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J
Jessica Plane
07/12/2025
Chemistry
Chemistry: Topic 1
116
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7 Dec 2025
•
Introduction to Chemistry: Understanding Atoms and the Periodic Table
J
Jessica Plane
@essicalane_jeu6oyvga
1 / 3
Structure of the Atom
Atoms are incredibly small, with a radius of just 0.1 nanometers (0.0000000001m). The nucleus sits at the center, containing positively charged protons and neutral neutrons. Negatively charged electrons orbit around this nucleus in shells or energy levels.
Protons and neutrons have a relative mass of 1, while electrons are about 2000 times smaller. Since atoms are neutral overall, they must contain equal numbers of protons and electrons. The atomic number represents the number of protons, while the mass number tells us the atom's weight.
Scientists' understanding of atomic structure evolved over time. Dalton (1766-1844) proposed atoms as tiny solid particles. Rutherford's (1871-1937) experiments revealed atoms were mostly empty space with a dense nucleus. Bohr (1885-1962) developed the concept of electron configuration, showing electrons orbit in specific shells.
Did you know? The first shell can hold only 2 electrons, the second and third shells up to 8, and the fourth shell can accommodate up to 18 electrons! This pattern determines an element's chemical properties.
Isotopes are versions of the same element with identical numbers of protons and electrons but different numbers of neutrons. While chemically identical, they have different physical properties like melting points and density. The relative atomic mass (RAM) of an element considers the weighted average of its isotope masses based on their natural abundance.
The Periodic Table and Chemical Classification
The periodic table organizes elements into groups (columns) and periods (rows). Elements in the same group share similar properties because they have the same number of electrons in their outer shell. As you move down a group, reactivity patterns emerge - Group 1 metals become more reactive, while Group 7 halogens become less reactive.
Mixtures contain two or more elements or compounds that aren't chemically combined. They can be separated through physical methods like chromatography (for inks and dyes), filtration (for insoluble solids in liquids), or distillation (for separating solutions). These separation techniques rely on differences in physical properties rather than chemical reactions.
Compounds form when elements chemically combine. When naming them, certain rules apply: for two-element compounds, use the metal name followed by the non-metal with "-ide" ending . For compounds with three elements including oxygen, the non-oxygen, non-metal element gets an "-ate" ending .
Remember this! When balancing chemical equations, the number of atoms of each element must be equal on both sides. Coefficients (numbers before formulas) multiply all atoms in that compound.
Different regions of the periodic table show distinct properties. Transition metals in the center form colorful compounds, have multiple ion forms, and make excellent catalysts. Noble gases (Group 0) have full outer electron shells, making them extremely unreactive. Halogens (Group 7) exist as diatomic molecules and become less reactive as you move down the group.
Reactivity Patterns and Electron Behavior
Elements achieve stability by obtaining a full outer electron shell, but they do this in different ways. Metals tend to lose electrons to form positive ions, while non-metals gain or share electrons to form negative ions or covalent bonds.
Metal reactivity increases as you move down and left in the periodic table. This happens because larger atoms hold their outer electrons less tightly, making them easier to lose during reactions. Metals at the bottom left of the table (like potassium and sodium) react vigorously, even explosively, with water.
Non-metal reactivity follows the opposite pattern, increasing as you move toward the top right. Smaller non-metal atoms more readily gain the electrons they need to achieve a full outer shell. This explains why fluorine (at the top of Group 7) is the most reactive halogen.
Quick tip: Think of the periodic table as a reactivity map - metals become more reactive as you go down and left, while non-metals become more reactive as you go up and right!
The electron arrangement in the outer shell largely determines how elements interact. Group 1 elements have just one outer electron they're eager to lose, while Group 7 elements need just one more electron to achieve a full shell. These patterns make chemistry more predictable and help us understand how compounds form.
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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
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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.
Anna
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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
Introduction to Chemistry: Understanding Atoms and the Periodic Table
J
Jessica Plane
@essicalane_jeu6oyvga
Diving into the world of atoms and the periodic table is like exploring the fundamental building blocks of everything around us. These tiny particles determine how substances behave, react, and combine to form everything from water to metals. Understanding this... Show more
Structure of the Atom
Atoms are incredibly small, with a radius of just 0.1 nanometers (0.0000000001m). The nucleus sits at the center, containing positively charged protons and neutral neutrons. Negatively charged electrons orbit around this nucleus in shells or energy levels.
Protons and neutrons have a relative mass of 1, while electrons are about 2000 times smaller. Since atoms are neutral overall, they must contain equal numbers of protons and electrons. The atomic number represents the number of protons, while the mass number tells us the atom's weight.
Scientists' understanding of atomic structure evolved over time. Dalton (1766-1844) proposed atoms as tiny solid particles. Rutherford's (1871-1937) experiments revealed atoms were mostly empty space with a dense nucleus. Bohr (1885-1962) developed the concept of electron configuration, showing electrons orbit in specific shells.
Did you know? The first shell can hold only 2 electrons, the second and third shells up to 8, and the fourth shell can accommodate up to 18 electrons! This pattern determines an element's chemical properties.
Isotopes are versions of the same element with identical numbers of protons and electrons but different numbers of neutrons. While chemically identical, they have different physical properties like melting points and density. The relative atomic mass (RAM) of an element considers the weighted average of its isotope masses based on their natural abundance.
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The Periodic Table and Chemical Classification
The periodic table organizes elements into groups (columns) and periods (rows). Elements in the same group share similar properties because they have the same number of electrons in their outer shell. As you move down a group, reactivity patterns emerge - Group 1 metals become more reactive, while Group 7 halogens become less reactive.
Mixtures contain two or more elements or compounds that aren't chemically combined. They can be separated through physical methods like chromatography (for inks and dyes), filtration (for insoluble solids in liquids), or distillation (for separating solutions). These separation techniques rely on differences in physical properties rather than chemical reactions.
Compounds form when elements chemically combine. When naming them, certain rules apply: for two-element compounds, use the metal name followed by the non-metal with "-ide" ending . For compounds with three elements including oxygen, the non-oxygen, non-metal element gets an "-ate" ending .
Remember this! When balancing chemical equations, the number of atoms of each element must be equal on both sides. Coefficients (numbers before formulas) multiply all atoms in that compound.
Different regions of the periodic table show distinct properties. Transition metals in the center form colorful compounds, have multiple ion forms, and make excellent catalysts. Noble gases (Group 0) have full outer electron shells, making them extremely unreactive. Halogens (Group 7) exist as diatomic molecules and become less reactive as you move down the group.
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Reactivity Patterns and Electron Behavior
Elements achieve stability by obtaining a full outer electron shell, but they do this in different ways. Metals tend to lose electrons to form positive ions, while non-metals gain or share electrons to form negative ions or covalent bonds.
Metal reactivity increases as you move down and left in the periodic table. This happens because larger atoms hold their outer electrons less tightly, making them easier to lose during reactions. Metals at the bottom left of the table (like potassium and sodium) react vigorously, even explosively, with water.
Non-metal reactivity follows the opposite pattern, increasing as you move toward the top right. Smaller non-metal atoms more readily gain the electrons they need to achieve a full outer shell. This explains why fluorine (at the top of Group 7) is the most reactive halogen.
Quick tip: Think of the periodic table as a reactivity map - metals become more reactive as you go down and left, while non-metals become more reactive as you go up and right!
The electron arrangement in the outer shell largely determines how elements interact. Group 1 elements have just one outer electron they're eager to lose, while Group 7 elements need just one more electron to achieve a full shell. These patterns make chemistry more predictable and help us understand how compounds form.
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?
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Most popular content: Separations and Purifications
Most popular content in Chemistry
Most popular content
English - inspector calls quotes and analysis
Quotes from every main character
English Literature10
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.9/5
App Store
4.8/5
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