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Mariya Akbar
12/12/2025
Chemistry
Atomic Structure
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12 Dec 2025
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Understanding Atomic Structure: Comprehensive AQA Year 1 Notes
Mariya Akbar
@mariyaakbar_x
1 / 4
Atomic Structure Basics
Every atom contains three key particles that you absolutely need to know. Protons have a positive charge and relative mass of 1, neutrons are neutral with the same mass, and electrons are negatively charged but incredibly light .
The nucleus sits at the atom's centre and contains all the protons and neutrons. This is where nearly all the atom's mass is concentrated. Electrons whiz around this nucleus in the space surrounding it.
Isotopes are atoms of the same element with different numbers of neutrons - they're like identical twins with different weights. The mass spectrometer is a brilliant piece of kit that separates these isotopes by firing electrons at them, accelerating the resulting ions, and timing how fast they travel.
Key Tip: Remember that lighter ions travel faster through the mass spectrometer - it's like comparing how quickly a tennis ball and a bowling ball would roll down the same hill!
How Scientists Discovered Atomic Structure
Scientists didn't always know what atoms looked like - it took centuries of brilliant detective work! Dalton started us off thinking atoms were solid, indivisible balls. Then Thomson discovered electrons and imagined atoms like plum puddings with negative electrons dotted throughout positive "dough."
Rutherford completely revolutionised our understanding by discovering that atoms are mostly empty space. His famous gold foil experiment showed that all the positive charge and mass is squashed into a tiny nucleus at the centre.
Bohr then suggested that electrons don't just float randomly - they travel in specific paths or orbitals around the nucleus. This electronic structure follows strict rules about how electrons fill up different energy levels.
The filling order gets a bit tricky because 4s fills before 3d - energy levels don't always follow the pattern you'd expect. When atoms form ions, they lose the highest energy electrons first, which explains why d-block elements behave the way they do.
Remember: Electrons are like students choosing seats - they fill up singly before pairing up, and they always pick the lowest energy "seats" first!
Ionisation Energy Fundamentals
Ionisation energy is simply the energy needed to rip an electron away from an atom - think of it as measuring how tightly an atom grips its electrons. The first ionisation energy removes the first electron, the second removes another from the resulting ion, and so on.
Three main factors control how much energy this takes. Nuclear charge matters because more protons mean stronger attraction to electrons. Distance from nucleus is crucial - electrons further away are easier to remove, like trying to hold onto something with a really long arm.
Shielding happens when inner electrons get between the nucleus and outer electrons, weakening the attraction. It's like trying to hear someone shouting when there's a crowd of people between you.
Going down any group in the periodic table, ionisation energy decreases. Even though nuclear charge increases, the atoms get so much bigger and have so much more shielding that electrons become much easier to remove.
Quick Check: If you're struggling with ionisation energy trends, just think about which electrons would be hardest to pull away - the ones closest to lots of protons, or the ones far away and shielded!
Ionisation Energy Across Periods
Moving across a period, ionisation energy generally increases because nuclear charge goes up whilst shielding stays roughly the same. The atoms also get smaller as increased nuclear charge pulls electrons closer, making them even harder to remove.
However, there are two important exceptions you need to spot. There's a drop between groups 2 and 3 because the outer electron jumps from an s-orbital to a higher-energy p-orbital, and the s-electrons provide extra shielding.
The drop between groups 5 and 6 happens for a different reason - electrons start pairing up in the p-orbitals. When two electrons share the same orbital, they repel each other, making one easier to remove than expected.
These patterns aren't just academic - they explain why certain elements form ions more easily than others and help predict chemical behaviour. Understanding these trends gives you a powerful tool for explaining reactivity patterns across the periodic table.
Exam Tip: Always mention nuclear charge, shielding, and atomic radius when explaining ionisation energy trends - these three factors explain almost every pattern you'll encounter!
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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
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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
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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.
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
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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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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
Understanding Atomic Structure: Comprehensive AQA Year 1 Notes
Mariya Akbar
@mariyaakbar_x
Atoms are the building blocks of everything around us, and understanding their structure is crucial for mastering chemistry. This topic covers how scientists discovered what atoms look like inside, how we can analyse them using advanced equipment, and why different... Show more
Atomic Structure Basics
Every atom contains three key particles that you absolutely need to know. Protons have a positive charge and relative mass of 1, neutrons are neutral with the same mass, and electrons are negatively charged but incredibly light .
The nucleus sits at the atom's centre and contains all the protons and neutrons. This is where nearly all the atom's mass is concentrated. Electrons whiz around this nucleus in the space surrounding it.
Isotopes are atoms of the same element with different numbers of neutrons - they're like identical twins with different weights. The mass spectrometer is a brilliant piece of kit that separates these isotopes by firing electrons at them, accelerating the resulting ions, and timing how fast they travel.
Key Tip: Remember that lighter ions travel faster through the mass spectrometer - it's like comparing how quickly a tennis ball and a bowling ball would roll down the same hill!
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How Scientists Discovered Atomic Structure
Scientists didn't always know what atoms looked like - it took centuries of brilliant detective work! Dalton started us off thinking atoms were solid, indivisible balls. Then Thomson discovered electrons and imagined atoms like plum puddings with negative electrons dotted throughout positive "dough."
Rutherford completely revolutionised our understanding by discovering that atoms are mostly empty space. His famous gold foil experiment showed that all the positive charge and mass is squashed into a tiny nucleus at the centre.
Bohr then suggested that electrons don't just float randomly - they travel in specific paths or orbitals around the nucleus. This electronic structure follows strict rules about how electrons fill up different energy levels.
The filling order gets a bit tricky because 4s fills before 3d - energy levels don't always follow the pattern you'd expect. When atoms form ions, they lose the highest energy electrons first, which explains why d-block elements behave the way they do.
Remember: Electrons are like students choosing seats - they fill up singly before pairing up, and they always pick the lowest energy "seats" first!
Sign up to see the contentIt's free!
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Ionisation Energy Fundamentals
Ionisation energy is simply the energy needed to rip an electron away from an atom - think of it as measuring how tightly an atom grips its electrons. The first ionisation energy removes the first electron, the second removes another from the resulting ion, and so on.
Three main factors control how much energy this takes. Nuclear charge matters because more protons mean stronger attraction to electrons. Distance from nucleus is crucial - electrons further away are easier to remove, like trying to hold onto something with a really long arm.
Shielding happens when inner electrons get between the nucleus and outer electrons, weakening the attraction. It's like trying to hear someone shouting when there's a crowd of people between you.
Going down any group in the periodic table, ionisation energy decreases. Even though nuclear charge increases, the atoms get so much bigger and have so much more shielding that electrons become much easier to remove.
Quick Check: If you're struggling with ionisation energy trends, just think about which electrons would be hardest to pull away - the ones closest to lots of protons, or the ones far away and shielded!
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Ionisation Energy Across Periods
Moving across a period, ionisation energy generally increases because nuclear charge goes up whilst shielding stays roughly the same. The atoms also get smaller as increased nuclear charge pulls electrons closer, making them even harder to remove.
However, there are two important exceptions you need to spot. There's a drop between groups 2 and 3 because the outer electron jumps from an s-orbital to a higher-energy p-orbital, and the s-electrons provide extra shielding.
The drop between groups 5 and 6 happens for a different reason - electrons start pairing up in the p-orbitals. When two electrons share the same orbital, they repel each other, making one easier to remove than expected.
These patterns aren't just academic - they explain why certain elements form ions more easily than others and help predict chemical behaviour. Understanding these trends gives you a powerful tool for explaining reactivity patterns across the periodic table.
Exam Tip: Always mention nuclear charge, shielding, and atomic radius when explaining ionisation energy trends - these three factors explain almost every pattern you'll encounter!
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 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