✓ Revise Right course
Ethene is C₂H₄, with the structural formula CH₂=CH₂, and propene is C₃H₆, or CH₂=CH-CH₃. Free GCSE AQA Chemistry notes with key terms and 9 questions answered.
Naming and Representing Alkenes is part of Reactions of alkenes and alcohols (chemistry only) in GCSE AQA Higher Chemistry.
4 min
Reading time
23
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
Ethene is C₂H₄, with the structural formula CH₂=CH₂, and propene is C₃H₆, or CH₂=CH-CH₃. Free GCSE AQA Chemistry notes with key terms and 9 questions answered.
AQA GCSE Chemistry mock draft, 26–31 Oct. Draft coverage: Chemistry Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside Reactions of alkenes and alcohols (chemistry only) while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Naming and Representing Alkenes before testing yourself from memory.
You've met alkanes - the saturated hydrocarbons with single bonds. Now we're stepping into the world of alkenes, where everything gets more reactive because of one key difference: the double bond.
Alkenes are hydrocarbons containing a C=C double bond. This double bond is what makes them unsaturated - they have fewer hydrogen atoms than the corresponding alkane. The double bond is also their reactive hotspot, making alkenes much more chemically active than alkanes.
For GCSE, you need to know the names and structures of the first four alkenes. Notice the pattern - each name ends in '-ene' instead of '-ane', telling you there's a double bond present.
Here are the four alkenes you must memorise:
Ethene (C₂H₄): The smallest alkene with 2 carbon atomsPropene (C₃H₆): 3 carbon atomsButene (C₄H₈): 4 carbon atomsPentene (C₅H₁₀): 5 carbon atoms
Notice the pattern in the molecular formulae: C₂H₄, C₃H₆, C₄H₈, C₅H₁₀. Each alkene has exactly double the number of hydrogens as carbons. This gives us the general formula CₙH₂ₙ for alkenes.
Alkenes can be represented in different ways, just like alkanes. You need to be comfortable switching between molecular formulae and structural formulae. The key is showing that crucial C=C double bond clearly.
Worked example: Drawing the Structure of Butene
Draw the structural formula for butene (C₄H₈)
Answer: H₂C=CH-CH₂-CH₃ or displayed as a structural formula showing all bonds
The structural formula shows you exactly how the atoms connect. Notice how the carbons in the double bond each have only one hydrogen attached, while the other carbons have 2 or 3 hydrogens to complete their four bonds.
Examiners can show you alkene structures in several formats. You need to recognise them all and convert between them confidently. Here's how the same molecule can look completely different on paper.
Molecular formula: C₃H₆ (just the numbers of each atom)Structural formula: CH₂=CH-CH₃ (shows the bonding pattern)Displayed formula: Shows every single bond as a line
The displayed formula is the most detailed - it shows every bond as a line and every atom. This makes it clear and systematic to count bonds and check your structure is correct. The structural formula is more compact but still shows the connectivity.
When drawing structures, always check your hydrogen count. Each carbon must have exactly 4 bonds. If you've drawn a carbon with 3 or 5 bonds, you've made an error.
Like all homologous series, alkenes follow predictable patterns. Once you spot these patterns, you can work out properties and formulae without memorising every single compound.
Each alkene differs from the next by exactly one CH₂ unit. Look at the molecular formulae: C₂H₄ → C₃H₆ → C₄H₈ → C₅H₁₀. Each step adds one carbon and two hydrogens.
The general formula CₙH₂ₙ captures this pattern perfectly. Whatever the number of carbons (n), the number of hydrogens is always exactly double. This is different from alkanes, which follow CₙH₂ₙ₊₂.
Alkene questions often test your ability to switch between different representations and spot patterns. Here's how to tackle them systematically and avoid the common traps that catch students out.
Common exam mistakes: Drawing carbons with the wrong number of bonds, confusing alkene and alkane formulae, and forgetting that the '-ene' suffix indicates a double bond. Always double-check your bond count - it's the fastest way to spot errors.
When the examiner asks you to 'draw the structure', they usually want the structural formula (like CH₂=CH₂) unless they specifically ask for a displayed formula. Practice both formats so you're ready for either.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-19.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The molecular formula of ethene is C₂H₄, and its structural formula is CH₂=CH₂. Ethene is the smallest alkene, with 2 carbon atoms joined by a C=C double bond.
The structural formula of propene is CH₂=CH-CH₃ and its molecular formula is C₃H₆. A displayed formula would also show every single bond as a line.
To draw butene, draw a chain of 4 carbon atoms, place a C=C double bond at the end, then add hydrogens so each carbon has 4 bonds. This gives H₂C=CH-CH₂-CH₃, where the carbons in the double bond have fewer hydrogens.
Each carbon atom forms exactly 4 bonds. In alkenes, the carbons in the double bond have fewer hydrogens attached; a carbon drawn with 3 or 5 bonds is a sign of an error.
The first four alkenes are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈) and pentene (C₅H₁₀). Each name ends in -ene, which shows there is a C=C double bond.
A displayed formula shows every atom and every bond as a line. It is the most detailed way to show a molecule, making it easy to count bonds and check that each carbon has exactly 4.
Alkene names use a prefix for the number of carbons and the ending -ene for the double bond; for example, the prefix but- always means 4 carbons, so butene has 4 carbons. Alkenes end in -ene instead of the -ane ending used for alkanes.
To check an alkene structure, make sure every carbon has exactly 4 bonds and the formula fits CₙH₂ₙ. A carbon with 3 or 5 bonds means there is an error, and drawing carbons with the wrong number of bonds is a common exam mistake.
The '-ene' ending in a name shows the compound is an alkene with a C=C double bond, instead of the '-ane' ending used for alkanes. For example, ethene, propene, butene and pentene are the first four alkenes.