Ion – IGCSE Chemistry Definition
IGCSE Chemistry definition of ion: a charged particle formed when an atom gains or loses electrons. Covers cations, anions, and ion formation.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
An ion is a charged particle formed when an atom gains or loses electrons. Ion formation is the basis of ionic bonding and electrolysis, making it one of the most connected concepts in the 0620 syllabus. You must know how ions form, how to predict their charges, and how to write their formulae.
The 0620 definition
An ion is an atom (or group of atoms) that has lost or gained one or more electrons, so it carries an overall electric charge.
- Atoms that lose electrons become positive ions (cations): they have more protons than electrons.
- Atoms that gain electrons become negative ions (anions): they have more electrons than protons.
Predicting ion charges
| Group | Electrons lost/gained | Ion charge | Example |
|---|---|---|---|
| I (1) | Loses 1 | +1 | Na+ |
| II (2) | Loses 2 | +2 | Mg²+ |
| III (3) | Loses 3 | +3 | Al³+ |
| V (15) | Gains 3 | -3 | N³- |
| VI (16) | Gains 2 | -2 | O²- |
| VII (17) | Gains 1 | -1 | Cl- |
Metals form cations by losing electrons. Non-metals form anions by gaining electrons. The aim is to achieve a full outer shell (the same electron arrangement as the nearest noble gas).
Polyatomic ions
Some ions are groups of atoms bonded together carrying a charge:
| Ion | Formula | Charge |
|---|---|---|
| Hydroxide | OH- | -1 |
| Nitrate | NO₃- | -1 |
| Sulfate | SO₄²- | -2 |
| Carbonate | CO₃²- | -2 |
| Ammonium | NH₄+ | +1 |
Ion formation example
Sodium atom (2, 8, 1) loses 1 electron to form Na+ (2, 8). The ion has 11 protons but only 10 electrons, so the overall charge is +1.
Chlorine atom (2, 8, 7) gains 1 electron to form Cl- (2, 8, 8). The ion has 17 protons but 18 electrons, so the overall charge is -1.
Worked exam question
A magnesium atom has the electron arrangement 2, 8, 2. (a) State the charge on a magnesium ion. (1) (b) Explain how this ion forms. (2) (c) Why does magnesium form a positive ion? (1)
Mark scheme
(a) 2+ / Mg²+ [1]
(b) Magnesium atom loses 2 electrons (from its outer shell) [1]; to achieve a stable electron arrangement / full outer shell of 2, 8 [1]
(c) It has lost electrons so it has more protons than electrons / there is an overall positive charge [1]
Common exam mistakes
- Saying an ion has “gained protons” to become positive. The number of protons never changes in a chemical reaction — only electrons are transferred.
- Writing ion charges in the wrong place. The charge goes as a superscript after the symbol: Mg²+, not Mg+2 or 2Mg+.
- Confusing the number of electrons in the ion with the number of protons. A Na+ ion still has 11 protons — it just has 10 electrons.
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