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18 Jan 2026
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Comprehensive Isomerism Guide for A-Level Chemistry
Daniel
@daniel_blja
1 / 10
Introduction to Isomerism
Ever wondered why some molecules with the same chemical formula behave completely differently? That's where isomerism comes in - it's the reason why chemistry gets really interesting!
Isomers are molecules that share the same molecular formula but have different structures. It's like having identical ingredients but following different recipes. There are two main types you need to master: structural isomerism and stereoisomerism.
💡 Quick Tip: Remember that same formula + different structure = isomers. This simple rule will help you spot them every time!
Structural Isomerism - The Big Three
Structural isomers have the same molecular formula but different arrangements of covalent bonds. There are three types that'll come up in your exams, and each one has its own personality.
Positional isomerism occurs when the functional group sits in different positions on the molecule. Think of but-1-ene versus but-2-ene - same carbon chain, but that double bond has moved house! This happens with alkenes , alcohols, and halogenoalkanes .
Chain isomerism is all about rearranging the carbon skeleton itself. Once you hit four carbon atoms, you can arrange them as a straight chain or add branches. More carbons = more possibilities for branching.
💡 Exam Alert: Alkanes don't show positional isomerism because they have no functional group to move around!
More Structural Isomerism Types
Chain isomerism gets more complex as molecules get bigger. With four carbons, you get two arrangements. Five carbons? Three ways. Six carbons gives you five different skeletons - the possibilities multiply quickly!
The key is understanding that branched molecules behave differently from straight-chain ones. Butane and methylpropane have the same formula (C₄H₁₀) but completely different shapes and properties.
Functional isomerism happens when molecules have the same formula but totally different functional groups. The classic example is cycloalkanes and alkenes - both have the formula CnH2n, but one's a ring and the other has a double bond.
💡 Memory Trick: More branching = lower boiling point. Branched molecules can't pack together as tightly, so weaker intermolecular forces!
Stereoisomerism - E-Z Isomerism
Here's where chemistry gets three-dimensional! Stereoisomerism occurs when molecules have the same structural formula but different spatial arrangements of atoms.
The type you need to know is E-Z stereoisomerism (also called geometrical isomerism). It happens because of restricted rotation around carbon-carbon double bonds. Unlike single bonds that can spin freely, double bonds are locked in place.
For E-Z isomerism to exist, you need two specific features: a C=C double bond and both carbon atoms attached to two different groups. If these conditions aren't met, no stereoisomerism occurs.
💡 Key Concept: Double bonds contain a σ-bond and a π-bond. The π-bond prevents rotation, which creates the different spatial arrangements!
The E-Z Naming System
The Cahn-Ingold-Prelog (CIP) system helps you name E-Z isomers correctly. First, identify the higher priority group on each carbon - this goes to whichever group has the atom with the higher atomic number attached to the double-bond carbon.
If the higher priority groups are on the same side, use the prefix Z (from German "zusammen" meaning together). If they're on opposite sides, use E (from German "entgegen" meaning opposite).
For example, in but-2-ene, if both CH₃ groups are on the same side, it's Z-but-2-ene. If they're on opposite sides, it's E-but-2-ene. Crab notation makes this easier to visualize - draw the C=C bond flat with the four groups arranged around it.
💡 Quick Check: Always use crab notation to predict if geometrical isomerism exists. If any carbon has two identical groups, no E-Z isomerism is possible!
Distinguishing Between Isomers
Isomers might have the same formula, but they don't have identical properties - and that's how chemists tell them apart! Melting and boiling points are the most reliable way to distinguish between structural isomers.
Here's the pattern you need to remember: straight-chain molecules have higher boiling points than their branched isomers. This happens because straight chains have larger surface areas, so they pack together more efficiently and experience stronger van der Waals forces.
Looking at the pentane isomers, pentane (straight chain) has the highest boiling point, while 2,2-dimethylpropane (most branched) has the lowest. The more compact and spherical the molecule, the weaker the intermolecular forces become.
💡 Exam Strategy: When asked to arrange isomers by boiling point, always put the most branched structure at the lowest temperature and the straight chain at the highest!
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This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
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Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
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Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
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I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
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Elisha
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This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
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Comprehensive Isomerism Guide for A-Level Chemistry
Daniel
@daniel_blja
Isomerism is one of the most important concepts in organic chemistry - it explains why molecules with identical formulas can have completely different properties. Think of it like having the same Lego pieces but building totally different structures with them.
Introduction to Isomerism
Ever wondered why some molecules with the same chemical formula behave completely differently? That's where isomerism comes in - it's the reason why chemistry gets really interesting!
Isomers are molecules that share the same molecular formula but have different structures. It's like having identical ingredients but following different recipes. There are two main types you need to master: structural isomerism and stereoisomerism.
💡 Quick Tip: Remember that same formula + different structure = isomers. This simple rule will help you spot them every time!
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Structural Isomerism - The Big Three
Structural isomers have the same molecular formula but different arrangements of covalent bonds. There are three types that'll come up in your exams, and each one has its own personality.
Positional isomerism occurs when the functional group sits in different positions on the molecule. Think of but-1-ene versus but-2-ene - same carbon chain, but that double bond has moved house! This happens with alkenes , alcohols, and halogenoalkanes .
Chain isomerism is all about rearranging the carbon skeleton itself. Once you hit four carbon atoms, you can arrange them as a straight chain or add branches. More carbons = more possibilities for branching.
💡 Exam Alert: Alkanes don't show positional isomerism because they have no functional group to move around!
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More Structural Isomerism Types
Chain isomerism gets more complex as molecules get bigger. With four carbons, you get two arrangements. Five carbons? Three ways. Six carbons gives you five different skeletons - the possibilities multiply quickly!
The key is understanding that branched molecules behave differently from straight-chain ones. Butane and methylpropane have the same formula (C₄H₁₀) but completely different shapes and properties.
Functional isomerism happens when molecules have the same formula but totally different functional groups. The classic example is cycloalkanes and alkenes - both have the formula CnH2n, but one's a ring and the other has a double bond.
💡 Memory Trick: More branching = lower boiling point. Branched molecules can't pack together as tightly, so weaker intermolecular forces!
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Stereoisomerism - E-Z Isomerism
Here's where chemistry gets three-dimensional! Stereoisomerism occurs when molecules have the same structural formula but different spatial arrangements of atoms.
The type you need to know is E-Z stereoisomerism (also called geometrical isomerism). It happens because of restricted rotation around carbon-carbon double bonds. Unlike single bonds that can spin freely, double bonds are locked in place.
For E-Z isomerism to exist, you need two specific features: a C=C double bond and both carbon atoms attached to two different groups. If these conditions aren't met, no stereoisomerism occurs.
💡 Key Concept: Double bonds contain a σ-bond and a π-bond. The π-bond prevents rotation, which creates the different spatial arrangements!
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The E-Z Naming System
The Cahn-Ingold-Prelog (CIP) system helps you name E-Z isomers correctly. First, identify the higher priority group on each carbon - this goes to whichever group has the atom with the higher atomic number attached to the double-bond carbon.
If the higher priority groups are on the same side, use the prefix Z (from German "zusammen" meaning together). If they're on opposite sides, use E (from German "entgegen" meaning opposite).
For example, in but-2-ene, if both CH₃ groups are on the same side, it's Z-but-2-ene. If they're on opposite sides, it's E-but-2-ene. Crab notation makes this easier to visualize - draw the C=C bond flat with the four groups arranged around it.
💡 Quick Check: Always use crab notation to predict if geometrical isomerism exists. If any carbon has two identical groups, no E-Z isomerism is possible!
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Distinguishing Between Isomers
Isomers might have the same formula, but they don't have identical properties - and that's how chemists tell them apart! Melting and boiling points are the most reliable way to distinguish between structural isomers.
Here's the pattern you need to remember: straight-chain molecules have higher boiling points than their branched isomers. This happens because straight chains have larger surface areas, so they pack together more efficiently and experience stronger van der Waals forces.
Looking at the pentane isomers, pentane (straight chain) has the highest boiling point, while 2,2-dimethylpropane (most branched) has the lowest. The more compact and spherical the molecule, the weaker the intermolecular forces become.
💡 Exam Strategy: When asked to arrange isomers by boiling point, always put the most branched structure at the lowest temperature and the straight chain at the highest!
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
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This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
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Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user