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22 Dec 2025
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Exploring Atomic Structure in Physics - Topic 4
Charlotte
@charlotte26
1 / 7
Atomic Structure Basics
Atoms are like tiny solar systems with three main particles that you absolutely need to know. Protons sit in the nucleus with a positive charge and mass of 1, whilst neutrons share the same space with no charge but the same mass. Electrons zoom around the outside in energy levels, carrying a negative charge and virtually no mass.
The nucleus is incredibly small but contains nearly all the atom's mass - imagine fitting all of London's population into a football! Electrons can only exist at specific distances from the nucleus, called energy levels or shells.
When scientists write atomic symbols, they show the mass number (total protons and neutrons) at the top and the atomic number (just protons) at the bottom. This tells you everything about that particular atom.
Quick Tip: Remember that the atomic number defines what element you're dealing with - change the number of protons and you get a completely different element!
Isotopes and Electron Behaviour
Isotopes are basically the same element wearing different outfits - they have identical numbers of protons but different numbers of neutrons. Carbon-12, carbon-13, and carbon-14 are perfect examples of this concept in action.
Atoms become charged when electrons and protons don't match up. More electrons than protons creates a negative charge, whilst more protons gives you a positive charge.
Here's where it gets interesting: electrons can jump between energy levels by absorbing or emitting electromagnetic radiation. When they absorb energy, they move further from the nucleus; when they release energy, they drop closer. If electrons gain enough energy, they escape completely, creating positive ions.
Remember: Electrons are picky - they'll only absorb specific amounts of energy to make these jumps between levels.
Development of Atomic Theory
Scientists didn't always understand atoms like we do today - the atomic model evolved through brilliant discoveries and experiments. Peer review remains crucial, especially for radiation research, because incorrect safety levels could literally kill people.
Dalton (1800) thought everything was made of tiny, indivisible spheres. JJ Thompson (1897) discovered electrons and created the "plum pudding model" - imagine electrons scattered through positive "pudding" like raisins in a cake.
Rutherford's famous experiment (1911) changed everything. He fired alpha particles at gold foil expecting them to pass straight through, but some bounced back! This proved atoms are mostly empty space with a dense, positive nucleus at the centre.
Fun Fact: Rutherford compared his surprise to firing a cannon at tissue paper and having the cannonball bounce back at you!
Modern Atomic Model and Radioactivity
Bohr (1913) discovered that electrons orbit in specific shells, creating the model we use today. James Chadwick later discovered neutrons, completing our understanding of atomic structure.
Radioactive decay happens when unstable nuclei break down randomly to become more stable, releasing radiation in the process. Activity measures how fast this decay occurs (in Becquerels), whilst count rate shows how many decays a detector records per second.
Alpha radiation is highly ionising but can't penetrate far - just 5cm through air. It's perfect for smoke detectors because it stays local. Beta radiation has medium penetration (50cm in air) and works brilliantly for testing metal thickness.
Gamma radiation barely ionises anything but penetrates almost everything - you need several centimetres of lead to stop it! This makes it ideal for medical imaging and X-rays.
Key Point: Higher ionisation means more dangerous to living tissue, but ironically, these particles can't travel as far.
Radiation Safety and Applications
Ionisation occurs when radiation knocks electrons out of atoms, which can damage living cells. Understanding contamination versus irradiation could save your life in a radiation emergency.
Contamination means radioactive material gets on or inside objects, continuing to emit radiation for ages. Irradiation simply means being exposed to radiation without becoming radioactive yourself - like getting an X-ray.
You can reduce radiation exposure through three simple methods: limit your time near sources, increase distance (radiation weakens quickly), and use proper shielding materials.
Each type of radiation has brilliant practical uses: alpha in smoke detectors, beta for testing material thickness, and gamma for medical procedures and sterilising equipment.
Safety First: Medical tools are often irradiated to kill bacteria without making them radioactive - they're perfectly safe to use afterwards.
Half-Life and Risk Assessment
Half-life tells you how long it takes for half the nuclei in a sample to decay, or for the count rate to halve. You can't predict when individual atoms will decay, but large samples follow predictable patterns.
Short half-life isotopes are like sprint runners - they decay quickly and become less dangerous fast, but can be immediately hazardous. Long half-life isotopes are marathon runners, staying weakly radioactive for ages and creating long-term risks.
Americium-241 in smoke detectors perfectly demonstrates this concept. With a 432-year half-life, it emits alpha particles steadily for decades without needing replacement.
The alpha particles can't travel far through air, so they stay contained within the detector. When smoke particles interrupt the radiation, the alarm sounds - clever engineering using atomic physics!
Real-World Connection: Your smoke detector contains radioactive material, but it's so well-designed that it poses virtually no risk to your health.
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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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Best app on earth! no words because it’s too good
Thomas R
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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
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
Exploring Atomic Structure in Physics - Topic 4
Charlotte
@charlotte26
Ever wondered what's inside an atom or how smoke detectors work? Atomic structure is everywhere around us, from the air we breathe to the technology we use daily. Understanding how atoms are built and behave helps explain everything from medical... Show more
Atomic Structure Basics
Atoms are like tiny solar systems with three main particles that you absolutely need to know. Protons sit in the nucleus with a positive charge and mass of 1, whilst neutrons share the same space with no charge but the same mass. Electrons zoom around the outside in energy levels, carrying a negative charge and virtually no mass.
The nucleus is incredibly small but contains nearly all the atom's mass - imagine fitting all of London's population into a football! Electrons can only exist at specific distances from the nucleus, called energy levels or shells.
When scientists write atomic symbols, they show the mass number (total protons and neutrons) at the top and the atomic number (just protons) at the bottom. This tells you everything about that particular atom.
Quick Tip: Remember that the atomic number defines what element you're dealing with - change the number of protons and you get a completely different element!
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Isotopes and Electron Behaviour
Isotopes are basically the same element wearing different outfits - they have identical numbers of protons but different numbers of neutrons. Carbon-12, carbon-13, and carbon-14 are perfect examples of this concept in action.
Atoms become charged when electrons and protons don't match up. More electrons than protons creates a negative charge, whilst more protons gives you a positive charge.
Here's where it gets interesting: electrons can jump between energy levels by absorbing or emitting electromagnetic radiation. When they absorb energy, they move further from the nucleus; when they release energy, they drop closer. If electrons gain enough energy, they escape completely, creating positive ions.
Remember: Electrons are picky - they'll only absorb specific amounts of energy to make these jumps between levels.
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Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Development of Atomic Theory
Scientists didn't always understand atoms like we do today - the atomic model evolved through brilliant discoveries and experiments. Peer review remains crucial, especially for radiation research, because incorrect safety levels could literally kill people.
Dalton (1800) thought everything was made of tiny, indivisible spheres. JJ Thompson (1897) discovered electrons and created the "plum pudding model" - imagine electrons scattered through positive "pudding" like raisins in a cake.
Rutherford's famous experiment (1911) changed everything. He fired alpha particles at gold foil expecting them to pass straight through, but some bounced back! This proved atoms are mostly empty space with a dense, positive nucleus at the centre.
Fun Fact: Rutherford compared his surprise to firing a cannon at tissue paper and having the cannonball bounce back at you!
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Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Modern Atomic Model and Radioactivity
Bohr (1913) discovered that electrons orbit in specific shells, creating the model we use today. James Chadwick later discovered neutrons, completing our understanding of atomic structure.
Radioactive decay happens when unstable nuclei break down randomly to become more stable, releasing radiation in the process. Activity measures how fast this decay occurs (in Becquerels), whilst count rate shows how many decays a detector records per second.
Alpha radiation is highly ionising but can't penetrate far - just 5cm through air. It's perfect for smoke detectors because it stays local. Beta radiation has medium penetration (50cm in air) and works brilliantly for testing metal thickness.
Gamma radiation barely ionises anything but penetrates almost everything - you need several centimetres of lead to stop it! This makes it ideal for medical imaging and X-rays.
Key Point: Higher ionisation means more dangerous to living tissue, but ironically, these particles can't travel as far.
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Radiation Safety and Applications
Ionisation occurs when radiation knocks electrons out of atoms, which can damage living cells. Understanding contamination versus irradiation could save your life in a radiation emergency.
Contamination means radioactive material gets on or inside objects, continuing to emit radiation for ages. Irradiation simply means being exposed to radiation without becoming radioactive yourself - like getting an X-ray.
You can reduce radiation exposure through three simple methods: limit your time near sources, increase distance (radiation weakens quickly), and use proper shielding materials.
Each type of radiation has brilliant practical uses: alpha in smoke detectors, beta for testing material thickness, and gamma for medical procedures and sterilising equipment.
Safety First: Medical tools are often irradiated to kill bacteria without making them radioactive - they're perfectly safe to use afterwards.
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Half-Life and Risk Assessment
Half-life tells you how long it takes for half the nuclei in a sample to decay, or for the count rate to halve. You can't predict when individual atoms will decay, but large samples follow predictable patterns.
Short half-life isotopes are like sprint runners - they decay quickly and become less dangerous fast, but can be immediately hazardous. Long half-life isotopes are marathon runners, staying weakly radioactive for ages and creating long-term risks.
Americium-241 in smoke detectors perfectly demonstrates this concept. With a 432-year half-life, it emits alpha particles steadily for decades without needing replacement.
The alpha particles can't travel far through air, so they stay contained within the detector. When smoke particles interrupt the radiation, the alarm sounds - clever engineering using atomic physics!
Real-World Connection: Your smoke detector contains radioactive material, but it's so well-designed that it poses virtually no risk to your health.
Sign up to see the contentIt's free!
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Join milions of students
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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?
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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