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Updated Mar 18, 2026
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OCR A-Level Chemistry: Atoms, Ions & Compounds Revision Notes
J
Jase George
@jasegeorge
1 / 6
Subatomic Particles and Isotopes
You've probably wondered what makes one element different from another - it's all about the tiny particles inside atoms! Protons carry a +1 charge, neutrons are neutral, and electrons have a -1 charge with almost no mass.
Isotopes are simply atoms of the same element that have different numbers of neutrons. Think of them as different versions of the same element - like different models of the same car. They have different masses but behave identically in chemical reactions.
Here's the key point: isotopes react in exactly the same way because they have the same number of electrons. The number of neutrons doesn't affect how atoms bond or react with other substances.
Quick Tip: Remember that isotopes are like twins with different weights - they look different but behave the same way!
Ions and Atomic Mass
When atoms gain or lose electrons, they become ions - charged particles that are crucial in chemistry. Cations are positive ions (fewer electrons than protons), while anions are negative ions (more electrons than protons).
Relative isotopic mass compares the mass of an isotope to one-twelfth of a carbon-12 atom. It's like using a standard ruler to measure everything else. Relative atomic mass (Ar) is the weighted average mass of all isotopes of an element.
Think of relative atomic mass as finding the average height of students in your year - you'd consider both how tall each student is and how many students there are of each height.
Memory Trick: Cations are "paws-itive" (they've lost electrons), anions are "negative" (they've gained electrons)!
Mass Spectrometry
Mass spectrometry is like a sophisticated sorting machine that separates isotopes by their mass. The process is surprisingly straightforward once you break it down.
First, the sample gets vaporised and ionised to create positive ions. These ions are then accelerated through the machine. Heavier ions move more slowly and are harder to deflect than lighter ones, causing separation.
The detector measures ions as a mass-to-charge ratio . The more abundant an isotope is, the stronger the signal it produces. This creates a mass spectrum - essentially a fingerprint showing which isotopes are present and in what amounts.
Real-World Connection: Mass spectrometry is used in forensic science to identify substances at crime scenes!
Calculating Relative Atomic Mass
Let's work through a real example with chlorine isotopes to show how straightforward these calculations can be. Chlorine has two main isotopes: Cl-35 (75.78% abundant) and Cl-37 (24.22% abundant).
The formula is: Ar = (abundance₁ × mass₁) + (abundance₂ × mass₂) / 100
For chlorine: Ar = (75.78 × 35) + (24.22 × 37) / 100 = 35.5
Common ions follow predictable patterns. Group 1 elements form +1 ions, Group 2 form +2 ions, and so on. Transition metals can form multiple different ions, which makes them more complex but also more versatile.
Exam Success: Always check your calculated atomic mass lies between the masses of the isotopes involved!
Important Ion Names
Memorising common ion names might seem tedious, but they're the vocabulary of chemistry - you'll use them constantly. Start with the most common ones you'll encounter in practicals and exams.
Key polyatomic ions include nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻), hydroxide (OH⁻), and ammonium (NH₄⁺). These appear in countless compounds and reactions.
Simple metal ions like zinc (Zn²⁺) and silver (Ag⁺) are equally important. Notice that silver is unusual - it forms a +1 ion rather than the +2 you might expect.
Study Strategy: Create flashcards for ion names and formulae - 10 minutes daily practice makes them automatic!
Balancing Chemical Equations
Balancing equations is like solving a puzzle - you need the same number of each type of atom on both sides. Let's walk through aluminium reacting with oxygen to form aluminium oxide.
Start by working out the correct formulae: Al³⁺ and O²⁻ ions combine to give Al₂O₃. Write the unbalanced equation: Al + O₂ → Al₂O₃.
Now balance systematically: 4Al + 3O₂ → 2Al₂O₃. Check your work - you should have 4 Al atoms and 6 O atoms on each side. Finally, add state symbols: 4Al(s) + 3O₂(g) → 2Al₂O₃(s).
Pro Tip: Always balance the most complex molecule first, then work through the simpler ones!
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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.
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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.
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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
OCR A-Level Chemistry: Atoms, Ions & Compounds Revision Notes
J
Jase George
@jasegeorge
Understanding atomic structure and chemical equations is fundamental to mastering chemistry. This guide covers the essential building blocks of atoms - from subatomic particles and isotopes to ions and balanced equations.
Subatomic Particles and Isotopes
You've probably wondered what makes one element different from another - it's all about the tiny particles inside atoms! Protons carry a +1 charge, neutrons are neutral, and electrons have a -1 charge with almost no mass.
Isotopes are simply atoms of the same element that have different numbers of neutrons. Think of them as different versions of the same element - like different models of the same car. They have different masses but behave identically in chemical reactions.
Here's the key point: isotopes react in exactly the same way because they have the same number of electrons. The number of neutrons doesn't affect how atoms bond or react with other substances.
Quick Tip: Remember that isotopes are like twins with different weights - they look different but behave the same way!
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Ions and Atomic Mass
When atoms gain or lose electrons, they become ions - charged particles that are crucial in chemistry. Cations are positive ions (fewer electrons than protons), while anions are negative ions (more electrons than protons).
Relative isotopic mass compares the mass of an isotope to one-twelfth of a carbon-12 atom. It's like using a standard ruler to measure everything else. Relative atomic mass (Ar) is the weighted average mass of all isotopes of an element.
Think of relative atomic mass as finding the average height of students in your year - you'd consider both how tall each student is and how many students there are of each height.
Memory Trick: Cations are "paws-itive" (they've lost electrons), anions are "negative" (they've gained electrons)!
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Mass Spectrometry
Mass spectrometry is like a sophisticated sorting machine that separates isotopes by their mass. The process is surprisingly straightforward once you break it down.
First, the sample gets vaporised and ionised to create positive ions. These ions are then accelerated through the machine. Heavier ions move more slowly and are harder to deflect than lighter ones, causing separation.
The detector measures ions as a mass-to-charge ratio . The more abundant an isotope is, the stronger the signal it produces. This creates a mass spectrum - essentially a fingerprint showing which isotopes are present and in what amounts.
Real-World Connection: Mass spectrometry is used in forensic science to identify substances at crime scenes!
Sign up to see the contentIt's free!
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Calculating Relative Atomic Mass
Let's work through a real example with chlorine isotopes to show how straightforward these calculations can be. Chlorine has two main isotopes: Cl-35 (75.78% abundant) and Cl-37 (24.22% abundant).
The formula is: Ar = (abundance₁ × mass₁) + (abundance₂ × mass₂) / 100
For chlorine: Ar = (75.78 × 35) + (24.22 × 37) / 100 = 35.5
Common ions follow predictable patterns. Group 1 elements form +1 ions, Group 2 form +2 ions, and so on. Transition metals can form multiple different ions, which makes them more complex but also more versatile.
Exam Success: Always check your calculated atomic mass lies between the masses of the isotopes involved!
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Important Ion Names
Memorising common ion names might seem tedious, but they're the vocabulary of chemistry - you'll use them constantly. Start with the most common ones you'll encounter in practicals and exams.
Key polyatomic ions include nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻), hydroxide (OH⁻), and ammonium (NH₄⁺). These appear in countless compounds and reactions.
Simple metal ions like zinc (Zn²⁺) and silver (Ag⁺) are equally important. Notice that silver is unusual - it forms a +1 ion rather than the +2 you might expect.
Study Strategy: Create flashcards for ion names and formulae - 10 minutes daily practice makes them automatic!
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
Balancing Chemical Equations
Balancing equations is like solving a puzzle - you need the same number of each type of atom on both sides. Let's walk through aluminium reacting with oxygen to form aluminium oxide.
Start by working out the correct formulae: Al³⁺ and O²⁻ ions combine to give Al₂O₃. Write the unbalanced equation: Al + O₂ → Al₂O₃.
Now balance systematically: 4Al + 3O₂ → 2Al₂O₃. Check your work - you should have 4 Al atoms and 6 O atoms on each side. Finally, add state symbols: 4Al(s) + 3O₂(g) → 2Al₂O₃(s).
Pro Tip: Always balance the most complex molecule first, then work through the simpler ones!
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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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
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
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