Class 9 Science Chapter 9 Atomic Foundations of Matter – NCERT Solutions & Easy Notes Part 3 | PJ Sir

Exploration – Grade 9 Science | Chapter 9

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

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

In Part 2, we understood how atoms combine through covalent and ionic bonds.

Now we will learn another very important skill:

How do we write the correct chemical formula of a compound?

For example:

Why is magnesium chloride written as MgCl₂ and not MgCl?

Why is aluminium oxide Al₂O₃?

Why do we use brackets in Ca(OH)₂?

Once you understand ions, valency and the criss-cross method, writing chemical formulae becomes much easier.

1. What is an Ion?

When an atom loses or gains electron(s), it can form a charged species called an ion.

There are two important types:

Cation → Positively charged ion

Examples:
Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺

Anion → Negatively charged ion

Examples:
Cl⁻, O²⁻, S²⁻

PJ Sir Trick

Remember:

Cation = Positive

Anion = Negative

2. What is Valency?

For writing chemical formulae, we need to know the valencies of the combining elements or ions.

Some common ions and their valencies given in the chapter are:

Valency 1

Na⁺ → Sodium
Li⁺ → Lithium
K⁺ → Potassium
Ag⁺ → Silver
Cl⁻ → Chloride
F⁻ → Fluoride
Br⁻ → Bromide
I⁻ → Iodide

Valency 2

Ca²⁺ → Calcium
Ba²⁺ → Barium
Mg²⁺ → Magnesium
Zn²⁺ → Zinc
O²⁻ → Oxide
S²⁻ → Sulfide

Valency 3

Al³⁺ → Aluminium

The chapter also lists ions such as Fe²⁺, Fe³⁺, Cu⁺ and Cu²⁺. Pasted markdown

PJ Sir Tip

Before writing a chemical formula, always ask:

“What is the valency of each combining species?”

If the valencies are correct, writing the formula becomes much easier.

3. What are Polyatomic Ions?

Some ions are formed by a group of atoms of two or more elements.

These are called polyatomic ions.

Important examples from the chapter are:

OH⁻ → Hydroxide → Valency 1

NO₃⁻ → Nitrate → Valency 1

HCO₃⁻ → Hydrogencarbonate → Valency 1

CO₃²⁻ → Carbonate → Valency 2

SO₄²⁻ → Sulfate → Valency 2

NH₄⁺ → Ammonium → Valency 1 chapter 2 acid base n salt

These ions are very important because we will use them while writing chemical formulae.

4. Naming Ionic Compounds

While naming an ionic compound:

First write the name of the cation.

Then write the name of the anion.

For simple anions, the name generally ends in -ide.

Examples:

NaCl → Sodium chloride

CaO → Calcium oxide

MgS → Magnesium sulfide

Ionic compounds are typically formed when metals combine with non-metals.

Polyatomic ion names generally do not end in -ide. Pasted markdown

5. How to Write Chemical Formulae?

Now comes the most important part.

The chapter gives a quick method using valencies.

For a covalent compound:

Step 1: Write the symbols of the constituent elements.

Step 2: Write their valencies.

Step 3: Cross over the valencies and write them as subscripts.

This is commonly called the criss-cross method. chapter 2 acid base n salt

6. Example: Hydrogen Chloride

Hydrogen = H

Chlorine = Cl

Valency:

H = 1

Cl = 1

Criss-cross:

H₁Cl₁

When the subscript is 1, we do not write it.

Therefore:

Formula = HCl

7. Example: Hydrogen Sulfide

Hydrogen = H

Sulfur = S

Valency:

H = 1

S = 2

Criss-cross the valencies:

H₂S₁

We do not write 1.

Therefore:

Formula = H₂S

8. Example: Carbon Tetrachloride

Carbon = C

Chlorine = Cl

Valency:

C = 4

Cl = 1

After criss-crossing:

CCl₄

Therefore:

Formula = CCl₄

These examples follow the formula-writing method given in the chapter. chapter 2 acid base n salt

9. PJ Sir Criss-Cross Trick

Suppose we have:

A with valency x

and

B with valency y

Cross the valencies:

AᵧBₓ

Simple rule:

“Valency dekho → Cross karo → Subscript banao.”

But remember:

If the final subscript is 1, we normally do not write it.

10. Writing Formulae of Ionic Compounds

For an ionic compound, write:

Cation first → Anion second

Then use their valencies to obtain the formula.

Let us understand through simple examples.

11. Sodium Chloride

Sodium ion = Na⁺

Chloride ion = Cl⁻

Valencies:

Na = 1

Cl = 1

Criss-cross:

Na₁Cl₁

Therefore:

NaCl

12. Magnesium Chloride

Magnesium ion = Mg²⁺

Chloride ion = Cl⁻

Valencies:

Mg = 2

Cl = 1

Criss-cross:

Mg₁Cl₂

Therefore:

MgCl₂

This also helps us understand why one Mg²⁺ ion combines with two Cl⁻ ions.

13. Aluminium Oxide

Aluminium ion = Al³⁺

Oxide ion = O²⁻

Valencies:

Al = 3

O = 2

Criss-cross:

Al₂O₃

Therefore:

Formula = Al₂O₃

PJ Sir Tip

Do not write the charge signs inside the final chemical formula.

We use the valencies to determine the required number of ions/atoms.

14. Formulae Containing Polyatomic Ions

Now comes a very important point.

Suppose a compound contains a polyatomic ion and more than one such ion is required.

Then the polyatomic ion is written inside brackets.

For example:

Calcium ion = Ca²⁺

Hydroxide ion = OH⁻

Valencies:

Ca = 2

OH = 1

Criss-cross gives:

Ca(OH)₂

Notice the brackets around OH.

The subscript 2 applies to the complete hydroxide ion. The chapter specifically uses brackets when more than one polyatomic ion is required in a formula. chapter 2 acid base n salt

15. Why are Brackets Important?

Compare:

Ca(OH)₂

Here there are:

1 Calcium atom

2 Oxygen atoms

2 Hydrogen atoms

The 2 outside the bracket applies to the entire OH group.

PJ Sir Trick

More than one polyatomic ion required?

Think:

“Group ko bracket mein rakho.”

16. One More Important Point – Simplest Ratio

For ionic compounds, the chemical formula represents the simplest whole-number ratio of the ions present in the compound. chapter 2 acid base n salt

So after criss-crossing, always check whether the numbers can be reduced to their simplest ratio.

Quick Formula Practice

Sodium chloride

Na⁺ + Cl⁻

Formula = NaCl

Magnesium chloride

Mg²⁺ + Cl⁻

Formula = MgCl₂

Aluminium oxide

Al³⁺ + O²⁻

Formula = Al₂O₃

Calcium hydroxide

Ca²⁺ + OH⁻

Formula = Ca(OH)₂

30-Second Revision

Remember these points:

1. Positively charged ion = Cation

2. Negatively charged ion = Anion

3. Some ions contain more than one atom and are called polyatomic ions.

4. Write the cation before the anion in an ionic compound.

5. First identify the valencies.

6. Cross the valencies to obtain subscripts.

7. Subscript 1 is normally not written.

8. Use brackets when more than one polyatomic ion is required.

9. Ionic compound formula represents the simplest whole-number ratio of ions.

Check Yourself – Important Questions

Q1. What is a cation?

A positively charged ion is called a cation.

Q2. What is an anion?

A negatively charged ion is called an anion.

Q3. What is a polyatomic ion?

An ion formed from a group of atoms of two or more elements is called a polyatomic ion.

Q4. Write the formula of hydroxide ion.

OH⁻

Q5. Write the formula of sulfate ion.

SO₄²⁻

Q6. Write the formula of ammonium ion.

NH₄⁺

Q7. Write the chemical formula of magnesium chloride.

Mg²⁺ and Cl⁻ combine in the required ratio.

Formula = MgCl₂

Q8. Write the chemical formula of aluminium oxide.

Al³⁺ and O²⁻ combine to give:

Formula = Al₂O₃

Q9. Why are brackets used in Ca(OH)₂?

Because two complete hydroxide ions are required. The subscript 2 applies to the entire OH group.

Q10. What does the chemical formula of an ionic compound represent?

It represents the simplest whole-number ratio of the ions present in the compound.

Message from PJ Sir

Do not try to memorise every chemical formula separately.

First understand:

Ion → Valency → Criss-Cross → Simplify → Correct Formula

Once this sequence becomes clear, writing chemical formulae becomes much easier.

Next Part

In Part 4 – Properties, Numericals & NCERT Solutions, we will complete Chapter 9 with:

Properties of Ionic & Covalent Compounds

Electrical Conductivity

Solubility

Melting & Boiling Points

Molecular Mass

Formula Unit Mass

Important Numericals

Important NCERT Questions & Answers

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
PART 2

Part 3 – Chemical Formulae Made Easy: Ions, Valency & Criss-Cross Method
You are reading Part 3.

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