Positive Ions (Cations)
Summary: A cation is a positive ion formed when a metal atom loses one or more outer-shell electrons, leaving it with more protons than electrons. Tags: igcse chemistry Created: 2026-07-18
A cation is a positively charged Ion formed when an atom — almost always a metal — loses one or more of its outer-shell electrons. Because electrons carry a negative charge, removing them leaves the particle with more protons than electrons, giving it an overall positive charge. Metal atoms lose electrons rather than gain them because they typically have only 1–3 valence electrons, and losing these is the easiest route to the stable, full outer shell of a noble gas Electronic Configuration (see Group 0 Noble Gases). The size of the positive charge equals the number of electrons lost, which for Groups I–III equals the group number. Cations are one half of every Ionic Bond, attracting Negative Ions (Anions) through strong electrostatic forces, and they also form the fixed lattice of positive ions in Metallic Bonding. During Electrolysis, cations migrate to the negative electrode (cathode), which is how they earned their name.
Formation of Positive Ions (Cations)
A metal atom forms a cation by losing all of its outer-shell electrons, so that the shell beneath — which is already full — becomes the new outer shell. The atom’s nucleus is unchanged: the number of protons stays the same, so the particle is now positive because protons > electrons.
The loss of electrons is written as an equation with the electrons on the right-hand side (products):
| Atom | Electron loss equation | Configuration change | Ion formed |
|---|---|---|---|
| Sodium | Na → Na⁺ + e⁻ | 2.8.1 → 2.8 | Na⁺ |
| Magnesium | Mg → Mg²⁺ + 2e⁻ | 2.8.2 → 2.8 | Mg²⁺ |
| Aluminium | Al → Al³⁺ + 3e⁻ | 2.8.3 → 2.8 | Al³⁺ |
In each case the resulting ion has the same Electronic Configuration as neon (2.8) — a full outer Electron Shell. Note that the ion is not a neon atom: it still has the original element’s number of protons.
Predicting the Charge from the Group Number
For metals in Groups I–III, the charge on the cation equals the group number, because the group number equals the number of outer-shell electrons available to lose:
| Group | Outer electrons | Electrons lost | Charge on ion | Examples |
|---|---|---|---|---|
| I | 1 | 1 | 1+ | Li⁺, Na⁺, K⁺ |
| II | 2 | 2 | 2+ | Mg²⁺, Ca²⁺, Ba²⁺ |
| III | 3 | 3 | 3+ | Al³⁺ |
This periodic pattern is one of the trends described in Periodic Table Trends.
Table of Common Cations
| Charge | Cations |
|---|---|
| 1+ | H⁺ (hydrogen), Li⁺ (lithium), Na⁺ (sodium), K⁺ (potassium), Ag⁺ (silver), NH₄⁺ (ammonium — polyatomic) |
| 2+ | Mg²⁺ (magnesium), Ca²⁺ (calcium), Ba²⁺ (barium), Zn²⁺ (zinc), Cu²⁺ (copper(II)), Fe²⁺ (iron(II)), Pb²⁺ (lead(II)) |
| 3+ | Al³⁺ (aluminium), Fe³⁺ (iron(III)), Cr³⁺ (chromium(III)) |
Two special cases are worth noting:
- Transition metals can form cations with more than one charge — the Roman numeral in the name gives the charge, e.g. iron(II) = Fe²⁺ and iron(III) = Fe³⁺.
- The ammonium ion (NH₄⁺) is a polyatomic cation: a group of covalently bonded atoms carrying an overall positive charge. It is the most important non-metal cation at IGCSE.
Why Losing Electrons, Not Gaining?
A Group I metal such as sodium would need to gain seven electrons to fill its outer shell but only needs to lose one to expose a full shell beneath. Losing the small number of outer electrons requires far less energy, so metals always form cations rather than anions. This is the mirror image of what happens with non-metals, which gain electrons to form Negative Ions (Anions). The complete picture of how cations and anions combine is covered in Ionic Bond and Ions and Ionic Bonds.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.4 (Ions and ionic bonds, Core point 1), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Na⁺ has gained a positive charge (gained a proton)“ | The nucleus never changes in a chemical reaction. Na⁺ is positive because it has lost an electron — 11 protons but only 10 electrons |
| ”Na⁺ becomes a neon atom because it has neon’s configuration (2.8)“ | Na⁺ has the same electron arrangement as neon but still has 11 protons, so it is a sodium ion, not neon |
| ”The electron loss equation should be Na + e⁻ → Na⁺“ | Electrons are lost, so they appear on the right: Na → Na⁺ + e⁻ |
| “Cations are always metal ions” | Almost always, but NH₄⁺ (ammonium) and H⁺ are important non-metal cations |
| ”Bigger charge means the atom lost electrons more easily” | The opposite — each successive electron is harder to remove; Al forms Al³⁺ only because the full-shell configuration it achieves is so stable |