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30 Dec 2025
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Understanding Mass Number, Atomic Number, and Isotopes in Chemistry
JJ
@jjstudymaster
1 / 6
Understanding Atomic Structure and Mass Spectrometry
Learning about atomic number and mass number is like having the secret code to unlock any atom's structure. The atomic number (Z) tells you exactly how many protons are in an atom's nucleus, while the mass number (A) gives you the total count of protons and neutrons combined.
Mass spectrometry is the detective tool that scientists use to identify elements and measure their masses with incredible precision. A time of flight mass spectrometer works by ionising atoms, accelerating them to the same kinetic energy, then measuring how long they take to drift through the instrument - heavier ions take longer to reach the detector.
This technique doesn't just identify elements; it also reveals the relative abundance of different isotopes in a sample. You'll need to interpret mass spectra and calculate relative atomic masses from isotopic data for your exams.
Key Insight: Mass spectrometry can determine both what elements are present and their relative molecular masses - making it essential for everything from forensics to space exploration.
Nuclear Symbols and Atomic Particles
The nuclear symbol is your roadmap to understanding any atom's composition. Written as ᴬₓX (where A is the mass number, Z is the atomic number, and X is the element symbol), it contains all the information you need to work out particle numbers.
Atomic number equals the number of protons, which defines what element you're dealing with. The mass number is the sum of protons and neutrons (the particles in the nucleus). To find neutrons, simply subtract: neutrons = mass number - atomic number.
Relative atomic mass compares an atom's mass to 1/12 the mass of a carbon-12 atom. This standardised scale allows scientists to compare masses across all elements consistently.
Pro Tip: Remember that the atomic number never changes for an element - it's like the element's fingerprint. Change the number of protons, and you get a completely different element!
Ions and Isotopes Explained
Ions are simply atoms that have gained or lost electrons, giving them an overall charge. When an atom becomes negatively charged, it has more electrons than protons. Conversely, positive ions have fewer electrons than protons.
For example, Br⁻ has gained one extra electron (36 electrons vs 35 protons), while Mg²⁺ has lost two electrons (10 electrons vs 12 protons). The key is that the number of protons never changes - only electrons move around.
Isotopes are different versions of the same element with identical numbers of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.
Remember: Isotopes behave identically in chemical reactions because they have the same number of electrons in their outer shell - it's only their mass that differs.
Working with Isotopes
Isotopes of an element share the same chemical properties because they have identical electron configurations. Chlorine-35 and chlorine-37 both have 17 protons and 17 electrons, but chlorine-35 has 18 neutrons whilst chlorine-37 has 20 neutrons.
Magnesium provides another brilliant example with three naturally occurring isotopes: ²⁴Mg (79%), ²⁵Mg (10%), and ²⁶Mg (11%). Each has 12 protons and 12 electrons, but they contain 12, 13, and 14 neutrons respectively.
The percentages in brackets show how abundant each isotope is in nature. These isotopic abundances are crucial for calculating relative atomic masses - you can't just average the mass numbers because some isotopes are much more common than others.
Exam Focus: You'll often be asked to calculate relative atomic mass using isotopic abundance data - practice this calculation as it appears frequently in A-level papers.
Calculating Relative Atomic and Molecular Mass
Relative atomic mass calculations require you to account for isotopic abundances. For chlorine (75% ³⁵Cl and 25% ³⁷Cl), the calculation is: (35 × 75 + 37 × 25) ÷ 100 = 35.5. Notice how the result isn't a whole number because it's a weighted average.
Relative molecular mass is straightforward - just add up all the relative atomic masses in the molecule. For C₂H₆O: (2 × 12.0) + (6 × 1.0) + (1 × 16.0) = 46.0.
You can use either percentages or decimals in your abundance calculations. Many students find decimals easier: (35 × 0.75) + (37 × 0.25) = 35.5.
Calculation Tip: Always check that your relative atomic mass falls between the lightest and heaviest isotopes - if it doesn't, you've made an error somewhere.
Relative Formula Mass for Ionic Compounds
Relative formula mass works exactly like relative molecular mass, except it's used for ionic compounds that don't form discrete molecules. You're still adding up atomic masses, but now you're dealing with formula units instead of molecules.
For CaF₂, you need one calcium ion (40.1) plus two fluoride ions (2 × 19.0), giving 78.1. With more complex compounds like CaCO₃, break down the carbonate ion: one calcium (40.1) + one carbon (12.0) + three oxygens (3 × 16.0) = 100.1.
The calculation method is identical whether you're dealing with molecules or ionic compounds - the only difference is the terminology. Don't let this confuse you in exams.
Memory Aid: Whether it's molecular mass or formula mass, you're always doing the same thing: adding up the atomic masses of everything in the compound.
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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.
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Elisha
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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
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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
Understanding Mass Number, Atomic Number, and Isotopes in Chemistry
JJ
@jjstudymaster
Ever wondered how we know the exact makeup of atoms and why some elements behave differently despite being the "same" element? This topic covers everything you need to know about atomic structure, from counting particles in atoms to understanding why... Show more
Understanding Atomic Structure and Mass Spectrometry
Learning about atomic number and mass number is like having the secret code to unlock any atom's structure. The atomic number (Z) tells you exactly how many protons are in an atom's nucleus, while the mass number (A) gives you the total count of protons and neutrons combined.
Mass spectrometry is the detective tool that scientists use to identify elements and measure their masses with incredible precision. A time of flight mass spectrometer works by ionising atoms, accelerating them to the same kinetic energy, then measuring how long they take to drift through the instrument - heavier ions take longer to reach the detector.
This technique doesn't just identify elements; it also reveals the relative abundance of different isotopes in a sample. You'll need to interpret mass spectra and calculate relative atomic masses from isotopic data for your exams.
Key Insight: Mass spectrometry can determine both what elements are present and their relative molecular masses - making it essential for everything from forensics to space exploration.
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Nuclear Symbols and Atomic Particles
The nuclear symbol is your roadmap to understanding any atom's composition. Written as ᴬₓX (where A is the mass number, Z is the atomic number, and X is the element symbol), it contains all the information you need to work out particle numbers.
Atomic number equals the number of protons, which defines what element you're dealing with. The mass number is the sum of protons and neutrons (the particles in the nucleus). To find neutrons, simply subtract: neutrons = mass number - atomic number.
Relative atomic mass compares an atom's mass to 1/12 the mass of a carbon-12 atom. This standardised scale allows scientists to compare masses across all elements consistently.
Pro Tip: Remember that the atomic number never changes for an element - it's like the element's fingerprint. Change the number of protons, and you get a completely different element!
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Ions and Isotopes Explained
Ions are simply atoms that have gained or lost electrons, giving them an overall charge. When an atom becomes negatively charged, it has more electrons than protons. Conversely, positive ions have fewer electrons than protons.
For example, Br⁻ has gained one extra electron (36 electrons vs 35 protons), while Mg²⁺ has lost two electrons (10 electrons vs 12 protons). The key is that the number of protons never changes - only electrons move around.
Isotopes are different versions of the same element with identical numbers of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.
Remember: Isotopes behave identically in chemical reactions because they have the same number of electrons in their outer shell - it's only their mass that differs.
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Working with Isotopes
Isotopes of an element share the same chemical properties because they have identical electron configurations. Chlorine-35 and chlorine-37 both have 17 protons and 17 electrons, but chlorine-35 has 18 neutrons whilst chlorine-37 has 20 neutrons.
Magnesium provides another brilliant example with three naturally occurring isotopes: ²⁴Mg (79%), ²⁵Mg (10%), and ²⁶Mg (11%). Each has 12 protons and 12 electrons, but they contain 12, 13, and 14 neutrons respectively.
The percentages in brackets show how abundant each isotope is in nature. These isotopic abundances are crucial for calculating relative atomic masses - you can't just average the mass numbers because some isotopes are much more common than others.
Exam Focus: You'll often be asked to calculate relative atomic mass using isotopic abundance data - practice this calculation as it appears frequently in A-level papers.
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Calculating Relative Atomic and Molecular Mass
Relative atomic mass calculations require you to account for isotopic abundances. For chlorine (75% ³⁵Cl and 25% ³⁷Cl), the calculation is: (35 × 75 + 37 × 25) ÷ 100 = 35.5. Notice how the result isn't a whole number because it's a weighted average.
Relative molecular mass is straightforward - just add up all the relative atomic masses in the molecule. For C₂H₆O: (2 × 12.0) + (6 × 1.0) + (1 × 16.0) = 46.0.
You can use either percentages or decimals in your abundance calculations. Many students find decimals easier: (35 × 0.75) + (37 × 0.25) = 35.5.
Calculation Tip: Always check that your relative atomic mass falls between the lightest and heaviest isotopes - if it doesn't, you've made an error somewhere.
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Relative Formula Mass for Ionic Compounds
Relative formula mass works exactly like relative molecular mass, except it's used for ionic compounds that don't form discrete molecules. You're still adding up atomic masses, but now you're dealing with formula units instead of molecules.
For CaF₂, you need one calcium ion (40.1) plus two fluoride ions (2 × 19.0), giving 78.1. With more complex compounds like CaCO₃, break down the carbonate ion: one calcium (40.1) + one carbon (12.0) + three oxygens (3 × 16.0) = 100.1.
The calculation method is identical whether you're dealing with molecules or ionic compounds - the only difference is the terminology. Don't let this confuse you in exams.
Memory Aid: Whether it's molecular mass or formula mass, you're always doing the same thing: adding up the atomic masses of everything in the compound.
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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