Formulae for Ionic Compounds

To build an ionic formula you first need to know the charge on each ion. For a single (monatomic) ion the charge usually follows its group in the periodic table; for a polyatomic ion (a charged group of atoms) you simply have to learn it.

Single-ion charges from the group:

Group	Ion type	Charge	Examples
1	metal (cation)	+1	Na⁺, K⁺, Li⁺
2	metal (cation)	+2	Mg²⁺, Ca²⁺
3	metal (cation)	+3	Al³⁺
5	non-metal (anion)	−3	N³⁻
6	non-metal (anion)	−2	O²⁻, S²⁻
7	non-metal (anion)	−1	Cl⁻, Br⁻, F⁻

Polyatomic ions — LEARN these five for Double Award:

Name	Formula	Charge
Ammonium	NH₄⁺	+1
Hydroxide	OH⁻	−1
Nitrate	NO₃⁻	−1
Carbonate	CO₃²⁻	−2
Sulfate	SO₄²⁻	−2

Transition metals are special. Metals like iron and copper can form ions of more than one charge, so the charge is written as a Roman numeral in the name: iron(II) = Fe²⁺, iron(III) = Fe³⁺, copper(II) = Cu²⁺. Read the name carefully — it tells you the charge.

Every ionic compound is electrically neutral — the total positive charge equals the total negative charge. There is a fast, reliable method to find the formula.

The swap-and-drop (cross-over) method:

1. Write the cation first, then the anion, each with its charge.
2. Swap and drop: the size of each ion's charge (ignore the + or −) becomes the subscript of the other ion.
3. Simplify the subscripts to the smallest whole-number ratio.
4. A subscript of 1 is never written.

Watch the simplifying step. Cross-over can give a ratio that is not yet simplest:

• Mg²⁺ + O²⁻ → cross-over gives Mg₂O₂ → simplify → MgO.
• Ca²⁺ + S²⁻ → cross-over gives Ca₂S₂ → simplify → CaS.

Whenever both charges are the same size, the ions combine 1:1 — so the subscripts cancel down to 1 each.

Check it works. After writing the formula, multiply each ion's charge by its subscript and confirm the totals cancel:

• Al₂O₃: positives = 2 × (+3) = +6; negatives = 3 × (−2) = −6. +6 − 6 = 0 ✓

Most exam formulae fall into a few simple patterns. Once you spot the pattern you can write the formula almost instantly.

Pattern A — equal and opposite charges → 1:1.

• Sodium chloride: Na⁺ + Cl⁻ → NaCl.
• Magnesium oxide: Mg²⁺ + O²⁻ → MgO.
• Calcium sulfide: Ca²⁺ + S²⁻ → CaS.

Pattern B — a +2 ion with a −1 ion → one of the +2, two of the −1.

• Magnesium chloride: Mg²⁺ + Cl⁻ → MgCl₂ (two Cl⁻ balance one Mg²⁺).
• Calcium bromide: Ca²⁺ + Br⁻ → CaBr₂.

Pattern C — a +1 ion with a −2 ion → two of the +1, one of the −2.

• Sodium oxide: Na⁺ + O²⁻ → Na₂O (two Na⁺ balance one O²⁻).
• Potassium sulfide: K⁺ + S²⁻ → K₂S.

Pattern D — a +3 ion with a −1 ion → one +3, three −1.

• Aluminium chloride: Al³⁺ + Cl⁻ → AlCl₃.
• Iron(III) chloride: Fe³⁺ + Cl⁻ → FeCl₃ (the (III) tells you Fe is +3).

Cation charge	Anion charge	Ratio	Example
+1	−1	1 : 1	NaCl
+2	−2	1 : 1	MgO
+2	−1	1 : 2	MgCl₂
+1	−2	2 : 1	Na₂O
+3	−1	1 : 3	AlCl₃
+3	−2	2 : 3	Al₂O₃

Reading transition-metal names. Because iron(II) is Fe²⁺ but iron(III) is Fe³⁺, iron(II) chloride is FeCl₂ while iron(III) chloride is FeCl₃ — the Roman numeral changes the formula.

A polyatomic ion is a charged group of atoms that stays together as a single unit (e.g. hydroxide OH⁻, sulfate SO₄²⁻). The cross-over rule works exactly the same way — just treat the whole group as one ion.

The bracket rule. If you need more than one polyatomic ion, you must put the whole ion in brackets and write the subscript outside the bracket. If you need only one, no brackets are used.

Worked through:

• Calcium hydroxide. Ca²⁺ + OH⁻ → need two OH⁻ to balance Ca²⁺ → Ca(OH)₂. The 2 multiplies everything inside: 1 Ca, 2 O, 2 H.
• Ammonium sulfate. NH₄⁺ + SO₄²⁻ → need two NH₄⁺ to balance SO₄²⁻ → (NH₄)₂SO₄. (Only one sulfate, so it has no brackets.)
• Aluminium sulfate. Al³⁺ + SO₄²⁻ → cross-over gives two Al and three SO₄ → Al₂(SO₄)₃. The three sulfates need brackets.
• Aluminium nitrate. Al³⁺ + NO₃⁻ → need three NO₃⁻ → Al(NO₃)₃.

Why brackets matter. Ca(OH)₂ means two whole hydroxide ions (2 O and 2 H). Writing "CaOH₂" would mean 1 O and 2 H — a completely different (and wrong) formula. The brackets show the subscript applies to the whole group.