Introduction

Imagine trying to build a LEGO model using only the number of bricks. Knowing that there are 20 bricks tells you very little about what the finished model looks like. Chemistry is similar. Simply knowing the number of atoms in a molecule is useful, but it doesn't tell us how those atoms are connected.

Chemists use several different ways to represent molecules, depending on the information they want to communicate. Some formulas simply tell us the number of each type of atom, while others show how atoms are joined together or even display every individual chemical bond. Learning to read and draw these different formulae is one of the most important skills in organic chemistry.

Different Ways to Represent Molecules

The same molecule can be represented in several different ways.

Each representation provides different information.

 

 

1. Molecular Formula

A molecular formula tells us:

• which elements are present
• how many atoms of each element are in one molecule

It does not show how the atoms are connected.

Examples

Notice that ethanol (C₂H₆O) and another compound called dimethyl ether also have the molecular formula C₂H₆O, even though they are different substances. This shows that a molecular formula alone cannot always identify a compound.

Worked Example 1

Write the molecular formula for a molecule containing:

• 3 carbon atoms
• 8 hydrogen atoms

Answer

Carbon first:

C₃

Hydrogen second:

H₈

Final answer:

C₃H₈

2. Structural Formula

A structural formula shows how the atoms are connected without drawing every bond.

Instead of drawing each line representing a bond, atoms are written in the order they are joined.

Examples

Notice how each CH₃ or CH₂ group is connected to the next one.

This makes structural formulas much quicker to write while still showing the overall arrangement of atoms.

Worked Example 2

Write the structural formula for propane.

We know propane contains three carbon atoms connected in a chain.

The two end carbons each have three hydrogens.

The middle carbon has two hydrogens.

Therefore:

CH₃CH₂CH₃

 

 

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3. Displayed Formula

A displayed formula shows:

• every atom
• every covalent bond

Each line represents one covalent bond.

Displayed formulas provide the greatest amount of structural information.

For example, methane can be drawn as:

     H
     |
H — C — H
     |
     H

Ethane:

H   H
|   |
H-C-C-H
|   |
H   H

Displayed formulas make it much easier to understand:

• molecular shape
• bonding
• functional groups
• reactions

 

 

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Comparing the Three Formula Types

Let's compare propane.

Molecular Formula

C₃H₈

Only tells us:

• 3 carbon atoms
• 8 hydrogen atoms

Structural Formula

CH₃CH₂CH₃

Also tells us:

• carbon atoms form a chain

Displayed Formula

Shows:

• every atom
• every single bond
• exact atom connectivity

Each representation gives progressively more information.

Converting Between Formula Types

Chemists often convert between these representations.

Example

Given:

Structural formula

CH₃CH₂OH

Step 1

Count the atoms.

Carbon:

2

Hydrogen:

3 + 2 + 1 = 6

Oxygen:

1

Therefore:

Molecular formula:

C₂H₆O

Example

Given:

Molecular formula

C₂H₆

Draw:

Structural formula

CH₃CH₃

Then convert to a displayed formula by drawing every carbon–hydrogen and carbon–carbon bond.

Why Different Formulae Are Useful

Different chemists use different representations depending on the situation.

Professional chemists often use structural formulas because they are compact while still providing important structural information.

Real-World Connection

Every medicine, fuel, plastic, flavouring, perfume, and vitamin has a chemical formula.

Pharmaceutical chemists often begin by examining displayed formulas to understand how a drug molecule may interact with proteins in the human body. Engineers designing new plastics frequently use structural formulas to compare different polymers, while molecular formulas are useful for calculating the amounts of reactants needed during manufacturing.

Did You Know?

Carbon compounds can often have the same molecular formula but completely different structures and properties. These compounds are called isomers. For example, ethanol and dimethyl ether both have the molecular formula C₂H₆O, yet one is a liquid used in beverages and fuel, while the other is a gas used as a propellant. You will explore isomers later in this course.

Key Terms

• Molecular formula — shows the number of each type of atom in a molecule.
• Structural formula — shows how atoms are connected.
• Displayed formula — shows every atom and every covalent bond.
• Connectivity — the way atoms are joined together within a molecule.
• Covalent bond — a bond formed when atoms share electrons.

Key Takeaways

• Molecular formulas show only the numbers of each type of atom.
• Structural formulas show how atoms are connected.
• Displayed formulas show every atom and every covalent bond.
• The same compound can be represented using different types of formulae.
• Chemists convert between molecular, structural, and displayed formulae depending on the information needed.
• Understanding these representations is a fundamental skill for studying all of organic chemistry.