Anion – IGCSE Chemistry Definition

An anion is a negatively charged ion. The term is especially important in the electrochemistry section of the 0620 syllabus, where you need to predict which ions migrate to which electrode during electrolysis. The name itself is a memory aid: anion goes to the anode.

The 0620 definition

An anion is a negatively charged ion, formed when an atom (or group of atoms) gains one or more electrons.

How anions form

Non-metal atoms gain electrons to achieve a full outer shell:

Atom	Electrons gained	Anion formed	Electron arrangement
Chlorine (2, 8, 7)	1	Cl⁻ (2, 8, 8)	Same as argon
Oxygen (2, 6)	2	O²⁻ (2, 8)	Same as neon
Nitrogen (2, 5)	3	N³⁻ (2, 8)	Same as neon

Common anions

Simple anions (single atom)

Name	Formula	Charge
Chloride	Cl⁻	-1
Bromide	Br⁻	-1
Iodide	I⁻	-1
Oxide	O²⁻	-2
Sulfide	S²⁻	-2

Polyatomic anions (group of atoms)

Name	Formula	Charge
Hydroxide	OH⁻	-1
Nitrate	NO₃⁻	-1
Sulfate	SO₄²⁻	-2
Carbonate	CO₃²⁻	-2

Anions in electrolysis

During electrolysis, anions are attracted to the anode (positive electrode). At the anode, they are oxidised — they lose electrons:

2Cl⁻ → Cl₂ + 2e⁻

4OH⁻ → 2H₂O + O₂ + 4e⁻

Worked exam question

During electrolysis of molten lead(II) bromide: (a) Name the anion present. (1) (b) To which electrode does this anion migrate? (1) (c) Write the half-equation for the reaction of this anion. (2)

Mark scheme

(a) Bromide ion / Br⁻ [1]

(b) The anode / positive electrode [1]

(c) 2Br⁻ → Br₂ + 2e⁻ [1 for equation, 1 for correct balancing]

Common exam mistakes

• Confusing anions with cations. Anions are negative (gained electrons). Cations are positive (lost electrons). Remember: anion has an “n” for negative.
• Saying anions go to the cathode. Anions are negative and are attracted to the positive electrode (anode).
• Forgetting polyatomic anions. Not all anions are single atoms — sulfate (SO₄²⁻) and nitrate (NO₃⁻) are anions too.