How to work out the formula of an ionic compound

Writing the correct formula of an ionic compound is a skill you use constantly in SPM Chemistry, in the mole concept, chemical formula and equation, in every equation you balance, and throughout salt preparation and qualitative analysis. Get the formula wrong and the rest of the question collapses. The good news is that there is a dependable method that works every single time, once you know the charges of the common ions. This guide teaches that method step by step.

The one idea behind every ionic formula

An ionic compound is built from a positive ion (a cation, usually a metal or ammonium) and a negative ion (an anion, usually a non-metal or an acid radical). These ions come from the giving and taking of electrons you meet in the chemical bond chapter. The single rule that fixes the formula is this: the compound must be electrically neutral. The total positive charge must exactly cancel the total negative charge, so the number of each ion is chosen to make the charges add up to zero.

Everything else is just knowing the ions and doing simple arithmetic.

Step 1: Know your common ions and their charges

You cannot work out a formula without the charges, so these must be memorised. A useful shortcut is that the group number often tells you the charge.

• +1 cations: sodium Na⁺, potassium K⁺, silver Ag⁺, hydrogen H⁺, ammonium NH₄⁺
• +2 cations: magnesium Mg²⁺, calcium Ca²⁺, zinc Zn²⁺, copper(II) Cu²⁺, iron(II) Fe²⁺, lead(II) Pb²⁺
• +3 cations: aluminium Al³⁺, iron(III) Fe³⁺
• −1 anions: chloride Cl⁻, bromide Br⁻, iodide I⁻, nitrate NO₃⁻, hydroxide OH⁻
• −2 anions: oxide O²⁻, sulfide S²⁻, sulfate SO₄²⁻, carbonate CO₃²⁻
• −3 anions: nitride N³⁻, phosphate PO₄³⁻

Notice that some metals have a Roman numeral, such as iron(II) and iron(III) or copper(II). That numeral is the charge, so it tells you exactly which ion to use. Keep a copy of the common ions and formulae table beside you while you practise, until the charges are automatic.

Step 2: Write the two ions with their charges

Put the cation first and the anion second, each with its charge clearly shown. For sodium chloride you write Na⁺ and Cl⁻. For calcium oxide you write Ca²⁺ and O²⁻. This ordering, metal first, is the same order you use to name and write the finished formula.

Step 3: Balance the charges with the criss-cross method

The fastest reliable trick is to criss-cross the size of each charge (ignoring the + and −) so that it becomes the subscript of the other ion.

Take aluminium oxide. The ions are Al³⁺ and O²⁻. Cross the 3 down to the oxide and the 2 down to the aluminium, giving Al₂O₃. Check: two Al³⁺ give +6, three O²⁻ give −6, total zero. Correct.

Step 4: Simplify to the smallest whole-number ratio

Criss-crossing sometimes gives numbers that share a common factor, and a formula must always be the simplest ratio. Calcium oxide looks like Ca₂O₂ after crossing Ca²⁺ and O²⁻, but both subscripts divide by 2, so the real formula is CaO. Whenever the two charges are equal, the ions simply pair one-to-one.

Step 5: Use brackets for polyatomic ions

When an ion made of several atoms, such as nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, hydroxide OH⁻ or ammonium NH₄⁺, is needed more than once, wrap it in brackets before adding the subscript. This keeps the whole group multiplied.

Calcium nitrate is a clear case. The ions are Ca²⁺ and NO₃⁻. Criss-crossing needs two nitrates, so you write Ca(NO₃)₂, never CaNO₃₂, which would wrongly change the nitrate itself. If only one polyatomic ion is needed, no brackets are used: sodium nitrate is simply NaNO₃.

Worked examples

Magnesium chloride. Ions Mg²⁺ and Cl⁻. Criss-cross: one Mg needs two Cl, giving MgCl₂. Check: +2 and 2 × (−1) = 0. Correct.

Aluminium sulfate. Ions Al³⁺ and SO₄²⁻. Criss-cross gives two aluminium and three sulfate. Because sulfate is polyatomic and appears three times, use brackets: Al₂(SO₄)₃. Check: 2 × (+3) = +6, 3 × (−2) = −6. Correct.

Ammonium carbonate. Ions NH₄⁺ and CO₃²⁻. Criss-cross needs two ammonium ions, so bracket it: (NH₄)₂CO₃. Check: 2 × (+1) = +2, one carbonate = −2. Correct.

Iron(III) hydroxide. Ions Fe³⁺ and OH⁻. Criss-cross gives three hydroxides: Fe(OH)₃. Check: +3 and 3 × (−1) = 0. Correct.

Common mistakes to avoid

• Forgetting the Roman numeral. Iron(II) sulfate is FeSO₄, but iron(III) sulfate is Fe₂(SO₄)₃. The numeral changes everything.
• Dropping brackets. Writing Ca(OH)₂ without brackets as CaOH₂ describes a different, wrong particle count.
• Not simplifying. Mg²⁺ with O²⁻ is MgO, not Mg₂O₂.
• Guessing charges. If you are unsure, look the ion up rather than write a formula that is wrong from the start.

Practise until it is automatic

This skill rewards drilling more than memorising. Write out ten compounds a day from a mixed list of cations and anions, then check each against the common ions and formulae reference. Once the charges are second nature, the criss-cross and simplify steps take only seconds. If formulae keep tripping you up, usually because the ion charges are not yet solid, that is a fast thing for a teacher to fix. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like the method drilled against your own weak spots.