Biology
Chemistry
AP Chemistry
English
AP English Literature
30
•
Dec 10, 2025
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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 specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
You can download the app in the Google Play Store and in the 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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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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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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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
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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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Improve your grades
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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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Improve your grades
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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 specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
You can download the app in the Google Play Store and in the 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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
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
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
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
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
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 app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
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