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Chemistry
honors chem 1 unit 3 study guide (periodic trends and electron structure)
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Nov 18, 2025
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Quantum mechanics revolutionized our understanding of atoms and electrons. This... Show more
The Bohr model correctly proposed that electrons exist in specific energy levels and can't be found between them. When electrons absorb energy (from flame or electricity), they jump to higher levels—an excited state. When they drop back to their ground state, they emit energy, usually as light.
The color of light depends on the energy transition. Red light comes from level 3→2 transitions, green from 4→2, blue from 5→2, and violet from 6→2. This is why different elements produce different colored flames!
Quantum theory changed our understanding of light itself. Light travels as both waves AND as discrete bundles called photons. Higher frequency light has higher photon energy, calculated using Planck's constant: Energy = 6.626 × 10⁻³⁴ J-sec × frequency.
Think about it: If light shows both wave and particle properties, could other things do the same? This led to the discovery that electrons also behave as waves, particularly when they're moving—a concept called wave-particle duality.
Heisenberg's uncertainty principle tells us something fascinating—it's impossible to know both an electron's exact position AND velocity at the same time. This isn't about measurement limitations; it's a fundamental property of reality that Schrödinger mapped through probability calculations.
Forget the neat electron orbits you've seen before! Electrons actually exist in orbitals—fuzzy, cloud-like regions where we're 90% certain to find them. These orbitals come in different shapes and correspond to specific energy levels.
The quantum mechanical model organizes electrons into a hierarchy. Each energy level (1-7) contains sublevels (s, p, d, f), which contain orbitals, which each hold up to 2 electrons. The s sublevel has 1 orbital (2 electrons max), p has 3 orbitals (6 electrons max), d has 5 orbitals (10 electrons max), and f has 7 orbitals (14 electrons max).
When filling orbitals, three important rules apply. The Aufbau principle says electrons fill lowest energy levels first (s before p, etc.). The Pauli exclusion principle states each orbital holds maximum 2 electrons with opposite spins. Hund's rule requires all orbitals in a sublevel to get one electron before any pairs up.
Pro tip: Notice how the periodic table's organization reflects electron configurations! The s-block elements are groups 1-2, p-block are groups 13-18, d-block are transition metals, and f-block are lanthanides and actinides.
Writing out full electron configurations can be tedious, but there's a shortcut! You can use the periodic table to determine configurations—the row indicates the sublevel, and the group number helps identify electron count. For even greater efficiency, you can use noble gas notation by putting the previous noble gas symbol in brackets.
Valence electrons are the most important electrons for chemistry—they're in the outermost energy level and participate in bonding. Elements in the same group have the same number of valence electrons . We represent these visually with Lewis dot structures that show valence electrons as dots around the element symbol.
Elements generally want to achieve a stable octet (8 valence electrons) like noble gases. When forming ions, atoms typically gain or lose the fewest electrons possible. Metals tend to lose electrons to form positive cations, while nonmetals gain electrons to form negative anions.
Remember: You can predict the charge of an ion by looking at its group number! Group 1 metals form 1+ ions, Group 2 form 2+ ions, and so on. Similarly, Group 17 (halogens) form 1- ions, and Group 16 form 2- ions.
Atomic radius increases down a group but decreases across a period. Why? Down a group, more energy levels mean electrons are further from the nucleus. Across a period, the increasing number of protons pulls electrons tighter despite having the same energy level.
When atoms form ions, their size changes. Anions (negative ions) are larger than their neutral atoms because extra electrons increase repulsion. Cations (positive ions) are smaller than their atoms because they've lost electrons, reducing repulsion.
Ionization energy (energy needed to remove an electron) increases across periods and decreases down groups. Smaller atoms hold their electrons more tightly! Each subsequent electron removal requires more energy, with a significant jump when removing core electrons.
Electronegativity measures an atom's ability to attract shared electrons. It follows the same pattern as ionization energy—increasing across periods and decreasing down groups. This helps explain reactivity patterns: metals (which lose electrons) become more reactive down groups, while nonmetals (which gain electrons) become more reactive across periods.
Exam tip: Remember these trends by thinking about "nuclear pull" versus "distance." The stronger the nuclear pull and the shorter the distance between the nucleus and outer electrons, the smaller the atom, the higher the ionization energy, and the greater the electronegativity.
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.
App Store
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
Quantum mechanics revolutionized our understanding of atoms and electrons. This model explains how electrons behave as both particles and waves, occupying probability-based orbitals rather than fixed paths, and helps us understand atomic properties and chemical behavior.
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The Bohr model correctly proposed that electrons exist in specific energy levels and can't be found between them. When electrons absorb energy (from flame or electricity), they jump to higher levels—an excited state. When they drop back to their ground state, they emit energy, usually as light.
The color of light depends on the energy transition. Red light comes from level 3→2 transitions, green from 4→2, blue from 5→2, and violet from 6→2. This is why different elements produce different colored flames!
Quantum theory changed our understanding of light itself. Light travels as both waves AND as discrete bundles called photons. Higher frequency light has higher photon energy, calculated using Planck's constant: Energy = 6.626 × 10⁻³⁴ J-sec × frequency.
Think about it: If light shows both wave and particle properties, could other things do the same? This led to the discovery that electrons also behave as waves, particularly when they're moving—a concept called wave-particle duality.
Heisenberg's uncertainty principle tells us something fascinating—it's impossible to know both an electron's exact position AND velocity at the same time. This isn't about measurement limitations; it's a fundamental property of reality that Schrödinger mapped through probability calculations.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Forget the neat electron orbits you've seen before! Electrons actually exist in orbitals—fuzzy, cloud-like regions where we're 90% certain to find them. These orbitals come in different shapes and correspond to specific energy levels.
The quantum mechanical model organizes electrons into a hierarchy. Each energy level (1-7) contains sublevels (s, p, d, f), which contain orbitals, which each hold up to 2 electrons. The s sublevel has 1 orbital (2 electrons max), p has 3 orbitals (6 electrons max), d has 5 orbitals (10 electrons max), and f has 7 orbitals (14 electrons max).
When filling orbitals, three important rules apply. The Aufbau principle says electrons fill lowest energy levels first (s before p, etc.). The Pauli exclusion principle states each orbital holds maximum 2 electrons with opposite spins. Hund's rule requires all orbitals in a sublevel to get one electron before any pairs up.
Pro tip: Notice how the periodic table's organization reflects electron configurations! The s-block elements are groups 1-2, p-block are groups 13-18, d-block are transition metals, and f-block are lanthanides and actinides.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Writing out full electron configurations can be tedious, but there's a shortcut! You can use the periodic table to determine configurations—the row indicates the sublevel, and the group number helps identify electron count. For even greater efficiency, you can use noble gas notation by putting the previous noble gas symbol in brackets.
Valence electrons are the most important electrons for chemistry—they're in the outermost energy level and participate in bonding. Elements in the same group have the same number of valence electrons . We represent these visually with Lewis dot structures that show valence electrons as dots around the element symbol.
Elements generally want to achieve a stable octet (8 valence electrons) like noble gases. When forming ions, atoms typically gain or lose the fewest electrons possible. Metals tend to lose electrons to form positive cations, while nonmetals gain electrons to form negative anions.
Remember: You can predict the charge of an ion by looking at its group number! Group 1 metals form 1+ ions, Group 2 form 2+ ions, and so on. Similarly, Group 17 (halogens) form 1- ions, and Group 16 form 2- ions.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Atomic radius increases down a group but decreases across a period. Why? Down a group, more energy levels mean electrons are further from the nucleus. Across a period, the increasing number of protons pulls electrons tighter despite having the same energy level.
When atoms form ions, their size changes. Anions (negative ions) are larger than their neutral atoms because extra electrons increase repulsion. Cations (positive ions) are smaller than their atoms because they've lost electrons, reducing repulsion.
Ionization energy (energy needed to remove an electron) increases across periods and decreases down groups. Smaller atoms hold their electrons more tightly! Each subsequent electron removal requires more energy, with a significant jump when removing core electrons.
Electronegativity measures an atom's ability to attract shared electrons. It follows the same pattern as ionization energy—increasing across periods and decreasing down groups. This helps explain reactivity patterns: metals (which lose electrons) become more reactive down groups, while nonmetals (which gain electrons) become more reactive across periods.
Exam tip: Remember these trends by thinking about "nuclear pull" versus "distance." The stronger the nuclear pull and the shorter the distance between the nucleus and outer electrons, the smaller the atom, the higher the ionization energy, and the greater the electronegativity.
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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From Unit 2, covers Solutions, Suspensions, and Colloids :)
ChemistryRadioactive chemicals, Decay, Nuclear Fission&Fusion
ChemistryDevelopment of Modern Atomic Theory
ChemistryDiscovery of atoms to 1937
ChemistryThis includes early chemical discoveries and atomic theory, electrons and other discoveries in the atomic physics.
ChemistryBasics of Atoms and theory including atom history.
ChemistryApp Store
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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
