Cation – IGCSE Chemistry Definition

A cation is a positively charged ion. In the 0620 syllabus, cations are central to ionic bonding, electrolysis, and tests for metal ions. The name itself helps: cation goes to the cathode.

The 0620 definition

A cation is a positively charged ion, formed when an atom (or group of atoms) loses one or more electrons.

How cations form

Metal atoms lose outer shell electrons to achieve a full outer shell:

Atom	Electrons lost	Cation formed	Electron arrangement
Sodium (2, 8, 1)	1	Na⁺ (2, 8)	Same as neon
Magnesium (2, 8, 2)	2	Mg²⁺ (2, 8)	Same as neon
Aluminium (2, 8, 3)	3	Al³⁺ (2, 8)	Same as neon
Calcium (2, 8, 8, 2)	2	Ca²⁺ (2, 8, 8)	Same as argon

Common cations

Name	Formula	Charge
Sodium	Na⁺	+1
Potassium	K⁺	+1
Magnesium	Mg²⁺	+2
Calcium	Ca²⁺	+2
Iron(II)	Fe²⁺	+2
Iron(III)	Fe³⁺	+3
Copper(II)	Cu²⁺	+2
Aluminium	Al³⁺	+3
Zinc	Zn²⁺	+2
Hydrogen	H⁺	+1
Ammonium	NH₄⁺	+1

Cations in electrolysis

During electrolysis, cations are attracted to the cathode (negative electrode). At the cathode, they gain electrons (are reduced):

Cu²⁺ + 2e⁻ → Cu

2H⁺ + 2e⁻ → H₂

For aqueous solutions, selective discharge determines whether the metal or hydrogen is produced.

Transition metal cations

Some metals can form more than one type of cation. Iron forms both Fe²⁺ (iron(II), green solutions) and Fe³⁺ (iron(III), brown/yellow solutions). Copper forms Cu²⁺ (copper(II), blue solutions). The Roman numeral in the name indicates the charge.

Worked exam question

During the electrolysis of copper(II) sulfate solution using carbon electrodes: (a) Name the cation that is discharged at the cathode. (1) (b) Write the half-equation. (1) (c) Describe what you would see at the cathode. (1)

Mark scheme

(a) Copper ion / Cu²⁺ [1]

(b) Cu²⁺ + 2e⁻ → Cu [1]

(c) A brown/pink/orange solid/coating/layer (of copper) is deposited on the electrode [1]

Common exam mistakes

• Confusing cations with anions. Cations are positive (lost electrons). Anions are negative (gained electrons).
• Saying cations go to the anode. Cations are positive and are attracted to the negative cathode.
• Forgetting that hydrogen (H⁺) is a cation. In aqueous electrolysis, H⁺ ions compete with metal cations for discharge at the cathode.