Formulae with polyatomic ions
A polyatomic ion goes into a formula exactly as an element does. Cross the valencies over, and treat the whole ion as one thing.
| Compound | Ions | Valencies | Chemical formula |
|---|---|---|---|
| Sodium nitrate | Na+ · NO3− | 1 · 1 | NaNO3 |
| Potassium carbonate | K+ · CO32− | 1 · 2 | K2CO3 |
| Magnesium nitrate | Mg2+ · NO3− | 2 · 1 | Mg(NO3)2 |
| Ammonium phosphate | NH4+ · PO43− | 1 · 3 | (NH4)3PO4 |
When to use brackets
When more than one polyatomic ion is included in the chemical formula of a compound, they are written inside brackets.
Look at the last two rows to see why the rule is needed. Magnesium nitrate holds two nitrate ions, so it is Mg(NO3)2 — the 2 applies to the whole NO3 group. Without the brackets, MgNO3 2 would say something quite different about how many oxygen atoms there are.
Sodium nitrate needs no brackets: there is only one nitrate ion in it, so NaNO3 is unambiguous.