Class 9 Science Chapter 8 Journey Inside the Atom – NCERT Solutions & Easy Notes Part 4 | PJ Sir

Part 4: Isotopes, Isobars & Average Atomic Mass

Part 1 me humne Dalton, Thomson aur Rutherford ke atomic models samjhe.

Part 2 me Bohr Model, electron, proton, neutron, atomic number aur mass number padha.

Part 3 me electronic configuration, 2n² rule, valence electrons, octet aur valency samjhi.

Ab Chapter 8 ke final part me hum padhenge:

Isotopes

Isobars

Average Atomic Mass

Important Numericals

Complete Chapter Revision

1. What Are Isotopes?

Hum jaante hain:

Atomic Number (Z) = Number of Protons

Aur:

Mass Number (A) = Number of Protons + Number of Neutrons

Kabhi-kabhi same element ke atoms me protons ki sankhya same hoti hai, lekin neutrons ki sankhya different hoti hai.

Aise atoms ko Isotopes kehte hain.

Definition of Isotopes

Atoms of the same element having the same atomic number but different mass numbers are called isotopes.

In simple words:

Same Element

Same Atomic Number

Same Number of Protons

but

Different Number of Neutrons

Different Mass Numbers

PJ Sir Trick

Remember:

Isotopes = Same Z, Different A

Z = Atomic Number

A = Mass Number

2. Why Do Isotopes Have the Same Atomic Number?

Atomic number depends upon the number of protons.

Isotopes belong to the same element, so they contain the same number of protons.

Therefore, their atomic number is the same.

But their number of neutrons is different.

Hence, their mass numbers are different.

3. Isotopes of Hydrogen

Hydrogen has three isotopes:

Protium

Deuterium

Tritium

Protium

Representation:

¹₁H

Protons = 1

Neutrons = 0

Mass Number = 1

Deuterium

Representation:

²₁H

Protons = 1

Neutrons = 1

Mass Number = 2

Tritium

Representation:

³₁H

Protons = 1

Neutrons = 2

Mass Number = 3

Notice carefully:

All three have:

Atomic Number = 1

But their mass numbers are:

1, 2 and 3

Therefore, they are isotopes of hydrogen.

4. Isotopes of Carbon

Carbon also occurs in different isotopic forms.

Important examples include:

Carbon-12

Carbon-13

Carbon-14

All carbon isotopes have:

Atomic Number = 6

Therefore:

Number of protons = 6

But their number of neutrons is different.

Carbon-12

Protons = 6

Neutrons = 12 – 6 = 6

Carbon-13

Protons = 6

Neutrons = 13 – 6 = 7

Carbon-14

Protons = 6

Neutrons = 14 – 6 = 8

PJ Sir Trick

For any isotope:

Neutrons = Mass Number – Atomic Number

n = A – Z

5. Chemical Behaviour of Isotopes

Chemical behaviour is strongly related to the electronic configuration of an atom.

Isotopes of the same element have the same atomic number.

Therefore, neutral isotopes have the same number of electrons and the same electronic configuration.

Hence, isotopes of an element generally show similar chemical properties.

However, because their masses are different, some of their physical properties can differ.

6. Uses of Isotopes

Different isotopes have several important applications.

For example, isotopes are used in areas such as:

Medicine

Scientific research

Determining the age of ancient materials

Nuclear applications

One well-known example is Carbon-14, which is used for estimating the age of materials that were once living.

7. Why Is Atomic Mass Sometimes a Decimal?

You may have noticed values such as:

Chlorine ≈ 35.5 u

This can look confusing.

How can atomic mass be a decimal when protons and neutrons occur as whole particles?

The answer lies in isotopes.

Many elements occur naturally as a mixture of different isotopes.

Therefore, the atomic mass shown for an element represents an average atomic mass based on the masses and relative abundance of its naturally occurring isotopes.

8. What Is Average Atomic Mass?

Average atomic mass is calculated as a weighted average of the masses of the naturally occurring isotopes of an element.

This means an isotope that is more abundant contributes more to the average.

General Formula

Average Atomic Mass =

(Mass₁ × Percentage Abundance₁ + Mass₂ × Percentage Abundance₂ + ...) ÷ 100

PJ Sir Trick

Don't take a simple average unless the isotopes are present in equal amounts.

Remember:

Mass × Abundance → Add → Divide by 100

9. Example of Average Atomic Mass

Suppose an element has two isotopes:

First isotope:

Mass = 10 u

Abundance = 20%

Second isotope:

Mass = 11 u

Abundance = 80%

Average Atomic Mass:

[(10 × 20) + (11 × 80)] ÷ 100

= (200 + 880) ÷ 100

= 1080 ÷ 100

= 10.8 u

Therefore:

Average Atomic Mass = 10.8 u

This shows why average atomic mass need not be a whole number.

10. Average Atomic Mass of Chlorine

Chlorine occurs naturally as a mixture of isotopes.

Because the isotopes occur in different proportions, the average atomic mass of naturally occurring chlorine comes close to:

35.5 u

The important concept here is:

Atomic mass shown for an element can represent the weighted average of its naturally occurring isotopes.

11. What Are Isobars?

Now consider atoms of different elements.

Sometimes two different elements may have the same mass number, even though their atomic numbers are different.

Such atoms are called:

Isobars

Definition of Isobars

Atoms of different elements having the same mass number but different atomic numbers are called isobars.

PJ Sir Trick

Isobars = Same A, Different Z

Compare:

Isotopes → Same Z, Different A

Isobars → Same A, Different Z

This is one of the easiest ways to remember the difference.

12. Example of Isobars

Consider:

⁴⁰₁₈Ar

and

⁴⁰₂₀Ca

For Argon:

Mass Number = 40

Atomic Number = 18

For Calcium:

Mass Number = 40

Atomic Number = 20

Both have:

Same Mass Number = 40

But:

Different Atomic Numbers

Therefore, they are isobars.

13. Isotopes vs Isobars

Isotopes

Same element

Same atomic number

Same number of protons

Different number of neutrons

Different mass number

Example:

¹₁H, ²₁H, ³₁H

Isobars

Different elements

Different atomic numbers

Different numbers of protons

Same mass number

Example:

⁴⁰₁₈Ar and ⁴⁰₂₀Ca

PJ Sir Super Trick

Just remember two lines:

ISO-TOPES → Same Z

ISO-BARS → Same A

14. Solved Numerical 1

An atom has:

Atomic Number = 17

Mass Number = 35

Find the number of protons, electrons and neutrons.

Solution

Atomic Number:

Z = 17

Therefore:

Protons = 17

For a neutral atom:

Electrons = 17

Neutrons:

n = A – Z

= 35 – 17

= 18

Answer

Protons = 17

Electrons = 17

Neutrons = 18

15. Solved Numerical 2

Two atoms have:

Atom X: Z = 6, A = 12

Atom Y: Z = 6, A = 14

Are they isotopes or isobars?

Solution

Both have:

Atomic Number = 6

But mass numbers are different:

12 and 14

Therefore:

X and Y are isotopes.

16. Solved Numerical 3

Two atoms have:

Atom X: Z = 18, A = 40

Atom Y: Z = 20, A = 40

Identify their relationship.

Solution

Mass Number is the same:

A = 40

Atomic Numbers are different:

18 and 20

Therefore:

They are isobars.

Important Questions & Answers

Q1. What are isotopes?

Answer: Atoms of the same element having the same atomic number but different mass numbers are called isotopes.

Q2. Why do isotopes have different mass numbers?

Answer: Isotopes have different numbers of neutrons, which causes their mass numbers to differ.

Q3. Name the three isotopes of hydrogen.

Answer: Protium, Deuterium and Tritium.

Q4. How many neutrons are present in deuterium?

Answer: Deuterium has 1 neutron.

Q5. How many neutrons are present in tritium?

Answer: Tritium has 2 neutrons.

Q6. Why do isotopes generally have similar chemical properties?

Answer: Isotopes of the same element have the same atomic number and therefore the same electronic configuration in their neutral atoms, resulting in similar chemical behaviour.

Q7. What is average atomic mass?

Answer: It is the weighted average of the masses of the naturally occurring isotopes of an element according to their relative abundance.

Q8. Why can average atomic mass be a decimal?

Answer: Because many elements occur as mixtures of isotopes with different masses and relative abundances.

Q9. What are isobars?

Answer: Atoms of different elements having the same mass number but different atomic numbers are called isobars.

Q10. Give an example of isobars.

Answer: ⁴⁰₁₈Ar and ⁴⁰₂₀Ca

Q11. What is the main difference between isotopes and isobars?

Answer: Isotopes have the same atomic number but different mass numbers, while isobars have the same mass number but different atomic numbers.

Q12. Write the formula for calculating neutrons.

Answer:

n = A – Z

30-Second Revision

Isotopes → Same Z, Different A

Isobars → Same A, Different Z

Hydrogen Isotopes → Protium, Deuterium, Tritium

Carbon Isotopes → Carbon-12, Carbon-13, Carbon-14

Neutrons → n = A – Z

Average Atomic Mass → Weighted Average of Isotopic Masses

More abundant isotope → Greater contribution to average mass

⁴⁰Ar & ⁴⁰Ca → Isobars

Complete Chapter 8 Revision Map

Now connect all four parts:

Dalton → Atomic Theory

↓

J. J. Thomson → Electron

↓

Thomson Model → Positive sphere + electrons

↓

Rutherford → Nucleus + mostly empty space

↓

Bohr → Fixed Energy Levels

↓

Electron, Proton & Neutron

↓

Atomic Number → Z = p

↓

Mass Number → A = p + n

↓

Electronic Configuration

↓

2n² Rule

↓

Valence Electrons

↓

Valency

↓

Isotopes

↓

Average Atomic Mass

↓

Isobars

This completes our journey inside the atom.

Read All Parts of Chapter 8

Part 1 – Journey of Atomic Models: Dalton to Rutherford
https://pjsirkota.blogspot.com/2026/09/class-9-journey-inside-atom-part-1.html

Part 2 – Bohr Model, Subatomic Particles, Atomic Number & Mass Number
https://pjsirkota.blogspot.com/2026/09/class-9-journey-inside-atom-part-2.html

Part 3 – Electronic Configuration & Valency Made Easy
https://pjsirkota.blogspot.com/2026/09/class-9-journey-inside-atom-part-3.html

Part 4 – Isotopes, Isobars & Average Atomic Mass
You are reading Part 4.

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
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Part 2 – Bohr Model, Subatomic Particles, Atomic Number & Mass Number
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Part 3 – Electronic Configuration, Valence Electrons & Valency
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Part 4 – Isotopes, Isobars & Average Atomic Mass
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