Structural Formulae

Introduction: how chemists show structure 🧪

students, in chemistry it is not enough to know what atoms are present in a substance. We also need to know how those atoms are connected. That is the purpose of structural formulae. A structural formula shows the arrangement of atoms in a molecule and helps chemists understand how substances behave, react, and are classified.

In IB Chemistry HL, structural formulae connect directly to Structure 3 — Classification of Matter because they help us separate elements, compounds, and mixtures, and they are especially important in organic chemistry, where the same atoms can make very different substances depending on their arrangement.

Learning goals

By the end of this lesson, students, you should be able to:

explain what structural formulae are and why chemists use them
identify common ways structural formulae are written
apply structural formulae to classify substances and recognize functional groups
connect structural formulae to broader patterns in chemistry, including periodicity and organic structure
use examples to interpret and compare compounds accurately

Think of it like this: a molecular formula tells you the ingredients, but a structural formula tells you the recipe layout 🍳

What is a structural formula?

A structural formula shows the connectivity of atoms in a molecule. In other words, it tells you which atoms are bonded to which. This is more informative than a molecular formula because many compounds can have the same molecular formula but different structures.

For example, the molecular formula $\mathrm{C_2H_6O}$ could represent either ethanol or dimethyl ether. Both have the same numbers of atoms, but their structural formulae are different, so their properties are different too.

Structural formulae are useful because they help chemists predict:

shape and bonding
boiling point and solubility
reactivity
whether a molecule has a particular functional group

A common point in IB Chemistry HL is that structure affects properties. That is a major pattern across the topic of classification of matter.

Structural formula vs molecular formula

A molecular formula gives the actual number of each type of atom in one molecule, such as $\mathrm{C_2H_6O}$.

A structural formula shows how those atoms are arranged, such as $\mathrm{CH_3CH_2OH}$ for ethanol.

This difference matters because molecules with the same molecular formula can be isomers. Isomers are compounds with the same molecular formula but different structural formulae.

For example:

ethanol: $\mathrm{CH_3CH_2OH}$
dimethyl ether: $\mathrm{CH_3OCH_3}$

Both are $\mathrm{C_2H_6O}$, but ethanol can form hydrogen bonds strongly because it has an $\mathrm{-OH}$ group, while dimethyl ether cannot hydrogen-bond to itself in the same way. That means their physical properties are different.

Common ways to write structural formulae

Chemists use several forms of structural representation depending on how much detail is needed.

1. Displayed formula

A displayed formula shows every atom and every covalent bond. It is the most detailed common form used at school level. For example, it can show all the bonds in methane, $\mathrm{CH_4}$, with carbon in the center and four single bonds to hydrogen.

Displayed formulae are helpful when you need to see:

valency
bond types
connectivity clearly

They can become long and crowded for larger molecules, which is why chemists often use shorter forms.

2. Structural formula

A structural formula is a condensed way of writing the bonding arrangement. Instead of drawing every bond, groups are written together.

Examples:

ethane: $\mathrm{CH_3CH_3}$
propanol: $\mathrm{CH_3CH_2CH_2OH}$
ethanoic acid: $\mathrm{CH_3COOH}$

This format is very common in IB questions because it is quick but still gives enough information to identify the compound.

3. Skeletal formula

A skeletal formula is often used for organic molecules with carbon chains or rings. Carbon atoms are shown as line ends or corners, and most hydrogen atoms attached to carbon are omitted. Heteroatoms such as $\mathrm{O}$, $\mathrm{N}$, and halogens are shown.

Skeletal formulae are extremely useful in organic chemistry because they make long carbon chains easier to read.

For example, a six-carbon chain can be drawn quickly as a zigzag line instead of writing every $\mathrm{C}$ and $\mathrm{H}$.

Structural formulae and classification of matter

Structural formulae are not just about drawing molecules neatly. They help classify substances in chemistry.

Elements and compounds

An element contains only one type of atom. A compound contains two or more different elements chemically bonded together.

Structural formulae help identify compounds because they show more than just the atoms present. For example:

$\mathrm{O_2}$ is an element, and its structural formula is $\mathrm{O=O}$
$\mathrm{H_2O}$ is a compound, and its structural formula shows two $\mathrm{O-H}$ bonds

This connection matters in Structure 3 because classification is based on composition and bonding.

Organic and inorganic compounds

Structural formulae are especially important in organic chemistry, which usually involves carbon compounds containing covalent bonds. They help identify carbon chains, branches, double bonds, and functional groups.

For example:

ethanol: $\mathrm{CH_3CH_2OH}$
propene: $\mathrm{CH_2=CHCH_3}$
ethanoic acid: $\mathrm{CH_3COOH}$

These structural formulae immediately show whether a molecule is an alcohol, alkene, or carboxylic acid.

Functional groups: the key to organic classification 🔍

A functional group is a specific group of atoms in a molecule that gives it characteristic chemical reactions.

Structural formulae are the fastest way to identify functional groups.

Common examples include:

alcohol: $\mathrm{-OH}$
alkene: $\mathrm{C=C}$
carboxylic acid: $\mathrm{-COOH}$
halogenoalkane: $\mathrm{-X}$, where $\mathrm{X}$ is a halogen such as $\mathrm{Cl}$, $\mathrm{Br}$, or $\mathrm{I}$
aldehyde: $\mathrm{-CHO}$
ketone: $\mathrm{>C=O}$

For example, compare these two compounds:

propan-1-ol: $\mathrm{CH_3CH_2CH_2OH}$
propanone: $\mathrm{CH_3COCH_3}$

They are different because one contains an alcohol group and the other contains a ketone group. Their structural formulae reveal why they react differently.

This is a major IB idea: structure determines function.

Isomerism and why structural formulae matter

One of the most important reasons chemists use structural formulae is to study isomers.

Structural isomerism

Structural isomers have the same molecular formula but different connections between atoms.

Example: $\mathrm{C_4H_{10}}$

butane: $\mathrm{CH_3CH_2CH_2CH_3}$
2-methylpropane: $\mathrm{CH_3CH(CH_3)CH_3}$

These compounds are both alkanes, but the atoms are connected differently. That changes their physical properties, such as boiling point, because branching affects intermolecular forces and how well molecules pack together.

Why this matters in real life

A structural change can completely change a substance’s use. In pharmaceuticals, for example, small differences in structure can change how a drug fits a biological target. In fuels, different isomers can burn differently. In polymers, structure affects strength and flexibility.

So structural formulae are not just drawings; they are a language for understanding chemical behavior.

How to use structural formulae in IB Chemistry HL

When you answer IB questions, students, you may be asked to:

draw a structural formula from a molecular formula
identify a functional group from a structure
compare two compounds with the same molecular formula
name a compound from its structural formula
predict properties using structure

Example 1: from molecular formula to structural formula

Suppose you are given $\mathrm{C_3H_8O}$.

Possible structural formulae include:

propan-1-ol: $\mathrm{CH_3CH_2CH_2OH}$
propan-2-ol: $\mathrm{CH_3CH(OH)CH_3}$
methoxyethane: $\mathrm{CH_3OCH_2CH_3}$

The molecular formula alone is not enough. You need the structural formula to know which compound is meant.

Example 2: identifying a functional group

If a structure contains $\mathrm{-COOH}$, it is a carboxylic acid. If it contains $\mathrm{C=C}$, it is an alkene. If it contains $\mathrm{-OH}$ attached to a saturated carbon chain, it is an alcohol.

Example 3: predicting properties

Compare ethanol and propane:

ethanol: $\mathrm{CH_3CH_2OH}$
propane: $\mathrm{CH_3CH_2CH_3}$

Ethanol can hydrogen bond because of $\mathrm{-OH}$, so it has a higher boiling point than propane, which only has London dispersion forces. Structural formulae help explain this difference.

Conclusion

Structural formulae are a core part of chemistry because they show how atoms are connected, not just what atoms are present. That makes them essential for understanding isomerism, functional groups, classification of matter, and the relationship between structure and properties.

For IB Chemistry HL, students, you should remember that structural formulae are more than drawings. They are evidence-based tools that help chemists classify substances, predict reactions, and explain patterns across chemistry. When you can read and write structural formulae confidently, you are better prepared for organic chemistry, bonding, and problem-solving throughout the course ✅

Study Notes

A structural formula shows how atoms are connected in a molecule.
A molecular formula shows only the number of each type of atom, such as $\mathrm{C_2H_6O}$.
Different compounds can have the same molecular formula but different structural formulae; these are isomers.
Common representations include displayed formulae, structural formulae, and skeletal formulae.
Structural formulae help identify functional groups such as $\mathrm{-OH}$, $\mathrm{C=C}$, $\mathrm{-COOH}$, and $\mathrm{-CHO}$.
Functional groups strongly influence physical and chemical properties.
Structural formulae help classify matter into elements and compounds, and they are essential in organic chemistry.
In IB Chemistry HL, you may need to draw, interpret, compare, and name compounds using structural formulae.
Structure and properties are linked: changing the arrangement of atoms can change boiling point, solubility, and reactivity.
Structural formulae are a key tool for understanding patterns across chemistry and for making accurate scientific comparisons.