Class 9 Science Chapter 9 Atomic Foundations of Matter – NCERT Solutions & Easy Notes Part 2 | PJ Sir

Exploration – Grade 9 Science | Chapter 9

Part 2: Chemical Bonding Made Easy – Covalent & Ionic Bonds

By Priyank Joshi (PJ Sir)
Chemistry Educator & Mentor | 18+ Years of Teaching Experience

In Part 1, we understood the Law of Conservation of Mass, Law of Constant Proportions and Dalton’s Atomic Theory.

Now comes one of the most interesting parts of Chemistry:

Why do atoms combine?
What is a chemical bond?
Why does hydrogen form H₂ while oxygen forms O₂?
How is common salt NaCl formed?

Let us understand all of this step by step.

1. What is a Molecule?

Atoms of the same element can combine to form a molecule of that element.

For example:

H + H → H₂

Two hydrogen atoms combine to form one hydrogen molecule.

Atoms of different elements can also combine to form molecules of compounds.

For example:

H + Cl → HCl

A molecule is an electrically neutral entity consisting of more than one atom that can exist independently and shows the properties of that substance. Pasted markdown

2. Why Do Atoms Combine?

Atoms generally combine to achieve a stable electronic configuration.

An atom having 8 electrons in its outermost shell is generally stable.

If the K-shell is the outermost shell, 2 electrons make it stable.

Atoms can achieve stability mainly by:

1. Sharing electrons

or

2. Transfer of electrons

The force that holds atoms together is called a chemical bond. Pasted markdown

PJ Sir Trick

Remember:

SHARE → Covalent Bond

TRANSFER → Ionic Bond

Just remember these two words and the basic difference becomes easy.

3. Covalent Bond – Sharing of Electrons

When atoms combine by sharing electrons, the bond formed between them is called a covalent bond. Pasted markdown

Let us understand this with examples.

4. Formation of Hydrogen Molecule – H₂

Atomic number of hydrogen = 1

Hydrogen has only 1 electron in its K-shell.

But the K-shell becomes stable with 2 electrons.

Therefore, each hydrogen atom needs one more electron.

Two hydrogen atoms share one electron each.

H + H → H₂

The shared pair of electrons holds the two hydrogen atoms together.

It is represented as:

H—H

Since one pair of electrons is shared, it forms a single covalent bond. Pasted markdown

PJ Sir Trick

1 shared pair = Single Bond

5. Formation of Chlorine Molecule – Cl₂

A chlorine atom has 7 electrons in its valence shell.

It needs 1 more electron to achieve a stable electronic configuration.

Therefore, two chlorine atoms share one electron each.

Cl + Cl → Cl₂

It can be represented as:

Cl—Cl

Since one pair of electrons is shared, Cl₂ contains a single covalent bond. Pasted markdown

6. Formation of Oxygen Molecule – O₂

An oxygen atom has 6 electrons in its valence shell.

It needs 2 more electrons to complete its octet.

Therefore, two oxygen atoms share two electrons each.

Two pairs of electrons are shared.

It is represented as:

O=O

Therefore, O₂ contains a double bond. Pasted markdown

PJ Sir Trick

1 shared pair → Single Bond

2 shared pairs → Double Bond

7. Formation of Hydrogen Chloride – HCl

Now let us combine atoms of two different elements.

Hydrogen needs 1 electron to complete its duplet.

Chlorine needs 1 electron to complete its octet.

So hydrogen and chlorine share one electron each.

H + Cl → HCl

It is represented as:

H—Cl

Because one pair of electrons is shared, HCl contains a single covalent bond. Pasted markdown

8. Formation of Water – H₂O

Oxygen needs 2 electrons to achieve a stable electronic configuration.

Each hydrogen atom needs 1 electron.

Therefore, two hydrogen atoms share electrons with one oxygen atom.

This produces:

H₂O

One molecule of water contains:

2 Hydrogen atoms + 1 Oxygen atom

This is why its chemical formula is H₂O. Pasted markdown

9. Naming Covalent Compounds

NCERT also introduces a prefix system for naming covalent compounds.

Some useful prefixes are:

Mono = 1
Di = 2
Tri = 3
Tetra = 4
Penta = 5
Hexa = 6

Examples:

CO → Carbon monoxide

CO₂ → Carbon dioxide

CS₂ → Carbon disulfide

PCl₃ → Phosphorus trichloride

SF₆ → Sulfur hexafluoride

The first element keeps its regular name, while the second element generally ends in -ide. NCERT also notes some naming exceptions and common names, such as H₂O being commonly known as water and NH₃ as ammonia. Pasted markdown

10. Ionic Bond – Transfer of Electrons

Now comes the second important type of bonding.

In a covalent bond, electrons are shared.

In an ionic bond, electrons are transferred.

Let us understand it using sodium chloride (NaCl) — common salt.

11. Formation of Sodium Ion – Na⁺

Atomic number of sodium = 11

Sodium has 1 electron in its valence shell.

Sodium can lose this electron to achieve a stable electronic configuration.

When sodium loses one electron:

Na → Na⁺ + e⁻

After losing the electron, sodium becomes positively charged.

This positively charged ion is called a sodium cation (Na⁺). Pasted markdown

PJ Sir Trick

Loss of electron → Positive ion → Cation

12. Formation of Chloride Ion – Cl⁻

Chlorine has 7 valence electrons.

It needs one more electron to achieve a stable electronic configuration.

Therefore, chlorine accepts one electron.

Cl + e⁻ → Cl⁻

It becomes negatively charged and is called a chloride anion (Cl⁻). Pasted markdown

PJ Sir Trick

Gain of electron → Negative ion → Anion

13. Formation of Sodium Chloride – NaCl

Now we have:

Na⁺ = Positive ion

Cl⁻ = Negative ion

Opposite charges attract each other.

The electrostatic force of attraction between these oppositely charged ions holds them together.

This attraction is called an ionic bond.

So:

Na⁺ + Cl⁻ → NaCl

Thus, sodium chloride is formed by the transfer of an electron from sodium to chlorine. Pasted markdown

14. What Are Ions?

A charged species formed when an atom loses or gains electron(s) is called an ion.

There are two important types:

Cation → Positively charged ion

Example: Na⁺

Anion → Negatively charged ion

Example: Cl⁻

Cations and anions are collectively called ions. Pasted markdown

15. Does NaCl Exist as a Single Molecule?

This is an important concept.

Ionic compounds generally do not remain as individual single units.

In sodium chloride, Na⁺ and Cl⁻ ions form a three-dimensional crystal structure with a regular repeating arrangement.

NCERT explains that each Na⁺ ion in the NaCl crystal is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions. Pasted markdown

For Class 9, remember:

Covalent compounds → sharing of electrons

Ionic compounds → transfer of electrons + attraction between oppositely charged ions

Covalent Bond vs Ionic Bond – Quick Comparison

Covalent Bond

Electrons are shared.

Examples: H₂, Cl₂, O₂, HCl, H₂O

Ionic Bond

Electrons are transferred.

Positive and negative ions are formed.

Example: NaCl

30-Second Revision

Before moving ahead, remember these points:

1. Atoms combine to become more stable.

2. The force holding atoms together is called a chemical bond.

3. Sharing of electrons forms a covalent bond.

4. H—H and Cl—Cl contain single covalent bonds.

5. O=O contains a double bond.

6. HCl and H₂O are covalent compounds.

7. Transfer of electrons can produce ions.

8. A positive ion is called a cation.

9. A negative ion is called an anion.

10. NaCl is formed through ionic bonding.

Check Yourself – Important Questions

Q1. What is a chemical bond?

A chemical bond is the force that holds atoms together.

Q2. What is a covalent bond?

A covalent bond is formed by the sharing of electrons between atoms.

Q3. Why does hydrogen form H₂?

Each hydrogen atom has one electron and needs one more to make its K-shell stable. Two hydrogen atoms therefore share electrons and form H₂.

Q4. Why is the bond in O₂ called a double bond?

Two oxygen atoms share two pairs of electrons. Therefore, they are connected by a double bond, represented as O=O.

Q5. What is an ionic bond?

An ionic bond is the electrostatic force of attraction between oppositely charged ions formed after electron transfer.

Q6. What is the difference between a cation and an anion?

A cation is positively charged, while an anion is negatively charged.

Q7. How is NaCl formed?

Sodium loses one electron to form Na⁺, while chlorine gains that electron to form Cl⁻. The attraction between Na⁺ and Cl⁻ forms an ionic bond and produces sodium chloride.

Q8. Which type of bond is present in HCl?

A single covalent bond.

Q9. Which type of bond is present in O₂?

A double covalent bond.

Q10. Why does neon normally neither gain, lose nor share electrons?

Its outermost shell already has a stable electronic configuration.

Message from PJ Sir

Chemical bonding becomes easy when you stop memorising compounds and start asking one question:

“Are the electrons being SHARED or TRANSFERRED?”

Once this is clear, the basic difference between covalent and ionic bonding becomes much easier to understand.

Next Part

In Part 3 – Chemical Formulae Made Easy, we will learn:

Common Ions
Valency
Polyatomic Ions
Writing Chemical Formulae
Criss-Cross Method
Covalent Compound Formulae
Ionic Compound Formulae

Reference: NCERT Exploration – Grade 9 Science, Chapter 9: Atomic Foundations of Matter.

This page contains independently written explanations, examples and learning support prepared by Priyank Joshi (PJ Sir) for students.

Complete Chapter 9 – Atomic Foundations of Matter

Part 1 – Laws of Chemical Combination & Dalton’s Atomic Theory
PART 1

Part 2 – Chemical Bonding Made Easy: Covalent & Ionic Bonds
You are reading Part 2.

Part 3 – Chemical Formulae Made Easy: Ions, Valency & Criss-Cross Method
PART 3

Part 4 – Properties, Molecular Mass & Formula Unit Mass
PART 4