The idea
The nucleus contains protons and neutrons; electrons occupy regions around it. The number of protons determines the element. Neutrons = A − Z. Forming an ion changes the electron count without changing the element: a positive ion has lost electrons, while a negative ion has gained them.
Worked example
A sodium-23 ion has atomic number 11 and charge +1. Find its proton, neutron and electron counts.
- Protons = 11.
- Neutrons = 23 − 11 = 12.
- A +1 charge means one fewer electron than protons, so electrons = 10.
A mistake to avoid
Do not subtract the ionic charge from the proton count. Ordinary ion formation changes electrons, not the nucleus.
Check your understanding
How many electrons does a chloride ion, Cl⁻, have if its atomic number is 17?
Show the explanation
It has 18 electrons: one more electron than its 17 protons.
Explore the lesson
This guide connects to What's Inside an Atom? in the IB Chemistry course. The first two published lessons per subject are free previews; further interactive lessons require an account.