The Criss-Cross Trick, Level 2

Why it matters

Some ions are gangs of atoms that travel together: sulphate SO₄²⁻, hydroxide OH⁻, ammonium NH₄⁺. When a gang needs a subscript, it goes in brackets. Time for the advanced league.

Predict first

Aluminium is 3+ and sulphate (SO₄) is 2−. The formula of aluminium sulphate is…

criss-cross: 3 and 2 swap → Al₂(SO₄)₃. The brackets say 'three complete sulphate teams', not 'three extra oxygens'.

• AlSO₄
• Al₂SO₄₃ (no brackets)
• Al₂(SO₄)₃ — correct
• Al₃(SO₄)₂

What you do

Build Al₂(SO₄)₃, Ca(OH)₂ and (NH₄)₂SO₄. Watch when brackets appear — and when they don't.

Check yourself

How many oxygen atoms are in Al₂(SO₄)₃?

Each SO₄ team carries 4 oxygens; there are 3 teams: 4 × 3 = 12. Brackets multiply everything inside.

Sodium hydroxide is NaOH, but calcium hydroxide is Ca(OH)₂. Why the brackets?

Ca²⁺ + 2 × OH⁻ = neutral. Without brackets, CaOH₂ would wrongly mean one O and two H.

The formula of ammonium sulphate is…

Two NH₄⁺ teams cancel one SO₄²⁻ team: (NH₄)₂SO₄. We keep the radical visible because it acts as one unit in reactions.

• 4
• 7
• 12 — correct
• 3
• Calcium needs two complete OH teams to cancel its 2+ charge — correct
• OH behaves differently near calcium
• NaOH₂ would explode
• Brackets are decorative
• NH₄SO₄
• (NH₄)₂SO₄ — correct
• NH₄(SO₄)₂
• N₂H₈SO₄ (expanded)