Biology
Chemistry
AP Chemistry
English
AP English Literature
Chemistry
Dec 2, 2025
104
3 pages
Dive into the fascinating world of atoms and atomic theory! This study explores the foundational concepts of atomic... Show more
The journey to understand atoms began with some key chemical laws. In 1774, Antoine Lavoisier demonstrated the Law of Conservation of Mass by heating tin in a sealed vessel and showing that mass remains the same before and after a chemical reaction. Soon after, Joseph Proust established the Law of Constant Composition, proving that compounds always contain the same proportions of elements by mass.
John Dalton, an English teacher, revolutionized chemistry with his Atomic Theory. He proposed that elements consist of tiny, indivisible particles called atoms that can't be created or destroyed. Dalton suggested that all atoms of an element are identical in mass and properties but different from atoms of other elements. His theory also stated that elements combine in simple numerical ratios to form compounds.
The study of electricity led to further atomic discoveries. Scientists found that objects can have either positive or negative electric charges, with opposite charges attracting each other and like charges repelling. Objects with equal numbers of positive and negative particles are electrically neutral.
💡 Try This Think about a chocolate chip cookie as a model for Dalton's "indivisible" atom. Later, we'll see how scientists discovered the atom could actually be split into smaller parts—just like you could separate the chocolate chips from the cookie dough!
The mystery of the atom began to unravel when J.J. Thomson discovered the electron in 1897. Using cathode-ray tubes (vacuum glass tubes with electricity passing through them), Thomson identified negatively charged particles that were part of all atoms. Later, Robert Millikan precisely measured the electron's charge through his famous oil-drop experiments.
The discovery path continued with Wilhelm Roentgen's identification of X-rays in 1895. This led Antoine Becquerel to discover radioactivity, and soon after, Ernest Rutherford identified alpha and beta particles emitted by radioactive substances. Paul Villard completed the trio by discovering highly penetrating gamma rays.
Thomson initially proposed the "Plum-Pudding" model of the atom—a sphere of positive charge with embedded electrons. However, this model was overturned by Rutherford's famous gold foil experiment around 1910. When Rutherford directed alpha particles at thin gold foil, most passed through, but some deflected dramatically or even bounced back. This surprising result led him to propose the nuclear model of the atom, where most mass and all positive charge concentrated in a tiny central nucleus.
🔍 Key Insight Rutherford's gold foil experiment showed that atoms are mostly empty space! If an atom were the size of a football stadium, the nucleus would be smaller than a marble at the center.
The modern view of atomic structure recognizes three fundamental particles electrons , protons , and neutrons (no charge). Electrons move in the vast space surrounding the nucleus, which contains the tightly packed protons and neutrons. Electrons are attracted to the nucleus by the electrical force between opposite charges.
Atoms are identified by two important numbers. The atomic number (Z) represents the number of protons in the nucleus, which defines which element the atom is. The mass number (A) is the total count of protons and neutrons combined. You can easily calculate the number of neutrons by subtracting neutrons = A - Z.
Isotopes are atoms of the same element (same number of protons) that contain different numbers of neutrons. For example, carbon-12, carbon-13, and carbon-14 are all isotopes of carbon. We represent isotopes using the notation X, where X is the element symbol.
Most elements in nature occur as mixtures of isotopes with constant compositions. The atomic weight of an element accounts for this by calculating a weighted average of its isotope masses. The standard unit for atomic mass is defined as 1/12 the mass of a carbon-12 atom.
⚖️ Think About It Isotopes are like siblings in a family—they share the same "last name" (element) but have different "weights" (mass numbers) due to their varying numbers of neutrons!
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.
5
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Radioactive chemicals, Decay, Nuclear Fission&Fusion
ChemistryDiscovery of atoms to 1937
ChemistryExplore the fundamentals of atomic structure and the periodic table in this comprehensive summary. Understand key concepts such as atomic number, mass number, subatomic particles, and the classification of elements into groups. This resource covers the development of atomic models, the properties of noble gases, halogens, and alkali metals, as well as periodic trends in reactivity and physical properties. Ideal for students studying chemistry.
Chemistryimportant scientists and experiments that contributed to the current atomic model
ChemistryStudy note about the electron configuration
ChemistryFrom Unit 2, covers Solutions, Suspensions, and Colloids :)
ChemistryApp 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
Dive into the fascinating world of atoms and atomic theory! This study explores the foundational concepts of atomic structure, from early chemical laws to the modern understanding of what makes up everything around us. You'll learn how scientists gradually pieced... Show more
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The journey to understand atoms began with some key chemical laws. In 1774, Antoine Lavoisier demonstrated the Law of Conservation of Mass by heating tin in a sealed vessel and showing that mass remains the same before and after a chemical reaction. Soon after, Joseph Proust established the Law of Constant Composition, proving that compounds always contain the same proportions of elements by mass.
John Dalton, an English teacher, revolutionized chemistry with his Atomic Theory. He proposed that elements consist of tiny, indivisible particles called atoms that can't be created or destroyed. Dalton suggested that all atoms of an element are identical in mass and properties but different from atoms of other elements. His theory also stated that elements combine in simple numerical ratios to form compounds.
The study of electricity led to further atomic discoveries. Scientists found that objects can have either positive or negative electric charges, with opposite charges attracting each other and like charges repelling. Objects with equal numbers of positive and negative particles are electrically neutral.
💡 Try This: Think about a chocolate chip cookie as a model for Dalton's "indivisible" atom. Later, we'll see how scientists discovered the atom could actually be split into smaller parts—just like you could separate the chocolate chips from the cookie dough!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The mystery of the atom began to unravel when J.J. Thomson discovered the electron in 1897. Using cathode-ray tubes (vacuum glass tubes with electricity passing through them), Thomson identified negatively charged particles that were part of all atoms. Later, Robert Millikan precisely measured the electron's charge through his famous oil-drop experiments.
The discovery path continued with Wilhelm Roentgen's identification of X-rays in 1895. This led Antoine Becquerel to discover radioactivity, and soon after, Ernest Rutherford identified alpha and beta particles emitted by radioactive substances. Paul Villard completed the trio by discovering highly penetrating gamma rays.
Thomson initially proposed the "Plum-Pudding" model of the atom—a sphere of positive charge with embedded electrons. However, this model was overturned by Rutherford's famous gold foil experiment around 1910. When Rutherford directed alpha particles at thin gold foil, most passed through, but some deflected dramatically or even bounced back. This surprising result led him to propose the nuclear model of the atom, where most mass and all positive charge concentrated in a tiny central nucleus.
🔍 Key Insight: Rutherford's gold foil experiment showed that atoms are mostly empty space! If an atom were the size of a football stadium, the nucleus would be smaller than a marble at the center.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The modern view of atomic structure recognizes three fundamental particles: electrons , protons , and neutrons (no charge). Electrons move in the vast space surrounding the nucleus, which contains the tightly packed protons and neutrons. Electrons are attracted to the nucleus by the electrical force between opposite charges.
Atoms are identified by two important numbers. The atomic number (Z) represents the number of protons in the nucleus, which defines which element the atom is. The mass number (A) is the total count of protons and neutrons combined. You can easily calculate the number of neutrons by subtracting: neutrons = A - Z.
Isotopes are atoms of the same element (same number of protons) that contain different numbers of neutrons. For example, carbon-12, carbon-13, and carbon-14 are all isotopes of carbon. We represent isotopes using the notation X, where X is the element symbol.
Most elements in nature occur as mixtures of isotopes with constant compositions. The atomic weight of an element accounts for this by calculating a weighted average of its isotope masses. The standard unit for atomic mass is defined as 1/12 the mass of a carbon-12 atom.
⚖️ Think About It: Isotopes are like siblings in a family—they share the same "last name" (element) but have different "weights" (mass numbers) due to their varying numbers of neutrons!
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.
5
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Radioactive chemicals, Decay, Nuclear Fission&Fusion
ChemistryDiscovery of atoms to 1937
ChemistryExplore the fundamentals of atomic structure and the periodic table in this comprehensive summary. Understand key concepts such as atomic number, mass number, subatomic particles, and the classification of elements into groups. This resource covers the development of atomic models, the properties of noble gases, halogens, and alkali metals, as well as periodic trends in reactivity and physical properties. Ideal for students studying chemistry.
Chemistryimportant scientists and experiments that contributed to the current atomic model
ChemistryStudy note about the electron configuration
ChemistryFrom Unit 2, covers Solutions, Suspensions, and Colloids :)
ChemistryApp 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
