electronic Configuration

Valence Shell

Valence Electrons

Octet & Stability

Valency

Important Examples & Questions

Continue with Part 3 → 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.

📚 Class 9 Science Chapter 8 – Journey Inside the Atom: Read All Parts

Part 1 – Journey of Atomic Models: Dalton, Thomson & Rutherford
READ PART 1 →

Part 2 – Bohr Model, Subatomic Particles, Atomic Number & Mass Number
You are reading Part 2

Part 3 – Electronic Configuration, Valence Electrons & Valency
READ PART 3 →

Part 4 – Isotopes, Isobars & Average Atomic Mass
READ PART 4 →