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Chelsea Tepsiri
05/12/2025
Chemistry
Chemistry Topic 1: Atomic structure and Periodic Table
368
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5 Dec 2025
•
Understanding Atomic Structure and the Periodic Table: Key Concepts
Chelsea Tepsiri
@chelseatep_
1 / 5
Atoms, Elements and Compounds
Think of atoms as the LEGO bricks of the universe - they're the smallest parts of elements that still keep their identity. Each element gets its own chemical symbol, like a name tag (think H for hydrogen, O for oxygen).
When two or more elements team up chemically, they form compounds. This isn't just mixing - it's like a chemical marriage that creates something completely new. You can only break up compounds using chemical reactions, not just by stirring or filtering.
Mixtures are totally different - imagine throwing different LEGO sets into one box. The pieces aren't stuck together, so you can separate them using simple methods like filtration or distillation. No chemical reactions needed!
Quick Tip: If you can separate it physically (like picking out raisins from cereal), it's a mixture. If you need a chemical reaction, it's a compound!
How We Discovered What Atoms Look Like
Scientists once thought atoms were like tiny, solid snooker balls that couldn't be broken apart. Boy, were they wrong! The discovery of electrons led to the "plum-pudding model" - imagine a positively charged cake with negatively charged raisins (electrons) scattered throughout.
Everything changed with the alpha particle scattering experiment. Scientists fired particles at thin gold foil and watched where they went. Most went straight through, but some bounced back at weird angles - even straight back! This was like throwing tennis balls at tissue paper and having some bounce back at your face.
This experiment revealed that atoms have a tiny, dense nucleus at their centre where all the positive charge hangs out. Niels Bohr then figured out that electrons orbit this nucleus like planets around the sun.
Later discoveries by James Chadwick revealed neutrons - particles with no charge that live in the nucleus alongside protons.
The Numbers Game: Atomic Structure
Here's where atoms get their ID cards! The atomic number tells you how many protons an atom has - it's like an atom's fingerprint because every element has a unique number. The mass number is the total count of protons plus neutrons.
Isotopes are like siblings in the atomic world - same element (same number of protons) but different numbers of neutrons. They're basically the same person wearing different weight clothing!
Electronic structure shows how electrons arrange themselves in shells around the nucleus. For sodium (Na), it's 2,8,1 - meaning 2 electrons in the first shell, 8 in the second, and 1 lonely electron in the outer shell.
Memory Hack: Think of electron shells like seats on a bus - the inner seats fill up first before anyone sits in the back!
Relative atomic mass is an average that accounts for all the different isotopes of an element and how common they are.
The Periodic Table: Chemistry's Greatest Cheat Sheet
The periodic table arranges all elements by atomic number, and here's the genius bit - elements with similar properties line up in columns called groups. It's like organising your music playlist where all the rock songs are together!
Mendeleev was the mastermind behind this organisation, though he initially used atomic mass. He was so confident in his pattern that he left gaps for elements that hadn't been discovered yet - and he was right!
Metals hang out on the left side and love giving away electrons to form positive ions. Non-metals chill on the right and prefer grabbing electrons to form negative ions. This behaviour determines how elements react with each other.
Group 1 (alkali metals) are the drama queens of chemistry - they have one electron in their outer shell and react explosively with water. The bigger they get (going down the group), the more reactive they become!
Safety Note: Never mess about with Group 1 metals in real life - they can literally explode when they touch water!
Special Groups: The Extremes of Chemistry
Group 0 (noble gases) are chemistry's loners - they have full outer shells (8 electrons, except helium with 2) and rarely react with anything. They're perfectly content on their own, which makes them super stable and unreactive.
Group 7 (halogens) are one electron short of happiness, so they're desperate to grab that missing electron. They'll react with metals to form ionic compounds or team up with non-metals to share electrons in covalent compounds.
As you travel down any group, patterns emerge. For halogens, molecular mass increases along with melting and boiling points - it's like they get heavier and need more energy to change state.
Pattern Spotter: The periodic table is full of trends - once you spot one pattern, you can predict properties of elements you've never studied!
The beauty of the periodic table lies in its predictability - master the patterns, and you've cracked the code of chemistry!
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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
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
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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
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
Understanding Atomic Structure and the Periodic Table: Key Concepts
Chelsea Tepsiri
@chelseatep_
Ever wondered what everything around you is made of? It all comes down to atoms - the tiny building blocks that make up your phone, your lunch, even you! Understanding atomic structure and the periodic table is like having a... Show more
Atoms, Elements and Compounds
Think of atoms as the LEGO bricks of the universe - they're the smallest parts of elements that still keep their identity. Each element gets its own chemical symbol, like a name tag (think H for hydrogen, O for oxygen).
When two or more elements team up chemically, they form compounds. This isn't just mixing - it's like a chemical marriage that creates something completely new. You can only break up compounds using chemical reactions, not just by stirring or filtering.
Mixtures are totally different - imagine throwing different LEGO sets into one box. The pieces aren't stuck together, so you can separate them using simple methods like filtration or distillation. No chemical reactions needed!
Quick Tip: If you can separate it physically (like picking out raisins from cereal), it's a mixture. If you need a chemical reaction, it's a compound!
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How We Discovered What Atoms Look Like
Scientists once thought atoms were like tiny, solid snooker balls that couldn't be broken apart. Boy, were they wrong! The discovery of electrons led to the "plum-pudding model" - imagine a positively charged cake with negatively charged raisins (electrons) scattered throughout.
Everything changed with the alpha particle scattering experiment. Scientists fired particles at thin gold foil and watched where they went. Most went straight through, but some bounced back at weird angles - even straight back! This was like throwing tennis balls at tissue paper and having some bounce back at your face.
This experiment revealed that atoms have a tiny, dense nucleus at their centre where all the positive charge hangs out. Niels Bohr then figured out that electrons orbit this nucleus like planets around the sun.
Later discoveries by James Chadwick revealed neutrons - particles with no charge that live in the nucleus alongside protons.
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
The Numbers Game: Atomic Structure
Here's where atoms get their ID cards! The atomic number tells you how many protons an atom has - it's like an atom's fingerprint because every element has a unique number. The mass number is the total count of protons plus neutrons.
Isotopes are like siblings in the atomic world - same element (same number of protons) but different numbers of neutrons. They're basically the same person wearing different weight clothing!
Electronic structure shows how electrons arrange themselves in shells around the nucleus. For sodium (Na), it's 2,8,1 - meaning 2 electrons in the first shell, 8 in the second, and 1 lonely electron in the outer shell.
Memory Hack: Think of electron shells like seats on a bus - the inner seats fill up first before anyone sits in the back!
Relative atomic mass is an average that accounts for all the different isotopes of an element and how common they are.
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The Periodic Table: Chemistry's Greatest Cheat Sheet
The periodic table arranges all elements by atomic number, and here's the genius bit - elements with similar properties line up in columns called groups. It's like organising your music playlist where all the rock songs are together!
Mendeleev was the mastermind behind this organisation, though he initially used atomic mass. He was so confident in his pattern that he left gaps for elements that hadn't been discovered yet - and he was right!
Metals hang out on the left side and love giving away electrons to form positive ions. Non-metals chill on the right and prefer grabbing electrons to form negative ions. This behaviour determines how elements react with each other.
Group 1 (alkali metals) are the drama queens of chemistry - they have one electron in their outer shell and react explosively with water. The bigger they get (going down the group), the more reactive they become!
Safety Note: Never mess about with Group 1 metals in real life - they can literally explode when they touch water!
Sign up to see the contentIt's free!
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By signing up you accept Terms of Service and Privacy Policy
Special Groups: The Extremes of Chemistry
Group 0 (noble gases) are chemistry's loners - they have full outer shells (8 electrons, except helium with 2) and rarely react with anything. They're perfectly content on their own, which makes them super stable and unreactive.
Group 7 (halogens) are one electron short of happiness, so they're desperate to grab that missing electron. They'll react with metals to form ionic compounds or team up with non-metals to share electrons in covalent compounds.
As you travel down any group, patterns emerge. For halogens, molecular mass increases along with melting and boiling points - it's like they get heavier and need more energy to change state.
Pattern Spotter: The periodic table is full of trends - once you spot one pattern, you can predict properties of elements you've never studied!
The beauty of the periodic table lies in its predictability - master the patterns, and you've cracked the code of chemistry!
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?
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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