Maths
Chemistry
Biology
English Language
History
Responding to change (a2 only)
Infection and response
Homeostasis and response
Energy transfers (a2 only)
Cell biology
Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
Organisation
Substance exchange
Bioenergetics
Genetic information & variation
Inheritance, variation and evolution
Genetics & ecosystems (a2 only)
Ecology
Cells
Show all topics
Britain & the wider world: 1745 -1901
1l the quest for political stability: germany, 1871-1991
The cold war
Inter-war germany
Medieval period: 1066 -1509
2d religious conflict and the church in england, c1529-c1570
2o democracy and nazism: germany, 1918-1945
1f industrialisation and the people: britain, c1783-1885
1c the tudors: england, 1485-1603
2m wars and welfare: britain in transition, 1906-1957
World war two & the holocaust
2n revolution and dictatorship: russia, 1917-1953
2s the making of modern britain, 1951-2007
World war one
Britain: 1509 -1745
Show all topics
Chemistry
7 Dec 2025
205
1 page
Neha Patel @nehapatel_
Understanding atoms is like learning the building blocks of everything around you - from the air you breathe... Show more
Think of atoms as the smallest LEGO pieces that make up everything in the universe. Each atom contains a tiny, dense nucleus at its centre, packed with protons (positive charge) and neutrons (no charge). Whizzing around this nucleus are electrons (negative charge) in energy shells.
Here's what makes atoms fascinating the nucleus is incredibly small compared to the whole atom - imagine a marble in the centre of a football stadium! The atomic number tells you how many protons an element has, whilst the mass number is the total of protons plus neutrons.
Isotopes are atoms of the same element that have different numbers of neutrons. They're like identical twins with slightly different weights - same chemical behaviour, but different masses. Scientists use relative atomic mass (RAM) because most elements exist as mixtures of isotopes.
Quick Tip Remember that atoms are electrically neutral because they have equal numbers of protons and electrons!
The story of atomic theory is like a detective mystery that took over a century to solve. John Dalton started things off, imagining atoms as solid, indivisible spheres. Then J.J. Thomson discovered electrons and proposed the "plum pudding model" - a ball of positive charge with electrons dotted throughout like raisins in a pudding.
Everything changed when Ernest Rutherford fired particles at gold foil in his famous experiment. Most particles went straight through, but some bounced back - proving that atoms have a tiny, dense, positively charged nucleus at their centre.
Niels Bohr then figured out that electrons orbit the nucleus at specific distances in energy shells, rather than randomly floating about. Finally, James Chadwick discovered neutrons in the nucleus, which explained why isotopes exist.
Did You Know It took 20 years after discovering the nucleus for scientists to fully accept the idea - that's how revolutionary it was!
Elements are pure substances made of just one type of atom - think gold, oxygen, or carbon. There are over 100 elements on the periodic table, each with its own unique atomic number. They're classified as metals or non-metals based on their properties.
Compounds form when two or more elements chemically bond together in fixed proportions. Water (H₂O) always contains exactly two hydrogen atoms for every oxygen atom. Compounds have completely different properties from their original elements - sodium (explosive metal) plus chlorine (poisonous gas) makes salt (essential for life)!
Mixtures are different - they're just elements or compounds mixed together without chemical bonding. Air is a mixture of gases, and you can separate mixtures using physical methods without breaking any chemical bonds.
Key Point Chemical reactions are needed to make or break compounds, but mixtures can be separated using physical techniques.
Scientists have developed brilliant methods to separate mixtures without using chemical reactions. Simple distillation works by heating a liquid mixture - the substance with the lower boiling point evaporates first, then condenses back to liquid in a separate container.
Fractional distillation is the heavy-duty version, used for separating crude oil into petrol, diesel, and other products. Filtration separates insoluble solids from liquids using filter paper, whilst crystallisation lets you recover dissolved solids by evaporating the solvent and cooling.
Chromatography is particularly useful for separating small amounts of dissolved substances and checking purity. You've probably seen this with ink on blotting paper - different coloured dyes travel at different speeds up the paper.
Lab Tip Always choose your separation method based on the physical properties of what you're trying to separate - solubility, boiling point, or particle size.
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.
You can download the app from Google Play Store and Apple App Store.
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 THE SCHOOLGPT. 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 THE SCHOOLGPT. 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
Neha Patel
@nehapatel_
Understanding atoms is like learning the building blocks of everything around you - from the air you breathe to your mobile phone. This foundational chemistry knowledge covers how atoms work, what makes elements different, and the clever ways scientists separate... Show more
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Think of atoms as the smallest LEGO pieces that make up everything in the universe. Each atom contains a tiny, dense nucleus at its centre, packed with protons (positive charge) and neutrons (no charge). Whizzing around this nucleus are electrons (negative charge) in energy shells.
Here's what makes atoms fascinating: the nucleus is incredibly small compared to the whole atom - imagine a marble in the centre of a football stadium! The atomic number tells you how many protons an element has, whilst the mass number is the total of protons plus neutrons.
Isotopes are atoms of the same element that have different numbers of neutrons. They're like identical twins with slightly different weights - same chemical behaviour, but different masses. Scientists use relative atomic mass (RAM) because most elements exist as mixtures of isotopes.
Quick Tip: Remember that atoms are electrically neutral because they have equal numbers of protons and electrons!
The story of atomic theory is like a detective mystery that took over a century to solve. John Dalton started things off, imagining atoms as solid, indivisible spheres. Then J.J. Thomson discovered electrons and proposed the "plum pudding model" - a ball of positive charge with electrons dotted throughout like raisins in a pudding.
Everything changed when Ernest Rutherford fired particles at gold foil in his famous experiment. Most particles went straight through, but some bounced back - proving that atoms have a tiny, dense, positively charged nucleus at their centre.
Niels Bohr then figured out that electrons orbit the nucleus at specific distances in energy shells, rather than randomly floating about. Finally, James Chadwick discovered neutrons in the nucleus, which explained why isotopes exist.
Did You Know: It took 20 years after discovering the nucleus for scientists to fully accept the idea - that's how revolutionary it was!
Elements are pure substances made of just one type of atom - think gold, oxygen, or carbon. There are over 100 elements on the periodic table, each with its own unique atomic number. They're classified as metals or non-metals based on their properties.
Compounds form when two or more elements chemically bond together in fixed proportions. Water (H₂O) always contains exactly two hydrogen atoms for every oxygen atom. Compounds have completely different properties from their original elements - sodium (explosive metal) plus chlorine (poisonous gas) makes salt (essential for life)!
Mixtures are different - they're just elements or compounds mixed together without chemical bonding. Air is a mixture of gases, and you can separate mixtures using physical methods without breaking any chemical bonds.
Key Point: Chemical reactions are needed to make or break compounds, but mixtures can be separated using physical techniques.
Scientists have developed brilliant methods to separate mixtures without using chemical reactions. Simple distillation works by heating a liquid mixture - the substance with the lower boiling point evaporates first, then condenses back to liquid in a separate container.
Fractional distillation is the heavy-duty version, used for separating crude oil into petrol, diesel, and other products. Filtration separates insoluble solids from liquids using filter paper, whilst crystallisation lets you recover dissolved solids by evaporating the solvent and cooling.
Chromatography is particularly useful for separating small amounts of dissolved substances and checking purity. You've probably seen this with ink on blotting paper - different coloured dyes travel at different speeds up the paper.
Lab Tip: Always choose your separation method based on the physical properties of what you're trying to separate - solubility, boiling point, or particle size.
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
0
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
App Store
Google Play
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 THE SCHOOLGPT. 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 THE SCHOOLGPT. 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
Sociology