Cation and Anion – IGCSE Chemistry Definitions

The terms cation and anion appear throughout electrolysis, ionic bonding, and qualitative analysis. Candidates who mix them up lose marks not because they do not understand the chemistry, but because they reverse the labels and send ions to the wrong electrode.

The 0620 definitions

A cation is a positively charged ion, formed when an atom loses one or more electrons. Metals form cations. Examples: Na⁺, Mg²⁺, Al³⁺, Fe²⁺, Fe³⁺, Cu²⁺, NH₄⁺.

An anion is a negatively charged ion, formed when an atom gains one or more electrons. Non-metals form anions. Examples: Cl⁻, Br⁻, O²⁻, S²⁻, OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻.

How cations form

A metal atom has a small number of electrons in its outer shell. It loses these electrons to achieve a stable electron configuration (the same as the nearest noble gas). Each electron lost gives the ion one more positive charge:

Metal atom	Electrons lost	Cation formed	Electron configuration
Na (2,8,1)	1	Na⁺ (2,8)	Same as neon
Mg (2,8,2)	2	Mg²⁺ (2,8)	Same as neon
Al (2,8,3)	3	Al³⁺ (2,8)	Same as neon
Ca (2,8,8,2)	2	Ca²⁺ (2,8,8)	Same as argon

The ammonium ion (NH₄⁺) is a polyatomic cation — it carries a positive charge even though it contains only non-metal atoms.

How anions form

A non-metal atom has a nearly full outer shell. It gains electrons to complete the shell:

Non-metal atom	Electrons gained	Anion formed	Electron configuration
Cl (2,8,7)	1	Cl⁻ (2,8,8)	Same as argon
O (2,6)	2	O²⁻ (2,8)	Same as neon
S (2,8,6)	2	S²⁻ (2,8,8)	Same as argon

Polyatomic anions include OH⁻, NO₃⁻, SO₄²⁻, and CO₃²⁻. Their charges are fixed and must be memorised.

Behaviour in electrolysis

During electrolysis, ions move toward the electrode with the opposite charge:

• Cations (positive) move to the cathode (negative electrode). Here they gain electrons — this is reduction.
• Anions (negative) move to the anode (positive electrode). Here they lose electrons — this is oxidation.

Memory aid: Cations go to the Cathode. Anions go to the Anode.

Example — electrolysis of molten lead(II) bromide:

• Cathode: Pb²⁺ + 2e⁻ → Pb (cation gains electrons, reduced)
• Anode: 2Br⁻ → Br₂ + 2e⁻ (anion loses electrons, oxidised)

Cations and anions in ionic bonding

An ionic bond is the electrostatic attraction between a cation and an anion. Sodium chloride consists of Na⁺ cations and Cl⁻ anions arranged in a giant ionic lattice. The strong forces between the oppositely charged ions give ionic compounds high melting points.

Cations and anions in qualitative analysis

Paper 6 and Paper 4 test the identification of ions:

Cation tests — add sodium hydroxide solution and observe the precipitate colour:

• Cu²⁺ → blue precipitate
• Fe²⁺ → green precipitate
• Fe³⁺ → red-brown precipitate

Anion tests — use specific reagents:

• Cl⁻ → add AgNO₃ and dilute HNO₃: white precipitate
• SO₄²⁻ → add BaCl₂ and dilute HCl: white precipitate
• CO₃²⁻ → add dilute acid: effervescence, gas turns limewater milky

See the full cation test and anion test pages.

Common exam mistakes

1. Reversing cation and anion. Remember: cation = positive = cathode. The C’s go together.
2. Saying “cations gain electrons to become positive”. Cations are already positive because they have already lost electrons. At the cathode, cations gain electrons back and are reduced to atoms.
3. Forgetting polyatomic ions. NH₄⁺ is a cation despite containing only non-metals. OH⁻ is an anion despite containing oxygen and hydrogen.
4. Writing the wrong charge on common ions. Memorise: Group I = 1+, Group II = 2+, Group VII = 1-, Group VI = 2-.