Describe atomic structure and calculate particle numbers
Relate proton number, nucleon number and charge to the particles in an atom or ion, and describe where mass and charge are found.
AS Level · Cambridge International Chemistry 9701, 2025–2027 syllabus. Electronic configurations are limited to ground-state atoms and ions of elements H to Kr. Written practice is self-checked; it does not replace cumulative exam practice or human subject-teacher review.
Before you begin
- Use atomic symbols and the Periodic Table, and distinguish an atom from an ion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the relative charge of an electron?
What you will learn
- Describe the nucleus and the space around it
- Use proton, neutron and electron charges and relative masses
- Calculate particle numbers from nuclide notation and ionic charge
A compact nucleus and a much larger region around it
An atom is mostly empty space. Nearly all of its mass and all of its positive charge are concentrated in a very small, dense nucleus containing protons and neutrons. Electrons occupy shells in the surrounding region; their negative charge balances the positive nuclear charge in a neutral atom.
Relative charges are proton +1, neutron 0 and electron −1. Relative masses are approximately proton 1, neutron 1 and electron 1/1840. These are relative values, not masses in kilograms. The nucleus is tiny compared with the whole atom, even though it contains almost all the atom's mass.
Use Z, A and charge as a particle-counting system
The proton number Z gives the number of protons and identifies the element. The nucleon number A is the total number of protons and neutrons, so neutrons = A − Z. A neutral atom has Z electrons. A positive ion has lost electrons; a negative ion has gained them.
For an ion with signed charge q, electron number = Z − q. For example, q = +2 means two fewer electrons than protons, whereas q = −1 means one more electron. Keep the element's proton number unchanged when it forms an ion.
Pause and explain
How many neutrons are in ³⁹₁₉K?
Worked example
Determine the numbers of protons, neutrons and electrons in ⁵⁶₂₆Fe²⁺.
Show the worked solution
- Z = 26, so there are 26 protons.
- Neutrons = A − Z = 56 − 26 = 30.
- A 2+ ion has lost two electrons: 26 − 2 = 24.
Answer 26 protons, 30 neutrons and 24 electrons.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For ³⁵₁₇Cl⁻, find the proton, neutron and electron numbers.
Give me a hint
Read Z and A first; then use the charge to find electrons.
Compare my reasoning
- Protons = Z = 17.
- Neutrons = 35 − 17.
- A 1− ion has one extra electron.
17 protons, 18 neutrons and 18 electrons.
Look for these in your work
- Proton number identified
- Neutrons calculated as A − Z
- Negative charge adds one electron
An ion has 13 protons, 14 neutrons and 10 electrons. Give its nuclide symbol and charge.
Give me a hint
The element is fixed by its proton number; compare proton and electron counts.
Compare my reasoning
- Z = 13 identifies aluminium.
- A = 13 + 14 = 27.
- There are three more protons than electrons.
²⁷₁₃Al³⁺.
Look for these in your work
- Element and nucleon number correct
- Charge +3 follows from the particle counts
A student says that a ²⁴₁₂Mg²⁺ ion has 12 protons, 12 neutrons and 14 electrons. Identify and correct the error.
Give me a hint
Only the electron count changes when the atom forms a positive ion.
Compare my reasoning
- Protons remain equal to Z.
- Neutrons remain A − Z.
- A 2+ charge means two fewer electrons than protons.
The ion has 12 protons and 12 neutrons, but 10 electrons. A 2+ ion has lost two electrons, so electron number is 12 − 2.
Look for these in your work
- Proton and neutron counts retained
- Electron count corrected using charge
Common mistakes
- Using A as the proton number
- Changing the number of protons when an atom becomes an ion
- Adding electrons to a positive ion
What can you explain now?
What is the relative charge of an electron?
Compare with the explanation
−1
The electron has one unit of negative relative charge.
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Make it stick
How do Z, A and ionic charge determine the numbers of protons, neutrons and electrons?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.