Polyatomic ions
A polyatomic ion is a collection of atoms of elements arranged in a certain pattern and carrying a charge.
It behaves as a single unit in a formula, with a valency of its own.
| Polyatomic ion | Chemical formula | Valency |
|---|---|---|
| Ammonium | NH4+ | 1 |
| Hydronium | H3O+ | 1 |
| Nitrate | NO3− | 1 |
| Hydrogen carbonate (bicarbonate) | HCO3− | 1 |
| Hydroxide | OH− | 1 |
| Permanganate | MnO4− | 1 |
| Hydrogen sulphate (bisulphate) | HSO4− | 1 |
| Chromate | CrO42− | 2 |
| Dichromate | Cr2O72− | 2 |
| Sulphate | SO42− | 2 |
| Carbonate | CO32− | 2 |
| Phosphate | PO43− | 3 |
Table 3.7 — common polyatomic ions.
The valency is the charge
Read the charge in the top right of each formula and you have the valency. One charge → valency 1. A 2− charge → valency 2. Phosphate's 3− → valency 3. There is nothing extra to memorise.
Note also that only two of the twelve are positive: ammonium and hydronium. The other ten are negative.