Class 9 Science Chapter 8 Journey Inside the Atom – NCERT Solutions & Easy Notes Part 3 | PJ Sir

Part 3: Electronic Configuration & Valency Made Easy

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

Part 2 me humne Bohr Model, subatomic particles, atomic number aur mass number padha.

Ab ek important question:

Atom me electrons different shells me arrange kaise hote hain?

Aur kisi atom ki valency kaise pata lagti hai?

Part 3 me hum Electronic Configuration, 2n² Rule, Valence Electrons, Octet aur Valency ko easy way me samjhenge.

1. Distribution of Electrons in Different Energy Levels

Bohr Model ke according electrons nucleus ke around different energy levels or shells me present hote hain.

These shells are represented as:

K, L, M, N...

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

Electrons ko shells me arrange karne ke liye Bohr and Bury ne kuch rules suggest kiye.

2. The 2n² Rule

Kisi shell me maximum kitne electrons aa sakte hain, ise calculate karne ka formula hai:

Maximum Number of Electrons = 2n²

Here:

n = Shell Number

K Shell

For K shell:

n = 1

Maximum electrons:

2 × 1² = 2

Therefore:

K shell → Maximum 2 electrons

L Shell

n = 2

2 × 2² = 8

Therefore:

L shell → Maximum 8 electrons

M Shell

n = 3

2 × 3² = 18

Therefore:

M shell → Maximum 18 electrons

So remember:

K → 2

L → 8

M → 18

PJ Sir Trick

Remember:

2n²

Put the shell number in place of n.

K = 1 → 2

L = 2 → 8

M = 3 → 18

Easy!

3. Important Rule for the Outermost Shell

According to the rules given in the chapter:

The maximum number of electrons that can be accommodated in the outermost shell is 8.

The first shell is an exception because it can contain a maximum of only 2 electrons.

4. Order of Filling of Electrons

Electrons are not filled randomly.

They are filled step-by-step, starting from the shell closest to the nucleus.

Order:

K → L → M → N...

The L shell starts filling after the K shell is complete, and the next shells fill progressively outward.

PJ Sir Trick

Think:

Inside First → Outside Later

So:

K first → L next → M next → N next

5. What Is Electronic Configuration?

The distribution of electrons among the various shells of an atom is called its:

Electronic Configuration

For example:

Hydrogen has atomic number 1.

Therefore, it has one electron.

Its electronic configuration is:

H = 1

Helium has two electrons:

He = 2

Lithium has three electrons.

K shell gets 2 electrons and the remaining 1 goes to L shell.

Therefore:

Li = 2, 1

6. Electronic Configuration of Important Elements

The chapter gives electronic distributions for the first eighteen elements. Some important examples are:

Hydrogen (H), Z = 1

Electronic Configuration:

Helium (He), Z = 2

Electronic Configuration:

Lithium (Li), Z = 3

Electronic Configuration:

2, 1

Carbon (C), Z = 6

Electronic Configuration:

2, 4

Nitrogen (N), Z = 7

Electronic Configuration:

2, 5

Oxygen (O), Z = 8

Electronic Configuration:

2, 6

Fluorine (F), Z = 9

Electronic Configuration:

2, 7

Neon (Ne), Z = 10

Electronic Configuration:

2, 8

Sodium (Na), Z = 11

Electronic Configuration:

2, 8, 1

Magnesium (Mg), Z = 12

Electronic Configuration:

2, 8, 2

Aluminium (Al), Z = 13

Electronic Configuration:

2, 8, 3

Silicon (Si), Z = 14

Electronic Configuration:

2, 8, 4

Phosphorus (P), Z = 15

Electronic Configuration:

2, 8, 5

Sulfur (S), Z = 16

Electronic Configuration:

2, 8, 6

Chlorine (Cl), Z = 17

Electronic Configuration:

2, 8, 7

Argon (Ar), Z = 18

Electronic Configuration:

2, 8, 8

7. What Is the Valence Shell?

The outermost shell containing electrons in an atom is called its:

Valence Shell

Example:

Sodium:

2, 8, 1

Its outermost occupied shell is the third shell.

Therefore, the third shell is its valence shell.

8. What Are Valence Electrons?

The electrons present in the valence shell are called:

Valence Electrons

Example:

Sodium = 2, 8, 1

Valence electrons = 1

Oxygen = 2, 6

Valence electrons = 6

Chlorine = 2, 8, 7

Valence electrons = 7

Argon = 2, 8, 8

Valence electrons = 8

PJ Sir Trick

Electronic configuration ka last number dekho.

Usually wahi aapko valence electrons batayega.

Example:

2, 8, 7 → 7 valence electrons

9. What Is an Octet?

If the outermost shell of an atom has 8 electrons, it is called an:

Octet

The chapter explains that atoms with a complete octet are generally more stable and largely unreactive.

Helium is an important case because its only shell becomes complete with 2 electrons.

So:

Helium → 2 → Stable

Neon → 2, 8 → Stable

Argon → 2, 8, 8 → Stable

10. Why Do Atoms Gain, Lose or Share Electrons?

Atoms having an incomplete valence shell are usually more reactive.

Such atoms may:

Lose electrons

Gain electrons

Share electrons

to achieve a more stable electronic configuration.

This concept becomes extremely important when you later study chemical bonding.

11. What Is Valency?

Valency tells us about the combining capacity of an atom.

The chapter defines valency in terms of the number of electrons that an atom gains, loses or shares to complete its octet.

Therefore:

Valency = Number of electrons gained, lost or shared to achieve a stable configuration

12. Valency When Valence Electrons Are Less Than 4

The chapter explains that generally, when an element has fewer than four electrons in its valence shell, it tends to lose electrons to achieve stability.

Example: Sodium

Electronic Configuration:

2, 8, 1

Valence electrons = 1

It can lose 1 electron.

Therefore:

Valency of Sodium = 1

Example: Magnesium

Electronic Configuration:

2, 8, 2

Valence electrons = 2

Therefore:

Valency of Magnesium = 2

13. Valency When Valence Electrons Are More Than 4

Generally, when an atom has more than four valence electrons, it tends to gain electrons to complete its octet.

Example: Oxygen

Electronic Configuration:

2, 6

Valence electrons = 6

It needs 2 more electrons to complete its octet.

Therefore:

Valency of Oxygen = 2

Example: Chlorine

Electronic Configuration:

2, 8, 7

Valence electrons = 7

It needs 1 electron.

Therefore:

Valency of Chlorine = 1

14. What Happens When There Are 4 Valence Electrons?

Carbon has electronic configuration:

2, 4

It has four valence electrons.

The chapter explains that carbon cannot easily gain or lose all four electrons.

Therefore, it can share four electrons with other atoms to complete its octet.

Hence:

Valency of Carbon = 4

15. PJ Sir Easy Valency Trick

For the examples studied here, use this quick pattern:

If valence electrons are:

1 → Valency 1

2 → Valency 2

3 → Valency 3

4 → Valency 4

For more than 4:

Valency = 8 – Valence Electrons

So:

5 valence electrons:

8 – 5 = 3

6 valence electrons:

8 – 6 = 2

7 valence electrons:

8 – 7 = 1

Complete outer shell:

Valency = 0

16. Important Examples

Sodium

Electronic Configuration = 2, 8, 1

Valence Electrons = 1

Valency = 1

Magnesium

Electronic Configuration = 2, 8, 2

Valence Electrons = 2

Valency = 2

Carbon

Electronic Configuration = 2, 4

Valence Electrons = 4

Valency = 4

Nitrogen

Electronic Configuration = 2, 5

Valence Electrons = 5

Valency = 3

Oxygen

Electronic Configuration = 2, 6

Valence Electrons = 6

Valency = 2

Fluorine

Electronic Configuration = 2, 7

Valence Electrons = 7

Valency = 1

Neon

Electronic Configuration = 2, 8

Complete outer shell.

Valency = 0

Chlorine

Electronic Configuration = 2, 8, 7

Valence Electrons = 7

Valency = 1

Argon

Electronic Configuration = 2, 8, 8

Complete octet.

Valency = 0

17. Why Are Noble Gas-Type Configurations Stable?

The chapter notes that elements with a complete octet—or helium with two electrons in its only shell—are largely unreactive and more stable.

Atoms with incomplete valence shells are generally more reactive and may gain, lose or share electrons to complete their outer shell.

Important Questions & Answers

Q1. What is electronic configuration?

Answer: The distribution of electrons among different shells of an atom is called its electronic configuration.

Q2. Write the formula for the maximum number of electrons in a shell.

Answer:

Maximum electrons = 2n²

where n is the shell number.

Q3. How many electrons can the K shell accommodate?

Answer: Maximum 2 electrons.

Q4. How many electrons can the L shell accommodate?

Answer: Maximum 8 electrons.

Q5. How many electrons can the M shell accommodate according to 2n²?

Answer: Maximum 18 electrons.

Q6. What is a valence shell?

Answer: The outermost shell containing electrons in an atom is called its valence shell.

Q7. What are valence electrons?

Answer: Electrons present in the valence shell are called valence electrons.

Q8. What is an octet?

Answer: A configuration of eight electrons in the outermost shell is called an octet.

Q9. Define valency.

Answer: The number of electrons gained, lost or shared by an atom to achieve a stable configuration is called its valency.

Q10. What is the electronic configuration of sodium?

Answer: 2, 8, 1

Q11. What is the valency of sodium?

Answer: 1

Q12. What is the electronic configuration and valency of oxygen?

Answer: Oxygen has electronic configuration 2, 6 and valency 2.

Q13. What is the electronic configuration and valency of carbon?

Answer: Carbon has electronic configuration 2, 4 and valency 4.

Q14. Why are atoms with a complete outer shell generally stable?

Answer: The chapter notes that atoms having a complete octet, or two electrons in helium's only shell, are largely unreactive and more stable.

30-Second Revision

Electronic Configuration = Distribution of electrons in shells

Maximum electrons in shell = 2n²

K = 2

L = 8

M = 18

Filling order = K → L → M → N

Valence Shell = Outermost occupied shell

Valence Electrons = Electrons in valence shell

8 outer electrons = Octet

Valency = Electrons gained, lost or shared for stability

Na = 2,8,1 → Valency 1

O = 2,6 → Valency 2

C = 2,4 → Valency 4

Cl = 2,8,7 → Valency 1

Ne/Ar → Complete outer shell → Valency 0

Read Previous Parts

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

What Will We Learn in Part 4?

In Part 4 – Isotopes, Isobars & Average Atomic Mass, we will study:

Isotopes

Isotopes of Hydrogen

Isotopes of Carbon

Uses of Isotopes

Average Atomic Mass

Weighted Average Numericals

Isobars

Important Questions

Complete Chapter Revision

Continue with Part 4 → 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
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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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