⚠️ Must Memorise
The syllabus does not provide polyatomic ion formulas in the Data Booklet. You must commit these to memory. Without them, you cannot balance any subsequent chemical equation.
| Polyatomic Ion | Formula | Charge |
|---|---|---|
| Ammonium | \(\text{NH}_4^+\) | 1+ |
| Hydroxide | \(\text{OH}^-\) | 1− |
| Nitrate | \(\text{NO}_3^-\) | 1− |
| Nitrite | \(\text{NO}_2^-\) | 1− |
| Hydrogencarbonate | \(\text{HCO}_3^-\) | 1− |
| Carbonate | \(\text{CO}_3^{2-}\) | 2− |
| Sulfate | \(\text{SO}_4^{2-}\) | 2− |
| Sulfite | \(\text{SO}_3^{2-}\) | 2− |
| Phosphate | \(\text{PO}_4^{3-}\) | 3− |
Building Neutral Ionic Formulas
When building ionic formulas, the total positive charge must equal the total negative charge to give a net-zero overall charge.
📐 Worked Example. Ammonium Sulfate
Ammonium = \(\text{NH}_4^+\) (1+) and Sulfate = \(\text{SO}_4^{2-}\) (2−)
Two ammonium ions needed to balance one sulfate ion:
\(\text{(NH}_4\text{)}_2\text{SO}_4\)
The brackets around NH₄ are mandatory. Writing NH₄₂SO₄ is a chemical impossibility and loses marks immediately.
⚠️ Bracket Rule
When you need more than one polyatomic ion, always use brackets around the ion before the subscript. Omitting brackets is a remarkably common error, even among advanced students.
Try the Balancing Act
Practise balancing chemical equations interactively - includes polyatomic ion formulas.