Negative Ions (Anions)
Summary: An anion is a negative ion formed when a non-metal atom gains one or more electrons into its outer shell, leaving it with more electrons than protons. Tags: igcse chemistry Created: 2026-07-18
An anion is a negatively charged Ion formed when an atom — almost always a non-metal — gains one or more electrons into its outer shell. Each added electron brings an extra negative charge with no change to the nucleus, so the particle ends up with more electrons than protons and carries an overall negative charge. Non-metal atoms gain electrons rather than lose them because they typically have 5–7 valence electrons, and gaining the few electrons needed to complete the shell is the easiest route to a stable noble gas Electronic Configuration (see Group 0 Noble Gases). The size of the negative charge equals the number of electrons gained, which for Groups VI and VII equals the group number subtracted from 8. Anions are the negative half of every Ionic Bond, held to Positive Ions (Cations) by strong electrostatic attraction. During Electrolysis, anions migrate to the positive electrode (anode), which is the origin of their name.
Formation of Negative Ions (Anions)
A non-metal atom forms an anion by gaining enough electrons to fill its outer shell. The nucleus is unchanged — the number of protons stays the same — so the particle becomes negative because electrons > protons.
The gain of electrons is written as an equation with the electrons on the left-hand side (reactants):
| Atom | Electron gain equation | Configuration change | Ion formed |
|---|---|---|---|
| Chlorine | Cl + e⁻ → Cl⁻ | 2.8.7 → 2.8.8 | Cl⁻ (chloride) |
| Oxygen | O + 2e⁻ → O²⁻ | 2.6 → 2.8 | O²⁻ (oxide) |
In each case the ion achieves a full outer Electron Shell — Cl⁻ has the configuration of argon (2.8.8) and O²⁻ has the configuration of neon (2.8). Notice also that the ion’s name changes: simple anions take the -ide ending (chloride, oxide, sulfide, nitride).
Predicting the Charge from the Group Number
For non-metals in Groups VI and VII, the charge on the anion equals the group number minus 8, because (8 − group number) electrons must be gained to complete the outer shell:
| Group | Outer electrons | Electrons gained | Charge on ion | Examples |
|---|---|---|---|---|
| V | 5 | 3 | 3− | N³⁻ (nitride), P³⁻ (phosphide) |
| VI | 6 | 2 | 2− | O²⁻ (oxide), S²⁻ (sulfide) |
| VII | 7 | 1 | 1− | F⁻, Cl⁻, Br⁻, I⁻ |
This is the mirror image of the group-number rule for Positive Ions (Cations) and is part of the wider pattern discussed in Periodic Table Trends.
Table of Common Anions
| Charge | Simple anions | Polyatomic anions |
|---|---|---|
| 1− | F⁻ (fluoride), Cl⁻ (chloride), Br⁻ (bromide), I⁻ (iodide) | OH⁻ (hydroxide), NO₃⁻ (nitrate), HCO₃⁻ (hydrogencarbonate) |
| 2− | O²⁻ (oxide), S²⁻ (sulfide) | SO₄²⁻ (sulfate), CO₃²⁻ (carbonate) |
| 3− | N³⁻ (nitride) | PO₄³⁻ (phosphate) |
Polyatomic anions are groups of atoms joined by covalent bonds that carry an overall negative charge and behave as a single unit in ionic compounds — for example, the nitrate ion in sodium nitrate (NaNO₃) or the sulfate ion in copper(II) sulfate (CuSO₄). Their names typically end in -ate when they contain oxygen.
Why Gaining Electrons, Not Losing?
A Group VII element such as chlorine would need to lose seven electrons to expose a full inner shell but only needs to gain one to complete its outer shell. Gaining one or two electrons is energetically far easier, so non-metals form anions rather than cations. When a metal and a non-metal react, the electrons lost by the metal are the very electrons gained by the non-metal — this electron transfer, and the Ionic Bond that results, is described in full in 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 |
|---|---|
| ”Cl⁻ is negative because it lost a proton” | The nucleus never changes in a chemical reaction. Cl⁻ is negative because it has gained an electron — 17 protons but 18 electrons |
| ”Cl⁻ becomes an argon atom because it has argon’s configuration (2.8.8)“ | Cl⁻ has the same electron arrangement as argon but still has 17 protons, so it is a chloride ion, not argon |
| ”The electron gain equation should be Cl → Cl⁻ + e⁻“ | Electrons are gained, so they appear on the left: Cl + e⁻ → Cl⁻ |
| “Chloride and chlorine are the same thing” | Chlorine (Cl or Cl₂) is the neutral element; chloride (Cl⁻) is the negative ion — the -ide ending signals an anion |
| ”Nitrate and nitride are interchangeable names” | Nitride is the simple ion N³⁻; nitrate is the polyatomic ion NO₃⁻ — completely different species |
| ”Polyatomic ions like SO₄²⁻ are held together ionically inside” | The atoms within a polyatomic ion are covalently bonded; the ion as a whole then forms ionic bonds with cations |