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Responding to change (a2 only)
Infection and response
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Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
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Britain & the wider world: 1745 -1901
1l the quest for political stability: germany, 1871-1991
The cold war
Inter-war germany
Medieval period: 1066 -1509
2d religious conflict and the church in england, c1529-c1570
2o democracy and nazism: germany, 1918-1945
1f industrialisation and the people: britain, c1783-1885
1c the tudors: england, 1485-1603
2m wars and welfare: britain in transition, 1906-1957
World war two & the holocaust
2n revolution and dictatorship: russia, 1917-1953
2s the making of modern britain, 1951-2007
World war one
Britain: 1509 -1745
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11 Dec 2025
•
Alexia C. Tario
@kikki_05
Understanding atomic structure is crucial for mastering chemistry - it's... Show more
Every atom is made up of three fundamental particles that determine its identity and behaviour. The nucleus contains protons and neutrons (charge 0, mass 1), whilst electrons orbit around it.
Your atomic number tells you exactly how many protons and electrons an atom has - they're always equal in a neutral atom. The atomic mass is simply the total number of protons plus neutrons, which gives you the atom's weight.
Isotopes are atoms of the same element that have different numbers of neutrons but identical numbers of protons and electrons. For example, carbon can exist as ¹²C, ¹³C, and ¹⁴C - all with 6 protons but 6, 7, or 8 neutrons respectively. Since chemical properties depend on electrons, all isotopes of an element behave identically in reactions.
Quick Tip: Remember that atomic number = protons = electrons, whilst atomic mass = protons + neutrons.
Relative atomic mass (Ar) isn't just a simple average - it's a weighted average that accounts for how common each isotope is in nature. The formula is: Ar = Σ(% abundance × isotopic mass) ÷ Σ(% abundance).
Let's see this in action with copper. Cu-63 makes up 69.2% of natural copper, whilst Cu-65 accounts for 30.8%. Using the formula: Ar = ÷ 100 = 63.6.
Mass spectrometry is the technique scientists use to identify these isotopes and their abundances. The process involves four key steps: ionisation (creating charged particles), acceleration (speeding them up), ion drift (separating by mass), and detection (measuring the results).
Real World: Mass spectrometry is used in everything from drug testing to space exploration!
Electrons don't just randomly orbit the nucleus - they occupy specific energy levels called shells. Think of these like the floors of a building, where each floor can only hold a certain number of residents.
The maximum number of electrons in each shell follows the 2n² rule, where n is the shell number. Shell 1 holds 2 electrons, shell 2 holds 8, shell 3 holds 18, and shell 4 holds 32 electrons maximum.
Within each shell, electrons occupy sub-shells (s, p, d, f) that have different shapes and capacities. The s sub-shell holds 2 electrons, p holds 6, d holds 10, and f holds 14. Shell 1 only has s, shell 2 has s and p, shell 3 has s, p, and d, whilst shell 4 contains all four types.
Memory Trick: The sub-shell capacities (2, 6, 10, 14) follow the pattern of adding 4 each time!
Orbitals have distinct shapes that determine where you're likely to find electrons. The s orbital is spherical, whilst p orbitals are dumbbell-shaped and come in three orientations (px, py, pz) pointing along different axes.
When writing electron configurations, you fill orbitals starting from the lowest energy level. For example, carbon (6 electrons) has the configuration 1s² 2s² 2p², showing exactly where each electron resides.
The filling pattern follows a specific order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. Notice that 4s fills before 3d because it's actually at a lower energy level despite being in the "fourth shell".
Study Tip: Draw out the orbital filling diagram - visual learners find this much easier than memorising the sequence!
Pauli's Exclusion Principle states that each orbital can hold a maximum of two electrons, and they must have opposite spins (represented as ↑↓). This prevents electrons from occupying the same space with identical properties.
When electrons enter orbitals of equal energy (like the three p orbitals), they prefer to occupy separate orbitals first before pairing up. This Hund's rule minimises electron repulsion and creates more stable atoms.
Shorthand notation makes writing electron configurations much quicker. Instead of writing out every orbital, you can use the nearest noble gas in brackets. For example, calcium (20 electrons) becomes 4s² rather than the full 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
Exam Tip: Learn the electron configurations of the first 20 elements - they appear in most chemistry exams!
Ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms, creating positive ions. It's measured in kJ/mol and tells you how tightly an atom holds onto its electrons.
The process can happen multiple times: first ionisation energy removes the first electron , second ionisation energy removes the second , and so on. Each successive ionisation requires more energy because you're removing electrons from increasingly positive ions.
Down a group, ionisation energy decreases dramatically. As you move from lithium to sodium to potassium, the atoms get larger and the outer electrons become further from the nucleus. This weaker nuclear attraction makes electrons easier to remove.
Key Insight: Ionisation energy patterns help explain why metals lose electrons easily whilst non-metals prefer to gain them!
Across a period, ionisation energy generally increases from left to right. As you move from lithium to neon, the nuclear charge increases whilst the atomic radius decreases slightly, creating a stronger pull on the electrons.
However, there are important exceptions to this trend. Boron has a lower ionisation energy than beryllium because its electron is in a higher energy p orbital. Similarly, oxygen's ionisation energy is slightly lower than nitrogen's due to electron repulsion in paired orbitals.
These periodic trends in ionisation energy help predict chemical behaviour. Elements with low ionisation energies (like sodium) readily form positive ions, whilst those with high ionisation energies (like fluorine) prefer to gain electrons instead.
Exam Focus: Learn the exceptions to the general trend - examiners love testing these anomalies!
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Alexia C. Tario
@kikki_05
Understanding atomic structure is crucial for mastering chemistry - it's the foundation that explains how elements behave and react. You'll explore everything from the basic parts of an atom to electron configurations and ionisation energy trends.
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Every atom is made up of three fundamental particles that determine its identity and behaviour. The nucleus contains protons and neutrons (charge 0, mass 1), whilst electrons orbit around it.
Your atomic number tells you exactly how many protons and electrons an atom has - they're always equal in a neutral atom. The atomic mass is simply the total number of protons plus neutrons, which gives you the atom's weight.
Isotopes are atoms of the same element that have different numbers of neutrons but identical numbers of protons and electrons. For example, carbon can exist as ¹²C, ¹³C, and ¹⁴C - all with 6 protons but 6, 7, or 8 neutrons respectively. Since chemical properties depend on electrons, all isotopes of an element behave identically in reactions.
Quick Tip: Remember that atomic number = protons = electrons, whilst atomic mass = protons + neutrons.
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Relative atomic mass (Ar) isn't just a simple average - it's a weighted average that accounts for how common each isotope is in nature. The formula is: Ar = Σ(% abundance × isotopic mass) ÷ Σ(% abundance).
Let's see this in action with copper. Cu-63 makes up 69.2% of natural copper, whilst Cu-65 accounts for 30.8%. Using the formula: Ar = ÷ 100 = 63.6.
Mass spectrometry is the technique scientists use to identify these isotopes and their abundances. The process involves four key steps: ionisation (creating charged particles), acceleration (speeding them up), ion drift (separating by mass), and detection (measuring the results).
Real World: Mass spectrometry is used in everything from drug testing to space exploration!
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Improve your grades
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By signing up you accept Terms of Service and Privacy Policy
Electrons don't just randomly orbit the nucleus - they occupy specific energy levels called shells. Think of these like the floors of a building, where each floor can only hold a certain number of residents.
The maximum number of electrons in each shell follows the 2n² rule, where n is the shell number. Shell 1 holds 2 electrons, shell 2 holds 8, shell 3 holds 18, and shell 4 holds 32 electrons maximum.
Within each shell, electrons occupy sub-shells (s, p, d, f) that have different shapes and capacities. The s sub-shell holds 2 electrons, p holds 6, d holds 10, and f holds 14. Shell 1 only has s, shell 2 has s and p, shell 3 has s, p, and d, whilst shell 4 contains all four types.
Memory Trick: The sub-shell capacities (2, 6, 10, 14) follow the pattern of adding 4 each time!
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Orbitals have distinct shapes that determine where you're likely to find electrons. The s orbital is spherical, whilst p orbitals are dumbbell-shaped and come in three orientations (px, py, pz) pointing along different axes.
When writing electron configurations, you fill orbitals starting from the lowest energy level. For example, carbon (6 electrons) has the configuration 1s² 2s² 2p², showing exactly where each electron resides.
The filling pattern follows a specific order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. Notice that 4s fills before 3d because it's actually at a lower energy level despite being in the "fourth shell".
Study Tip: Draw out the orbital filling diagram - visual learners find this much easier than memorising the sequence!
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Pauli's Exclusion Principle states that each orbital can hold a maximum of two electrons, and they must have opposite spins (represented as ↑↓). This prevents electrons from occupying the same space with identical properties.
When electrons enter orbitals of equal energy (like the three p orbitals), they prefer to occupy separate orbitals first before pairing up. This Hund's rule minimises electron repulsion and creates more stable atoms.
Shorthand notation makes writing electron configurations much quicker. Instead of writing out every orbital, you can use the nearest noble gas in brackets. For example, calcium (20 electrons) becomes 4s² rather than the full 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
Exam Tip: Learn the electron configurations of the first 20 elements - they appear in most chemistry exams!
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Ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms, creating positive ions. It's measured in kJ/mol and tells you how tightly an atom holds onto its electrons.
The process can happen multiple times: first ionisation energy removes the first electron , second ionisation energy removes the second , and so on. Each successive ionisation requires more energy because you're removing electrons from increasingly positive ions.
Down a group, ionisation energy decreases dramatically. As you move from lithium to sodium to potassium, the atoms get larger and the outer electrons become further from the nucleus. This weaker nuclear attraction makes electrons easier to remove.
Key Insight: Ionisation energy patterns help explain why metals lose electrons easily whilst non-metals prefer to gain them!
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Across a period, ionisation energy generally increases from left to right. As you move from lithium to neon, the nuclear charge increases whilst the atomic radius decreases slightly, creating a stronger pull on the electrons.
However, there are important exceptions to this trend. Boron has a lower ionisation energy than beryllium because its electron is in a higher energy p orbital. Similarly, oxygen's ionisation energy is slightly lower than nitrogen's due to electron repulsion in paired orbitals.
These periodic trends in ionisation energy help predict chemical behaviour. Elements with low ionisation energies (like sodium) readily form positive ions, whilst those with high ionisation energies (like fluorine) prefer to gain electrons instead.
Exam Focus: Learn the exceptions to the general trend - examiners love testing these anomalies!
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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ChemistryIt’s talks about the creators of each atom and what they ere made for.
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ChemistryDiscussed here are the behavior of electrons, quantum mechanical theory, and electromagnetic radiation.
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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
Sociology