Read nuclear notation
The top number is mass number and the bottom number is atomic number. Use them to find protons, neutrons and electrons in a neutral atom, then explain why isotopes have similar chemical properties.
GCSE Physics resources
AQA GCSE Atomic structure
Atoms, Isotopes and Nuclear Notation
The top number is mass number and the bottom number is atomic number. Use them to find protons, neutrons and electrons in a neutral atom, then explain why isotopes have similar chemical properties.
Start here
The quick revision snapshot
The top number is mass number and the bottom number is atomic number. Use them to find protons, neutrons and electrons in a neutral atom, then explain why isotopes have similar chemical properties.
By the end, you should be able to…
- Explain read nuclear notation using precise physics language.
- Apply the relevant equations, diagrams or practical method to an exam-style situation.
- Use the mini quizzes and practice answers to identify and correct a misconception.
Core learning content
Learn the idea before you practise it
Start with this short teaching sequence for Atoms, Isotopes and Nuclear Notation. Read the model, use the visual, then test your recall before moving into examples and exam practice.
Build the picture
Atomic number counts protons and identifies the element. Mass number counts protons plus neutrons, so neutrons are found by subtraction. Isotopes have the same electron arrangement when neutral, which is why they have similar chemical behaviour despite different masses.
Use the right relationship
Choose a relationship because of the physical change in the question, not because it is familiar. Activity after half-lives: remaining activity = initial activity × (½)ⁿ (Bq or counts/s). Mass number: A = protons + neutrons (no unit). Atomic number: Z = number of protons (no unit). Keep the unit next to each value while you substitute.
What a strong answer does
Write the two numbers beside the symbol, state what each counts and subtract atomic number from mass number for neutrons. Before writing, underline the command word and identify the information that matters. Keep a calculation as equation, substitution and answer with unit; keep an explanation as a linked chain using because, so or therefore. Finish by checking that the final sentence answers the exact context in the question and that the number, direction or conclusion agrees with the evidence given.
Retrieval practice
Quick checks
Answer without looking back first. The feedback explains the model, so a wrong answer still helps you learn.
Mini quiz 1
Check the core idea
Mini quiz 2
Apply the model
Mini quiz 3
Use exam precision
Try it
Atom and isotope builder
How can the same element have different masses?
What to notice: the readout and written explanation remain available if you do not use the controls.
Worked examples
See the method being built
Cover the answer, attempt the method and then compare your physics language as well as the number.
Example 1 · Read notation
For carbon-14, atomic number 6, find protons, neutrons and electrons in a neutral atom.
- Protons = atomic number.
- Neutrons = 14 − 6.
- Neutral electrons = protons.
Show the answer
It has 6 protons, 8 neutrons and 6 electrons.
Example 2 · Identify isotopes
Compare chlorine-35 and chlorine-37.
- Check atomic number.
- Compare mass number.
- State what changes.
Show the answer
They have the same number of protons but different numbers of neutrons, so they are isotopes of chlorine.
Example 3 · Ion calculation
A magnesium atom has 12 protons and forms Mg²⁺. State its electrons.
- A neutral atom has 12 electrons.
- 2+ means two electrons lost.
- Subtract 2.
Show the answer
Mg²⁺ has 10 electrons.
Misconception clinic
Common wrong turns
Each of these is tempting because it sounds almost right. Replace it with the precise physics before you meet it in a question.
- Reversing atomic and mass number.
- Saying isotopes have different protons.
- Forgetting electrons equal protons only in a neutral atom.
Independent practice
Exam-style questions
Use the working space first. Reveal the guidance only after you have committed to an answer, then improve the exact part that would gain the next mark.
1. State the charge on a proton.
[1 mark]Show answer and marking guidance
+1.
2. An atom has mass number 23 and atomic number 11. Find neutrons.
[2 marks]Show answer and marking guidance
23 − 11 = 12.
3. Define isotope.
[2 marks]Show answer and marking guidance
Atoms of the same element with the same number of protons but different numbers of neutrons.
4. State the number of electrons in a neutral atom with atomic number 8.
[1 mark]Show answer and marking guidance
8.
5. Explain why isotopes have similar chemical properties.
[3 marks]Show answer and marking guidance
They have the same electron arrangement when neutral.
6. An ion has 17 protons and 18 electrons. State its charge.
[2 marks]Show answer and marking guidance
−1, because it has one extra electron.
Questions pupils ask
GCSE Physics FAQs
What is the key idea in Atoms, Isotopes and Nuclear Notation?
Atomic-structure questions combine a model of the atom, radiation properties and careful use of nuclear notation. Always distinguish irradiation from contamination.
How should I practise Atoms, Isotopes and Nuclear Notation?
Read the snapshot, attempt the worked examples without looking, complete the mini quizzes and then use the practice questions to check your wording and method.
Next step
Keep your revision moving
Physics sticks when you move between explanation, retrieval and application.