Introduction

Chemists use a special language to describe substances and chemical reactions. Instead of writing the full names of elements every time, they use chemical formulae to show exactly which elements are present in a substance and how many atoms of each element it contains. Chemical formulae are like mathematical shorthand that allows scientists around the world to communicate clearly and efficiently.

Understanding chemical formulae is an essential skill in chemistry. By learning how to read and write formulae, you can identify the elements in a compound, count the number of atoms present, and prepare for balancing chemical equations and studying chemical reactions.

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What Is a Chemical Formula?

A chemical formula is a combination of chemical symbols and numbers that shows:

• The elements present in a substance.
• The number of atoms of each element.

Examples:

Each formula represents one type of compound.

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Figure 1. Chemical formulae show the elements present in a compound and the number of atoms of each.

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Reading Chemical Formulae

To interpret a chemical formula:

Step 1

Identify the chemical symbols.

These tell you which elements are present.

Step 2

Read the subscripts.

These tell you how many atoms of each element are present.

If there is no subscript, the number of atoms is 1.

Example:

CO₂

Contains:

• 1 carbon atom
• 2 oxygen atoms

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Counting Atoms

Examples:

Remember:

No subscript means one atom.

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Figure 2. Subscripts indicate the number of atoms of each element in a compound.

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What Is a Subscript?

A subscript is the small number written after and below a chemical symbol.

It tells us how many atoms of that element are present.

Examples:

H₂O

• 2 hydrogen atoms
• 1 oxygen atom

CO₂

• 1 carbon atom
• 2 oxygen atoms

C₆H₁₂O₆

Contains:

• 6 carbon atoms
• 12 hydrogen atoms
• 6 oxygen atoms

Subscripts are part of the chemical formula and must not be changed unless the substance itself changes.

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What Is a Coefficient?

A coefficient is a whole number written in front of a chemical formula.

It tells us how many molecules or formula units are present.

Example:

3H₂O

Means:

Three water molecules.

Total atoms:

Hydrogen:

3 × 2 = 6

Oxygen:

3 × 1 = 3

Coefficients multiply every atom in the formula.

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Figure 3. Subscripts count atoms within one molecule, while coefficients count the number of molecules.

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Subscripts vs Coefficients

Understanding this difference is essential when balancing chemical equations.

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Writing Chemical Formulae

Chemists use chemical symbols to write the correct formula for compounds.

Some common examples include:

The order of the elements and the subscripts must always be written correctly.

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Representing Compounds Accurately

A chemical formula gives an exact description of a compound.

For example:

H₂O

always represents water.

Changing the formula changes the substance.

Example:

CO

Carbon monoxide

CO₂

Carbon dioxide

Although both compounds contain carbon and oxygen, they are different substances with different properties.

Accurate chemical formulae are essential for clear scientific communication.

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Figure 4. Even small changes in a chemical formula create a different compound with different properties.

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Why Chemical Formulae Matter

Chemical formulae allow scientists to:

• Describe compounds accurately.
• Count atoms.
• Predict chemical reactions.
• Balance equations.
• Communicate internationally.

Every area of chemistry depends on correctly interpreting and writing chemical formulae.

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Worked Example

Question

Determine the number of atoms of each element in 2CaCl₂.

Solution

One formula unit of CaCl₂ contains:

• 1 calcium atom
• 2 chlorine atoms

The coefficient is 2, so multiply each amount by 2.

Total atoms:

• Calcium: 2 × 1 = 2
• Chlorine: 2 × 2 = 4

Answer: 2 calcium atoms and 4 chlorine atoms.

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Real-World Connection

Chemical formulae appear in many everyday products. Fertiliser bags list compounds such as NH₄NO₃ (ammonium nitrate), food labels may include NaCl (table salt), and medicines often contain active ingredients represented by chemical formulae. Scientists, pharmacists, engineers, and doctors all rely on accurate chemical formulae to communicate safely and precisely.

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Did You Know?

The chemical formula for glucose, C₆H₁₂O₆, contains 24 atoms in a single molecule:

• 6 carbon atoms
• 12 hydrogen atoms
• 6 oxygen atoms

Plants produce millions of glucose molecules every second during photosynthesis, providing the energy that supports nearly all life on Earth.

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Key Terms

Atom – The smallest particle of an element that retains its chemical properties.

Chemical formula – A combination of chemical symbols and numbers showing the types and numbers of atoms in a substance.

Chemical symbol – A one- or two-letter abbreviation representing an element.

Coefficient – A number written before a chemical formula indicating the number of molecules or formula units.

Compound – A pure substance made of two or more different elements chemically bonded together.

Formula unit – The simplest ratio of ions in an ionic compound.

Molecule – A group of atoms chemically bonded together.

Subscript – A small number written after a chemical symbol indicating the number of atoms of that element in one molecule or formula unit.

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Key Takeaways

• A chemical formula shows the types and numbers of atoms in a compound.
• Chemical symbols identify the elements present, while subscripts show how many atoms of each element are present.
• A missing subscript means there is one atom of that element.
• Coefficients count the number of molecules or formula units and multiply every atom in the formula.
• Writing chemical formulae accurately is essential for describing compounds and chemical reactions.
• Understanding chemical formulae is a key foundation for studying chemical bonding, reactions, and equations.