Radicals: Atoms That Travel in Gangs

Why it matters

Some atoms refuse to travel alone. Sulphate — one sulphur and four oxygens — moves through every reaction as a single gang of five, wearing one shared 2− badge. Chemistry calls these gangs radicals, and they criss-cross just like single atoms.

Predict first

Copper sulphate (CuSO₄) dissolves in water. What happens to the sulphate part — one sulphur plus four oxygens?

A radical is an atom or a group of atoms that behaves as a single unit with a positive or negative charge. SO₄²⁻ stays together — and its charge is its valency, so it criss-crosses exactly like a single atom.

• It travels as one charged unit: the SO₄²⁻ radical — correct
• It splits into separate sulphur and oxygen atoms
• The oxygen escapes as O₂ gas

What you do

Criss-cross with whole radicals: treat NH₄⁺ (valency 1), CO₃²⁻ (2), NO₃⁻ (1) and SO₄²⁻ (2) as single unbreakable pieces. If a compound radical is taken more than once, lock it in brackets.

Check yourself

What exactly is a radical?

Simple radicals are single charged atoms (Mg²⁺); compound radicals are charged groups of different atoms (SO₄²⁻, NH₄⁺) that act as one unit through every reaction.

The carbonate radical is CO₃²⁻. Its valency is…

The charge on a radical is its valency. CO₃²⁻ carries 2−, so its combining power is 2 — which is why calcium carbonate is simply CaCO₃: valencies 2 and 2 cancel.

Why does aluminium sulphate need brackets: Al₂(SO₄)₃?

Criss-cross Al³⁺ with SO₄²⁻: two aluminiums, three sulphates. When a group of atoms takes a valency number greater than 1, it is enclosed in parentheses — (SO₄)₃ means 3 sulphurs and 12 oxygens.

• An atom or group of atoms behaving as a single unit with a positive or negative charge — correct
• Any very reactive metal
• One half of a diatomic molecule like O₂
• 2 — correct
• 3
• 4
• The whole SO₄ group is taken three times, so it is bracketed before the subscript — correct
• Brackets show that sulphate is a gas
• The 3 multiplies only the nearest oxygen atoms