A polyatomic ion is a group of atoms that stays together and carries one overall charge. In formulas they act as one unit, which is why you'll see brackets: Ca(OH)₂ has two hydroxide ions.
Start with the ones you'll see most
| Ion | Formula |
|---|---|
| ammonium | NH₄⁺ |
| hydroxide | OH⁻ |
| nitrate | NO₃⁻ |
| sulfate | SO₄²⁻ |
| carbonate | CO₃²⁻ |
| phosphate | PO₄³⁻ |
| acetate | CH₃COO⁻ |
Ammonium is the only common positive one.
The -ate / -ite pattern
Learn the -ate ion; the -ite ion has one less oxygen and the same charge:
- sulfate SO₄²⁻ → sulfite SO₃²⁻
- nitrate NO₃⁻ → nitrite NO₂⁻
The chlorine family adds two prefixes:
| Name | Formula |
|---|---|
| perchlorate | ClO₄⁻ |
| chlorate | ClO₃⁻ |
| chlorite | ClO₂⁻ |
| hypochlorite | ClO⁻ |
Adding hydrogen
Putting H⁺ on a negative ion raises its charge by one: carbonate CO₃²⁻ becomes hydrogen carbonate HCO₃⁻ (also called bicarbonate).
What you do have to memorize
The charge on each -ate ion and how many oxygens it has. There's no rule that gives you NO₃⁻ but SO₄²⁻; that part is just practice, which is what this deck is for.