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.

AQA 8463AQA 8464 GCSE PhysicsCombined Science: Trilogy FoundationHigher

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The 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:

  • Read nuclear notation.
  • Apply the atomic structure model, evidence or method to an unfamiliar exam context.
  • Recognise and correct this wrong turn: Reversing atomic and mass number.

Before you revise

Diagnostic question

Choose an answer from memory. Your result tells you what to watch for in the guide.

Which statement correctly summarises atoms, isotopes and nuclear notation?

Core revision guide

Learn the model, then use it

Atoms, Isotopes and Nuclear Notation questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.

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.

Build the physics 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.

Relationships used in this guide

Mass number: A = protons + neutrons (no unit). Atomic number: Z = number of protons (no unit). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.

Nuclide notation and two carbon isotopes show the same six protons but different neutron numbers.
Atomic number fixes the element; isotopes differ in neutron number and therefore mass number.Open the full-size exam diagram
Mass numberA = protons + neutronsno unit
Atomic numberZ = number of protonsno unit

Misconception clinic

Replace the tempting answer

Read each belief, say what is wrong with it, then compare your correction.

Tempting idea: Reversing atomic and mass number.

Use instead: 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.

Tempting idea: Saying isotopes have different protons.

Use instead: 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.

Tempting idea: Forgetting electrons equal protons only in a neutral atom.

Use instead: Write the two numbers beside the symbol, state what each counts and subtract atomic number from mass number for neutrons.

Retrieval practice

Quick checks

1. Which exam method is most reliable for atoms, isotopes and nuclear notation?

2. Which statement correctly applies atoms, isotopes and nuclear notation to a question?

3. A pupil writes: “Reversing atomic and mass number.” Which replacement is accurate?

Worked examples

See the method being built

These examples expose the difference between a secure read nuclear notation method and the common error “Reversing atomic and mass number.”.

Example 1 · Read notation

For carbon-14, atomic number 6, find protons, neutrons and electrons in a neutral atom.

  1. Protons = atomic number.
  2. Neutrons = 14 − 6.
  3. 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.

  1. Check atomic number.
  2. Compare mass number.
  3. 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.

  1. A neutral atom has 12 electrons.
  2. 2+ means two electrons lost.
  3. Subtract 2.
Show the answer

Mg²⁺ has 10 electrons.

Exam precision

Exam technique: Read nuclear notation

Do this: Write the two numbers beside the symbol, state what each counts and subtract atomic number from mass number for neutrons.

Independent practice

Exam-style questions

Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.

1. State the charge on a proton.

[1 mark]
Show marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: +1.

2. An atom has mass number 23 and atomic number 11. Find neutrons.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award one mark for the correct relationship, one for a valid substitution and one for the final answer with its unit.

Model answer: 23 − 11 = 12.

3. Define isotope.

[2 marks]
Show marking guidance and model answer

Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.

Model answer: 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 marking guidance and model answer

Marking guidance: Award one mark for the precise statement shown in the model answer.

Model answer: 8.

5. Explain why isotopes have similar chemical properties.

[3 marks]
Show marking guidance and model answer

Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.

Model answer: They have the same electron arrangement when neutral.

6. An ion has 17 protons and 18 electrons. State its charge.

[2 marks]
Show marking guidance and model answer

Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.

Model answer: −1, because it has one extra electron.

Questions pupils ask

Atoms, Isotopes and Nuclear Notation FAQs

What is the main idea in 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.

What mistake should I avoid in Atoms, Isotopes and Nuclear Notation?

Reversing atomic and mass number. 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.

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