Who this is for: Cambridge IGCSE Chemistry (0620) students who want organic formulae, functional groups and homologous-series terminology to become a clear toolkit instead of interchangeable diagram labels.
What query it owns: how to understand and revise formulae, functional groups and terminology in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the formulae and functional groups revision-guide angle, while Tutopiya’s Formulae, Functional Groups and Terminology subtopic page owns the learning resource and the free quiz owns the practice.

Before you can name compounds or predict reactions, you need the vocabulary: molecular, structural and displayed formulae; functional groups that define homologous series; and terms like saturated, unsaturated and homologous series. Cambridge IGCSE Chemistry (0620) tests whether you can read and write these accurately. This guide covers the formulae types, the functional groups on the syllabus, and how examiners phrase the questions.

Key takeaways

• Molecular formula shows atom numbers only (e.g. C₂H₆); structural formula shows connectivity (e.g. CH₃CH₃).
• Displayed formula shows every bond; general formula applies to a whole homologous series (e.g. CₙH₂ₙ₊₂ for alkanes).
• Functional group = the atom/group that determines chemical properties (–OH, –COOH, C=C, etc.).
• Homologous series = family of compounds with the same functional group and similar properties.
• Saturated = single C–C bonds only; unsaturated = contains C=C double bond.

What are formulae and functional groups in Cambridge IGCSE Chemistry?

Organic chemistry uses several formula types to represent the same molecule at different levels of detail. Functional groups identify which homologous series a compound belongs to and predict its reactions. Mastering this terminology unlocks every other Organic Chemistry subtopic on the 0620 syllabus.

Read the full notes on Tutopiya’s Formulae, Functional Groups and Terminology subtopic page before attempting questions.

The core ideas you must master

Term / formula type	What it shows	Example (ethanol)
Molecular formula	Atom count only	C₂H₆O
Structural formula	Atom connectivity	CH₃CH₂OH
Displayed formula	All bonds drawn	H–C–C–O–H with all H atoms shown
General formula	Pattern for a series	CₙH₂ₙ₊₁OH (alcohols)
Functional group –OH	Alcohol	Ethanol
Functional group –COOH	Carboxylic acid	Ethanoic acid
Functional group C=C	Alkene	Ethene
Homologous series	Same functional group, trend in properties	Alkanes, alkenes, alcohols

Functional groups on the 0620 syllabus

Homologous series	Functional group	General formula
Alkanes	C–C and C–H only (saturated)	CₙH₂ₙ₊₂
Alkenes	C=C (unsaturated)	CₙH₂ₙ
Alcohols	–OH	CₙH₂ₙ₊₁OH
Carboxylic acids	–COOH	CₙH₂ₙ₊₁COOH

Formulae and terminology in past-paper wording

Command word	What the question wants	Typical stem
Draw	Displayed or structural formula	”Draw the displayed formula of ethanol.”
State	Name a formula type or group	”State the functional group in ethene.”
Define	Precise syllabus definition	”Define homologous series.”
Deduce	Work out a formula from information	”Deduce the molecular formula of this compound.”
Identify	Name the series from a formula	”Identify the homologous series of CH₃CH₂COOH.”

Worked exam-style stems (how to answer the wording)

1. “Define homologous series.” A family of similar compounds with the same functional group, showing a trend in physical properties and sharing the same general formula. Reward: functional group + general formula + trend.
2. “Draw the displayed formula of ethene.” Two carbon atoms joined by a double bond, each bonded to two hydrogen atoms. Reward: C=C shown + four H atoms correctly placed.
3. “State the general formula of the alkanes.” CₙH₂ₙ₊₂. Reward: correct formula.

Test yourself with the Formulae, Functional Groups and Terminology quiz once you have worked through a few examples.

How formulae connect to the rest of Organic Chemistry

This subtopic underpins Naming Organic Compounds, Alkanes, Alkenes, Alcohols and Carboxylic Acids. The Cambridge IGCSE Chemistry resource hub links all units.

Common mistakes students make

• Confusing molecular and structural formulae in draw questions.
• Forgetting to show the double bond in displayed alkene formulae.
• Calling any organic compound a “functional group” — the group is the reactive part (e.g. –OH), not the whole molecule.
• Mixing up general formulae: CₙH₂ₙ for alkenes, not alkanes.
• Omitting trend in properties when defining homologous series.

When you need more support

If formula-drawing and functional-group identification keep costing marks, work through the Formulae, Functional Groups and Terminology quiz to find the gap, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

What is the difference between structural and displayed formulae? Structural formula shows connectivity in condensed form (CH₃CH₂OH); displayed formula shows every atom and bond.

What is a functional group? The atom or group of atoms responsible for the characteristic reactions of a homologous series.

What does saturated mean in organic chemistry? All carbon–carbon bonds are single bonds — no C=C double bonds.

How do I revise formulae and functional groups effectively? Learn one series at a time with its general formula and functional group, practise drawing displayed formulae, then take the quiz.

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