Chemical Formulae: Understanding Composition of Compounds For Class 7

Have you ever seen formulas like H₂O or CO₂ and wondered what they mean? These are called chemical formulae, and they help us understand what a substance is made of. Instead of writing long names, scientists use these short forms to represent compounds clearly and quickly.

This page aims to help students understand what chemical formulae are, how to write them step by step, and how they are used in everyday chemistry. 

By learning this, students can easily read and write chemical equations with confidence.

Table of Contents 

• What are Chemical Formulae
• Writing a Chemical Formula

 What are Chemical Formulae?

A chemical formula is a notation made up of chemical symbols of the elements present in a compound and numerical subscripts that indicate the number of atoms of each element present in a molecule of the compound.

A chemical formula provides information about the atomic composition of a substance. For example, one molecule of carbon dioxide contains one atom of carbon and two atoms of oxygen. So, the chemical formula of carbon dioxide is CO₂. The number 2 after O indicates that there are two atoms of carbon in one molecule of carbon dioxide.

Similarly, common salt (sodium chloride) is made up of sodium and chlorine. One molecule of common salt contains 1 atom of sodium (Na) and 1 atom of chlorine (Cl). So, the formula of common salt is NaCl.

Thus, we get the following information from the chemical formula of a compound:

1. It denotes various elements present in the compound.
2. It also indicates the number of atoms of each kind of element present in a molecule.

Writing a Chemical Formula

By following the steps given below, we can write the chemical formula of a compound.

1. Write the symbols of the elements that form the compound.
2. Write their valencies on top of their symbols as superscripts.
3. If the valencies are identical, cancel the valencies. If the valencies are different, then interchange the valency.

Example 1: Formula for the compound water

Steps to writing a chemical formula of water using the criss-cross method

Step 1
Write the symbols of the elements present in the compound.
H O

Step 2
Write the valencies of the constituent elements below the symbols.

H O
1  2

Step 3
Interchange the valency and write them as subscripts.
H₁ O₂

Step 4
Cancel down ratio if necessary, otherwise retain them as they are.
H₂O (this is the formula for a water molecule)

Let's discuss the Formula for the compound Magnesium oxide

Step 1
Write the symbols of the elements present in the compound.
Mg O

Step 2
Write the valencies of the constituent elements below the symbols.
Mg  O
2      2

Step 3
Interchange the valency and write them as subscripts.
Mg₂ O₂

Step 4
Cancel out the ratio (Dividing by 2).
MgO is the formula for magnesium oxide.

We have learned that chemical formulae are essential tools in chemistry that help represent compounds simply and clearly. They provide important information about the elements and their quantities in a substance. By understanding how to write chemical formulae, students can easily solve problems and build a strong foundation in chemistry.