Every ionic compound is electrically neutral: the total positive charge from metal ions must exactly cancel the total negative charge from non-metal ions. Knowing each ion's charge lets you work out the correct formula for any ionic compound — a skill tested from KS3 through to GCSE and A-level.
What is an ionic formula?
A formula tells you the ratio of atoms (or ions) in a compound. In an ionic compound, the formula shows the simplest ratio of positive ions (cations) to negative ions (anions) that produces an overall charge of zero.
For example, sodium chloride is written NaCl — one Na⁺ (+1) and one Cl⁻ (−1), giving a total charge of 0. Magnesium chloride is written MgCl₂ — one Mg²⁺ (+2) and two Cl⁻ (2 × −1 = −2), also giving zero overall.
What charges do common ions carry?
Learn these before trying to write formulae:
| Ion | Symbol | Charge |
|---|---|---|
| Sodium | Na⁺ | +1 |
| Potassium | K⁺ | +1 |
| Silver | Ag⁺ | +1 |
| Magnesium | Mg²⁺ | +2 |
| Calcium | Ca²⁺ | +2 |
| Zinc | Zn²⁺ | +2 |
| Iron(II) | Fe²⁺ | +2 |
| Iron(III) | Fe³⁺ | +3 |
| Aluminium | Al³⁺ | +3 |
| Chloride | Cl⁻ | −1 |
| Bromide | Br⁻ | −1 |
| Iodide | I⁻ | −1 |
| Hydroxide | OH⁻ | −1 |
| Nitrate | NO₃⁻ | −1 |
| Oxide | O²⁻ | −2 |
| Sulfate | SO₄²⁻ | −2 |
| Carbonate | CO₃²⁻ | −2 |
| Phosphate | PO₄³⁻ | −3 |
The group number in the periodic table gives a quick guide: Group 1 metals form +1 ions; Group 2 form +2; Group 6 non-metals form −2; Group 7 form −1.
How do you balance charges to get the formula?
Step-by-step method:
- Write down the symbols and charges of both ions.
- Find the lowest common multiple (LCM) of the two charges (ignoring signs).
- Divide the LCM by each ion's charge to find how many of each ion you need.
- Write the formula with subscript numbers. If a polyatomic ion (like SO₄²⁻) needs more than one, put it in brackets: (SO₄)₂.
- Check: total positive charge + total negative charge = 0.
Worked examples
Example 1 — Calcium chloride:
- Ca²⁺ (charge +2) and Cl⁻ (charge −1)
- LCM of 2 and 1 = 2
- Ca: 2 ÷ 2 = 1 calcium ion; Cl: 2 ÷ 1 = 2 chloride ions
- Formula: CaCl₂
- Check: (+2) + 2 × (−1) = 0 ✓
Example 2 — Aluminium oxide:
- Al³⁺ (charge +3) and O²⁻ (charge −2)
- LCM of 3 and 2 = 6
- Al: 6 ÷ 3 = 2 aluminium ions; O: 6 ÷ 2 = 3 oxide ions
- Formula: Al₂O₃
- Check: 2 × (+3) + 3 × (−2) = +6 − 6 = 0 ✓
Example 3 — Iron(III) sulfate:
- Fe³⁺ (charge +3) and SO₄²⁻ (charge −2)
- LCM of 3 and 2 = 6
- Fe: 6 ÷ 3 = 2 iron ions; SO₄: 6 ÷ 2 = 3 sulfate ions
- Formula: Fe₂(SO₄)₃
- Check: 2 × (+3) + 3 × (−2) = +6 − 6 = 0 ✓
Note the brackets around SO₄ because the subscript 3 applies to the whole sulfate group, not just the oxygen.
Example 4 — Sodium hydroxide:
- Na⁺ (charge +1) and OH⁻ (charge −1)
- Charges cancel immediately: 1 Na⁺ and 1 OH⁻
- Formula: NaOH
- Check: (+1) + (−1) = 0 ✓
Can you use the "criss-cross" shortcut?
The criss-cross method swaps the charge numbers across to give the subscripts:
- Ca²⁺ and Cl⁻ → subscripts become Ca₁Cl₂ → CaCl₂
- Al³⁺ and O²⁻ → subscripts become Al₂O₃ → Al₂O₃
Important: always simplify. If the criss-cross gives Ca₂O₂, simplify to CaO. The criss-cross method is a shortcut, not a rule — the LCM method always works and always gives the simplest formula directly.
Frequently asked questions
Why does the formula show the simplest ratio rather than actual numbers?
Ionic compounds are giant lattice structures: Na⁺ and Cl⁻ ions alternate throughout a three-dimensional crystal, with billions of ions in each grain of salt. The formula NaCl does not mean "one sodium and one chloride" — it means the ratio is 1:1 throughout the lattice. This simplest whole-number ratio is called the empirical formula and is the conventional way to represent ionic compounds.
What does it mean when a polyatomic ion needs brackets in the formula?
Brackets indicate that the subscript applies to the whole group of atoms inside them, not just the last atom. For example, Ca(OH)₂ contains one calcium ion and two hydroxide groups — that is, two oxygen atoms and two hydrogen atoms, not one. Without brackets, CaOH₂ would incorrectly suggest one oxygen and two hydrogens with no implied grouping. Always use brackets when a polyatomic ion appears more than once in a formula.
How do I know whether to use iron(II) or iron(III)?
The Roman numeral tells you which ion is present. Iron(II) is Fe²⁺; iron(III) is Fe³⁺. If the question names the compound "iron(II) sulfate", use Fe²⁺. If it says "iron(III) chloride", use Fe³⁺. Without the Roman numeral, use context: rust is mainly iron(III) oxide (Fe₂O₃); iron nails are often coated in iron(II) sulfate solutions (light green). At KS3, the Roman numeral will always be given; at GCSE you may need to deduce it from the formula.
Why are some formulae the same as the "criss-cross" result and some need simplifying?
The LCM method always produces the correct simplest formula. The criss-cross method produces the correct result when the charges share no common factor (e.g. +3 and −2 share no factor other than 1, so Al₂O₃ is already simplified). When charges do share a factor, criss-cross gives an unsimplified formula that must be reduced. For example, Ca²⁺ and O²⁻: criss-cross gives Ca₂O₂, but the correct simplified formula is CaO. The LCM method avoids this error automatically.
For KS3 chemistry with Professor Curie — building from ion charges to the particle model of ionic structures — visit aitutors.me.