Polyatomic Ions Not in Data Booklet
The IB syllabus does not provide polyatomic ion formulas or charges in the Data Booklet. You are expected to know nitrate, sulfate, carbonate, hydrogencarbonate, phosphate, hydroxide, and ammonium by heart.
| 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.
Constructing Neutral Polyatomic Formulas
Ammonium = NH₄⁺ (1+) and Sulfate = SO₄²⁻ (2−). To achieve electrical neutrality, two NH₄⁺ cations are required for every one SO₄²⁻ anion: (NH₄)₂SO₄.
The Parentheses / Bracket Rule
When you need more than one polyatomic ion to balance charge, you must put the polyatomic ion in brackets before adding the subscript: e.g. Mg(OH)₂ (1 Mg, 2 O, 2 H), NOT MgOH₂ (which would mean 1 Mg, 1 O, 2 H).
Try the Balancing Act
Practise balancing chemical equations interactively - includes polyatomic ion formulas.
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