These polyatomic ions appear in formulae and equations throughout GCSE Chemistry. When there are two or more of a polyatomic ion in a formula, it goes in brackets: Ca(OH)₂, Fe₂(SO₄)₃.
| Ion Name | Formula | Charge |
|---|---|---|
| Ammonium | NH₄⁺ | +1 |
| Hydroxide | OH⁻ | −1 |
| Nitrate | NO₃⁻ | −1 |
| Carbonate | CO₃²⁻ | −2 |
| Sulfate | SO₄²⁻ | −2 |
| Phosphate | PO₄³⁻ | −3 |
Example: Ca²⁺ + CO₃²⁻ → CaCO₃ (charges balance, 1:1 ratio). Mg²⁺ + OH⁻ → Mg(OH)₂ (need 2 OH⁻ to balance 2+ charge, hence brackets).
This deep dive covers Common Ions You Must Know within Formulae, Equations and Conservation of Mass for GCSE Chemistry. Revise Formulae, Equations and Conservation of Mass in Quantitative Chemistry for GCSE Chemistry with 13 exam-style questions and 12 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 3 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Formulae, Equations and Conservation of Mass
Which of the following is the correct formula for oxygen gas?
Explain why mass is conserved in a chemical reaction.